emacs/doc/misc/cl.texi

\input texinfo    @c -*-texinfo-*-
@setfilename ../../info/cl.info
@settitle Common Lisp Extensions
@include docstyle.texi
@include emacsver.texi

@copying
This file documents the GNU Emacs Common Lisp emulation package.

Copyright @copyright{} 1993, 2001--2024 Free Software Foundation, Inc.

@quotation
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with no
Invariant Sections, with the Front-Cover Texts being ``A GNU Manual'',
and with the Back-Cover Texts as in (a) below.  A copy of the license
is included in the section entitled ``GNU Free Documentation License''.

(a) The FSF's Back-Cover Text is: ``You have the freedom to copy and
modify this GNU manual.''
@end quotation
@end copying

@dircategory Emacs lisp libraries
@direntry
* CL-Lib: (cl).                 Partial Common Lisp support for Emacs Lisp.
@end direntry

@finalout

@titlepage
@sp 6
@center @titlefont{Common Lisp Extensions}
@sp 4
@center For GNU Emacs Lisp
@sp 1
@center as distributed with Emacs @value{EMACSVER}
@sp 5
@center Dave Gillespie
@center daveg@@synaptics.com
@page
@vskip 0pt plus 1filll
@insertcopying
@end titlepage

@contents

@ifnottex
@node Top
@top GNU Emacs Common Lisp Emulation

@insertcopying
@end ifnottex

@menu
* Overview::             Basics, usage, organization, naming conventions.
* Printing::             Human friendly printing with @code{cl-prin1}.
* Program Structure::    Arglists, @code{cl-eval-when}.
* Predicates::           Type predicates and equality predicates.
* Control Structure::    Assignment, conditionals, blocks, looping.
* Macros::               Destructuring, compiler macros.
* Declarations::         @code{cl-proclaim}, @code{cl-declare}, etc.
* Symbols::              Property lists, creating symbols.
* Numbers::              Predicates, functions, random numbers.
* Sequences::            Mapping, functions, searching, sorting.
* Lists::                Functions, substitution, sets, associations.
* Structures::           @code{cl-defstruct}.
* Assertions::           Assertions and type checking.

Appendices
* Efficiency Concerns::            Hints and techniques.
* Common Lisp Compatibility::      All known differences with Steele.
* Porting Common Lisp::            Hints for porting Common Lisp code.
* Obsolete Features::              Obsolete features.
* GNU Free Documentation License:: The license for this documentation.

Indexes
* Function Index::                 An entry for each documented function.
* Variable Index::                 An entry for each documented variable.
* Concept Index::                  An entry for each concept.
@end menu

@node Overview
@chapter Overview

@noindent
This document describes a set of Emacs Lisp facilities borrowed from
Common Lisp.  All the facilities are described here in detail.  While
this document does not assume any prior knowledge of Common Lisp, it
does assume a basic familiarity with Emacs Lisp.

Common Lisp is a huge language, and Common Lisp systems tend to be
massive and extremely complex.  Emacs Lisp, by contrast, is rather
minimalist in the choice of Lisp features it offers the programmer.
As Emacs Lisp programmers have grown in number, and the applications
they write have grown more ambitious, it has become clear that Emacs
Lisp could benefit from many of the conveniences of Common Lisp.

The @dfn{CL} package adds a number of Common Lisp functions and
control structures to Emacs Lisp.  While not a 100% complete
implementation of Common Lisp, it adds enough functionality
to make Emacs Lisp programming significantly more convenient.

Some Common Lisp features have been omitted from this package
for various reasons:

@itemize @bullet
@item
Some features are too complex or bulky relative to their benefit
to Emacs Lisp programmers.  CLOS and Common Lisp streams are fine
examples of this group.  (The separate package EIEIO implements
a subset of CLOS functionality.  @xref{Top, , Introduction, eieio, EIEIO}.)

@item
Other features cannot be implemented without modification to the
Emacs Lisp interpreter itself, such as multiple return values,
case-insensitive symbols, and complex numbers.
This package generally makes no attempt to emulate these features.

@end itemize

This package was originally written by Dave Gillespie,
@file{daveg@@synaptics.com}, as a total rewrite of an earlier 1986
@file{cl.el} package by Cesar Quiroz.  Care has been taken to ensure
that each function is defined efficiently, concisely, and with minimal
impact on the rest of the Emacs environment.  Stefan Monnier added the
file @file{cl-lib.el} and rationalized the namespace for Emacs 24.3.

@menu
* Usage::                How to use this package.
* Organization::         The package's component files.
* Naming Conventions::   Notes on function names.
@end menu

@node Usage
@section Usage

@noindent
This package is distributed with Emacs, so there is no need
to install any additional files in order to start using it.  Lisp code
that uses features from this package should simply include at
the beginning:

@example
(require 'cl-lib)
@end example

@noindent
You may wish to add such a statement to your init file, if you
make frequent use of features from this package.

Code that only uses macros from this package can enclose the above in
@code{eval-when-compile}.  Internally, this library is divided into
several files, @pxref{Organization}.  Your code should only ever load
the main @file{cl-lib} file, which will load the others as needed.

@node Organization
@section Organization

@noindent
The Common Lisp package is organized into four main files:

@table @file
@item cl-lib.el
This is the main file, which contains basic functions
and information about the package.  This file is relatively compact.

@item cl-extra.el
This file contains the larger, more complex or unusual functions.
It is kept separate so that packages which only want to use Common
Lisp fundamentals like the @code{cl-incf} function won't need to pay
the overhead of loading the more advanced functions.

@item cl-seq.el
This file contains most of the advanced functions for operating
on sequences or lists, such as @code{cl-delete-if} and @code{cl-assoc}.

@item cl-macs.el
This file contains the features that are macros instead of functions.
Macros expand when the caller is compiled, not when it is run, so the
macros generally only need to be present when the byte-compiler is
running (or when the macros are used in uncompiled code).  Most of the
macros of this package are isolated in @file{cl-macs.el} so that they
won't take up memory unless you are compiling.
@end table

The file @file{cl-lib.el} includes all necessary @code{autoload}
commands for the functions and macros in the other three files.
All you have to do is @code{(require 'cl-lib)}, and @file{cl-lib.el}
will take care of pulling in the other files when they are
needed.

There is another file, @file{cl.el}, which was the main entry point to
this package prior to Emacs 24.3.  Nowadays, it is replaced by
@file{cl-lib.el}.  The two provide the same features (in most cases),
but use different function names (in fact, @file{cl.el} mainly just
defines aliases to the @file{cl-lib.el} definitions).  Where
@file{cl-lib.el} defines a function called, for example,
@code{cl-incf}, @file{cl.el} uses the same name but without the
@samp{cl-} prefix, e.g., @code{incf} in this example.  There are a few
exceptions to this.  First, functions such as @code{cl-defun} where
the unprefixed version was already used for a standard Emacs Lisp
function.  In such cases, the @file{cl.el} version adds a @samp{*}
suffix, e.g., @code{defun*}.  Second, there are some obsolete features
that are only implemented in @file{cl.el}, not in @file{cl-lib.el},
because they are replaced by other standard Emacs Lisp features.
Finally, in a very few cases the old @file{cl.el} versions do not
behave in exactly the same way as the @file{cl-lib.el} versions.
@xref{Obsolete Features}.
@c There is also cl-mapc, which was called cl-mapc even before cl-lib.el.
@c But not autoloaded, so maybe not much used?

The old file @file{cl.el}, as well as the even older
@file{cl-compat.el}, are deprecated and will be removed in a future
version of Emacs.  Any existing code that uses them should be updated
to use @file{cl-lib.el} instead.

@node Naming Conventions
@section Naming Conventions

@noindent
Except where noted, all functions defined by this package have the
same calling conventions as their Common Lisp counterparts, and
names that are those of Common Lisp plus a @samp{cl-} prefix.

Internal function and variable names in the package are prefixed
by @code{cl--}.  Here is a complete list of functions prefixed by
@code{cl-} that were @emph{not} taken from Common Lisp:

@example
cl-callf           cl-callf2          cl-defsubst
cl-letf            cl-letf*
@end example

@c This is not uninteresting I suppose, but is of zero practical relevance
@c to the user, and seems like a hostage to changing implementation details.
The following simple functions and macros are defined in @file{cl-lib.el};
they do not cause other components like @file{cl-extra} to be loaded.

@example
cl-evenp           cl-oddp            cl-minusp
cl-plusp           cl-endp            cl-subst
cl-copy-list       cl-list*           cl-ldiff
cl-rest            cl-decf [1]        cl-incf [1]
cl-acons           cl-adjoin [2]      cl-pairlis
cl-pushnew [1,2]   cl-declaim         cl-proclaim
cl-caaar@dots{}cl-cddddr                  cl-first@dots{}cl-tenth
cl-mapcar [3]
@end example

@noindent
[1] Only when @var{place} is a plain variable name.

@noindent
[2] Only if @code{:test} is @code{eq}, @code{equal}, or unspecified,
and @code{:key} is not used.

@noindent
[3] Only for one sequence argument or two list arguments.

@node Printing
@chapter Printing

@noindent
This chapter describes some enhancements to Emacs Lisp's
@dfn{printing}, the action of representing Lisp objects in text form.
The functions documented here are intended to produce output more for
human readers than the standard printing functions such as
@code{prin1} and @code{princ} (@pxref{Output Functions,,,elisp,GNU
Emacs Lisp Reference Manual}).

Several of these functions have a parameter @var{stream}; this
specifies what to do with the characters printing produces.  For
example, it might be a buffer, a marker, @code{nil} (meaning use
standard output), or @code{t} (use the echo area).  @xref{Output
Streams,,,elisp,GNU Emacs Lisp Reference Manual}, for a full
description.

@defvar cl-print-readably
When this variable is non-@code{nil}, @code{cl-prin1} and other
functions described here try to produce output which can later be read
by the Lisp reader (@pxref{Input Functions,,,elisp,GNU Emacs Lisp
Reference Manual}).
@end defvar

@defvar cl-print-compiled
This variable controls how to print byte-compiled functions.  Valid
values are:
@table @code
@item nil
The default: Just an internal hex identifier is printed.
@item static
The internal hex identifier together with the function's constant
vector are printed.
@item disassemble
The byte code gets disassembled.
@item raw
The raw form of the function is printed by @code{prin1}.
@end table

Sometimes, a button is set on the output to allow you to disassemble
the function.  See @code{cl-print-compile-button}.
@end defvar

@defvar cl-print-compile-button
When this variable is non-@code{nil} and a byte-compiled function has
been printed to a buffer, you can click with the mouse or type
@key{RET} on that output to disassemble the code.  This doesn't apply
when @code{cl-print-compiled} is set to @code{disassemble}.
@end defvar

@defvar cl-print-string-length
The maximum length of a string to print before abbreviating it.  A
value of @code{nil}, the default, means no limit.

When the CL printing functions abbreviate a string, they print the
first @code{cl-print-string-length} characters of the string, followed
by ``@enddots{}''.  When the printing is to a buffer, you can click
with the mouse or type @key{RET} on this ellipsis to expand the
string.

This variable has effect only in the @code{cl-prin*} functions, not in
primitives such as @code{prin1}.
@end defvar

@defun cl-prin1 object &option stream
@code{cl-print1} prints @var{object} on @var{stream} (see above)
according to its type and the settings described above.  The variables
@code{print-length} and @code{print-level} and the other standard
Emacs settings also affect the printing (@pxref{Output
Variables,,,elisp,GNU Emacs Lisp Reference Manual}).
@end defun

@defun cl-prin1-to-string object
This function is like @code{cl-prin1}, except the output characters
are returned as a string from this function rather than being passed
to a stream.
@end defun

@defun cl-print-to-string-with-limit print-function value limit
This function returns a string containing a printed representation of
@var{value}.  It attempts to get the length of the returned string
under @var{limit} characters with successively more restrictive
settings of @code{print-level}, @code{print-length}, and
@code{cl-print-string-length}.  It uses @var{print-function} to print,
a function which should take the arguments @var{value} and a stream
(see above), and which should respect @code{print-length},
@code{print-level}, and @code{cl-print-string-length}.  @var{limit}
may be @code{nil} or zero, in which case @var{print-function} will be
called with these settings bound to @code{nil}; it can also be
@code{t}, in which case @var{print-function} will be called with their
current values.

Use this function with @code{cl-prin1} to print an object, possibly
abbreviating it with one or more ellipses to fit within the size
limit.
@end defun

@defun cl-print-object object stream
This function prints @var{object} on @var{stream} (see above).  It is
actually a @code{cl-defgeneric} (@pxref{Generic Functions,,,elisp,GNU
Emacs Lisp Reference Manual}), which is defined for several types of
@var{object}.  Normally, you just call @code{cl-prin1} to print an
@var{object} rather than calling this function directly.

You can write @code{cl-print-object} @code{cl-defmethod}s for other
types of @var{object}, thus extending @code{cl-prin1}.  If such a
method uses ellipses, you should also write a
@code{cl-print-object-contents} method for the same type.  For
examples of these methods, see @file{emacs-lisp/cl-print.el} in the
Emacs source directory.
@end defun

@defun cl-print-object-contents object start stream
This function replaces an ellipsis in @var{stream} beginning at
@var{start} with the text from the partially printed @var{object} it
represents.  It is also a @code{cl-defgeneric} defined for several
types of @var{object}.  @var{stream} is a buffer containing the text
with the ellipsis.  @var{start} specifies the starting position of the
ellipsis in a manner dependent on the type; it will have been obtained
from a text property on the ellipsis, having been put there by
@code{cl-print-insert-ellipsis}.
@end defun

@defun cl-print-insert-ellipsis object start stream
This function prints an ellipsis (``@dots{}'') to @var{stream} (see
above).  When @var{stream} is a buffer, the ellipsis will be given the
@code{cl-print-ellipsis} text property.  The value of the text
property will contain state (including @var{start}) in order to print
the elided part of @var{object} later.  @var{start} should be
@code{nil} if the whole @var{object} is being elided, otherwise it
should be an index or other pointer into the internals of @var{object}
which can be passed to @code{cl-print-object-contents} at a later
time.
@end defun

@defvar cl-print-expand-ellipsis-function
This variable holds a function which expands an ellipsis in the
current buffer.  The function takes four arguments: @var{begin} and
@var{end}, which are the bounds of the ellipsis; @var{value}, which is
the value of the @code{cl-print-ellipsis} text property on the
ellipsis (typically set earlier by @code{cl-prin1}); and
@var{line-length}, the desired maximum length of the output.  Its
return value is the buffer position after the expanded text.
@end defvar

@deffn Command cl-print-expand-ellipsis &optional button
This command expands the ellipsis at point.  Non-interactively, if
@var{button} is non-@code{nil}, it should be either a buffer position
or a button made by @code{cl-print-insert-ellipsis}
(@pxref{Buttons,,,elisp,GNU Emacs Lisp Reference Manual}), which
indicates the position of the ellipsis.  The return value is the
buffer position after the expanded text.
@end deffn

@node Program Structure
@chapter Program Structure

@noindent
This section describes features of this package that have to
do with programs as a whole: advanced argument lists for functions,
and the @code{cl-eval-when} construct.

@menu
* Argument Lists::       @code{&key}, @code{&aux}, @code{cl-defun}, @code{cl-defmacro}.
* Time of Evaluation::   The @code{cl-eval-when} construct.
@end menu

@node Argument Lists
@section Argument Lists
@cindex &key
@cindex &aux

@noindent
Emacs Lisp's notation for argument lists of functions is a subset of
the Common Lisp notation.  As well as the familiar @code{&optional}
and @code{&rest} markers, Common Lisp allows you to specify default
values for optional arguments, and it provides the additional markers
@code{&key} and @code{&aux}.

Since argument parsing is built-in to Emacs, there is no way for
this package to implement Common Lisp argument lists seamlessly.
Instead, this package defines alternates for several Lisp forms
which you must use if you need Common Lisp argument lists.

@defmac cl-defun name arglist body@dots{}
This form is identical to the regular @code{defun} form, except
that @var{arglist} is allowed to be a full Common Lisp argument
list.  Also, the function body is enclosed in an implicit block
called @var{name}; @pxref{Blocks and Exits}.
@end defmac

@defmac cl-iter-defun name arglist body@dots{}
This form is identical to the regular @code{iter-defun} form, except
that @var{arglist} is allowed to be a full Common Lisp argument
list.  Also, the function body is enclosed in an implicit block
called @var{name}; @pxref{Blocks and Exits}.
@end defmac

@defmac cl-defsubst name arglist body@dots{}
This is just like @code{cl-defun}, except that the function that
is defined is automatically proclaimed @code{inline}, i.e.,
calls to it may be expanded into in-line code by the byte compiler.
This is analogous to the @code{defsubst} form;
@code{cl-defsubst} uses a different method (compiler macros) which
works in all versions of Emacs, and also generates somewhat more
@c For some examples,
@c see https://lists.gnu.org/r/emacs-devel/2012-11/msg00009.html
efficient inline expansions.  In particular, @code{cl-defsubst}
arranges for the processing of keyword arguments, default values,
etc., to be done at compile-time whenever possible.
@end defmac

@cindex &allow-other-keys
@defmac cl-defmacro name arglist body@dots{}
This is identical to the regular @code{defmacro} form,
except that @var{arglist} is allowed to be a full Common Lisp
argument list.  The @code{&environment} keyword is supported as
described in Steele's book @cite{Common Lisp, the Language}.
The @code{&whole} keyword is supported only
within destructured lists (see below); top-level @code{&whole}
cannot be implemented with the current Emacs Lisp interpreter.
The macro expander body is enclosed in an implicit block called
@var{name}.
@end defmac

@defmac cl-function symbol-or-lambda
This is identical to the regular @code{function} form,
except that if the argument is a @code{lambda} form then that
form may use a full Common Lisp argument list.
@end defmac

Also, all forms (such as @code{cl-flet} and @code{cl-labels}) defined
in this package that include @var{arglist}s in their syntax allow
full Common Lisp argument lists.

Note that it is @emph{not} necessary to use @code{cl-defun} in
order to have access to most CL features in your function.
These features are always present; @code{cl-defun}'s only
difference from @code{defun} is its more flexible argument
lists and its implicit block.

The full form of a Common Lisp argument list is

@example
(@var{var}@dots{}
 &optional (@var{var} @var{initform} @var{svar})@dots{}
 &rest @var{var}
 &key ((@var{keyword} @var{var}) @var{initform} @var{svar})@dots{}
 &aux (@var{var} @var{initform})@dots{})
@end example

Each of the five argument list sections is optional.  The @var{svar},
@var{initform}, and @var{keyword} parts are optional; if they are
omitted, then @samp{(@var{var})} may be written simply @samp{@var{var}}.

The first section consists of zero or more @dfn{required} arguments.
These arguments must always be specified in a call to the function;
there is no difference between Emacs Lisp and Common Lisp as far as
required arguments are concerned.

The second section consists of @dfn{optional} arguments.  These
arguments may be specified in the function call; if they are not,
@var{initform} specifies the default value used for the argument.
(No @var{initform} means to use @code{nil} as the default.)  The
@var{initform} is evaluated with the bindings for the preceding
arguments already established; @code{(a &optional (b (1+ a)))}
matches one or two arguments, with the second argument defaulting
to one plus the first argument.  If the @var{svar} is specified,
it is an auxiliary variable which is bound to @code{t} if the optional
argument was specified, or to @code{nil} if the argument was omitted.
If you don't use an @var{svar}, then there will be no way for your
function to tell whether it was called with no argument, or with
the default value passed explicitly as an argument.

The third section consists of a single @dfn{rest} argument.  If
more arguments were passed to the function than are accounted for
by the required and optional arguments, those extra arguments are
collected into a list and bound to the ``rest'' argument variable.
Common Lisp's @code{&rest} is equivalent to that of Emacs Lisp.
Common Lisp accepts @code{&body} as a synonym for @code{&rest} in
macro contexts; this package accepts it all the time.

The fourth section consists of @dfn{keyword} arguments.  These
are optional arguments which are specified by name rather than
positionally in the argument list.  For example,

@example
(cl-defun foo (a &optional b &key c d (e 17)))
@end example

@noindent
defines a function which may be called with one, two, or more
arguments.  The first two arguments are bound to @code{a} and
@code{b} in the usual way.  The remaining arguments must be
pairs of the form @code{:c}, @code{:d}, or @code{:e} followed
by the value to be bound to the corresponding argument variable.
(Symbols whose names begin with a colon are called @dfn{keywords},
and they are self-quoting in the same way as @code{nil} and
@code{t}.)

For example, the call @code{(foo 1 2 :d 3 :c 4)} sets the five
arguments to 1, 2, 4, 3, and 17, respectively.  If the same keyword
appears more than once in the function call, the first occurrence
takes precedence over the later ones.  Note that it is not possible
to specify keyword arguments without specifying the optional
argument @code{b} as well, since @code{(foo 1 :c 2)} would bind
@code{b} to the keyword @code{:c}, then signal an error because
@code{2} is not a valid keyword.

You can also explicitly specify the keyword argument; it need not be
simply the variable name prefixed with a colon.  For example,

@example
(cl-defun bar (&key (a 1) ((baz b) 4)))
@end example

@noindent

specifies a keyword @code{:a} that sets the variable @code{a} with
default value 1, as well as a keyword @code{baz} that sets the
variable @code{b} with default value 4.  In this case, because
@code{baz} is not self-quoting, you must quote it explicitly in the
function call, like this:

@example
(bar :a 10 'baz 42)
@end example

@cindex &allow-other-keys
Ordinarily, it is an error to pass an unrecognized keyword to
a function, e.g., @code{(foo 1 2 :c 3 :goober 4)}.  You can ask
Lisp to ignore unrecognized keywords, either by adding the
marker @code{&allow-other-keys} after the keyword section
of the argument list, or by specifying an @code{:allow-other-keys}
argument in the call whose value is non-@code{nil}.  If the
function uses both @code{&rest} and @code{&key} at the same time,
the ``rest'' argument is bound to the keyword list as it appears
in the call.  For example:

@example
(cl-defun find-thing (thing thing-list &rest rest &key need &allow-other-keys)
  (or (apply 'cl-member thing thing-list :allow-other-keys t rest)
      (if need (error "Thing not found"))))
@end example

@noindent
This function takes a @code{:need} keyword argument, but also
accepts other keyword arguments which are passed on to the
@code{cl-member} function.  @code{allow-other-keys} is used to
keep both @code{find-thing} and @code{cl-member} from complaining
about each others' keywords in the arguments.

The fifth section of the argument list consists of @dfn{auxiliary
variables}.  These are not really arguments at all, but simply
variables which are bound to @code{nil} or to the specified
@var{initforms} during execution of the function.  There is no
difference between the following two functions, except for a
matter of stylistic taste:

@example
(cl-defun foo (a b &aux (c (+ a b)) d)
  @var{body})

(cl-defun foo (a b)
  (let ((c (+ a b)) d)
    @var{body}))
@end example

@cindex destructuring, in argument list
Argument lists support @dfn{destructuring}.  In Common Lisp,
destructuring is only allowed with @code{defmacro}; this package
allows it with @code{cl-defun} and other argument lists as well.
In destructuring, any argument variable (@var{var} in the above
example) can be replaced by a list of variables, or more generally,
a recursive argument list.  The corresponding argument value must
be a list whose elements match this recursive argument list.
For example:

@example
(cl-defmacro dolist ((var listform &optional resultform)
                   &rest body)
  @dots{})
@end example

This says that the first argument of @code{dolist} must be a list
of two or three items; if there are other arguments as well as this
list, they are stored in @code{body}.  All features allowed in
regular argument lists are allowed in these recursive argument lists.
In addition, the clause @samp{&whole @var{var}} is allowed at the
front of a recursive argument list.  It binds @var{var} to the
whole list being matched; thus @code{(&whole all a b)} matches
a list of two things, with @code{a} bound to the first thing,
@code{b} bound to the second thing, and @code{all} bound to the
list itself.  (Common Lisp allows @code{&whole} in top-level
@code{defmacro} argument lists as well, but Emacs Lisp does not
support this usage.)

One last feature of destructuring is that the argument list may be
dotted, so that the argument list @code{(a b . c)} is functionally
equivalent to @code{(a b &rest c)}.

If the optimization quality @code{safety} is set to 0
(@pxref{Declarations}), error checking for wrong number of
arguments and invalid keyword arguments is disabled.  By default,
argument lists are rigorously checked.

@node Time of Evaluation
@section Time of Evaluation

@noindent
Normally, the byte-compiler does not actually execute the forms in
a file it compiles.  For example, if a file contains @code{(setq foo t)},
the act of compiling it will not actually set @code{foo} to @code{t}.
This is true even if the @code{setq} was a top-level form (i.e., not
enclosed in a @code{defun} or other form).  Sometimes, though, you
would like to have certain top-level forms evaluated at compile-time.
For example, the compiler effectively evaluates @code{defmacro} forms
at compile-time so that later parts of the file can refer to the
macros that are defined.

@defmac cl-eval-when (situations@dots{}) forms@dots{}
This form controls when the body @var{forms} are evaluated.
The @var{situations} list may contain any set of the symbols
@code{compile}, @code{load}, and @code{eval} (or their long-winded
ANSI equivalents, @code{:compile-toplevel}, @code{:load-toplevel},
and @code{:execute}).

The @code{cl-eval-when} form is handled differently depending on
whether or not it is being compiled as a top-level form.
Specifically, it gets special treatment if it is being compiled
by a command such as @code{byte-compile-file} which compiles files
or buffers of code, and it appears either literally at the
top level of the file or inside a top-level @code{progn}.

For compiled top-level @code{cl-eval-when}s, the body @var{forms} are
executed at compile-time if @code{compile} is in the @var{situations}
list, and the @var{forms} are written out to the file (to be executed
at load-time) if @code{load} is in the @var{situations} list.

For non-compiled-top-level forms, only the @code{eval} situation is
relevant.  (This includes forms executed by the interpreter, forms
compiled with @code{byte-compile} rather than @code{byte-compile-file},
and non-top-level forms.)  The @code{cl-eval-when} acts like a
@code{progn} if @code{eval} is specified, and like @code{nil}
(ignoring the body @var{forms}) if not.

The rules become more subtle when @code{cl-eval-when}s are nested;
consult Steele (second edition) for the gruesome details (and
some gruesome examples).

Some simple examples:

@example
;; Top-level forms in foo.el:
(cl-eval-when (compile)           (setq foo1 'bar))
(cl-eval-when (load)              (setq foo2 'bar))
(cl-eval-when (compile load)      (setq foo3 'bar))
(cl-eval-when (eval)              (setq foo4 'bar))
(cl-eval-when (eval compile)      (setq foo5 'bar))
(cl-eval-when (eval load)         (setq foo6 'bar))
(cl-eval-when (eval compile load) (setq foo7 'bar))
@end example

When @file{foo.el} is compiled, these variables will be set during
the compilation itself:

@example
foo1  foo3  foo5  foo7      ; 'compile'
@end example

When @file{foo.elc} is loaded, these variables will be set:

@example
foo2  foo3  foo6  foo7      ; 'load'
@end example

And if @file{foo.el} is loaded uncompiled, these variables will
be set:

@example
foo4  foo5  foo6  foo7      ; 'eval'
@end example

If these seven @code{cl-eval-when}s had been, say, inside a @code{defun},
then the first three would have been equivalent to @code{nil} and the
last four would have been equivalent to the corresponding @code{setq}s.

Note that @code{(cl-eval-when (load eval) @dots{})} is equivalent
to @code{(progn @dots{})} in all contexts.  The compiler treats
certain top-level forms, like @code{defmacro} (sort-of) and
@code{require}, as if they were wrapped in @code{(cl-eval-when
(compile load eval) @dots{})}.
@end defmac

Emacs includes two special forms related to @code{cl-eval-when}.
@xref{Eval During Compile,,,elisp,GNU Emacs Lisp Reference Manual}.
One of these, @code{eval-when-compile}, is not quite equivalent to
any @code{cl-eval-when} construct and is described below.

The other form, @code{(eval-and-compile @dots{})}, is exactly
equivalent to @samp{(cl-eval-when (compile load eval) @dots{})}.

@defmac eval-when-compile forms@dots{}
The @var{forms} are evaluated at compile-time; at execution time,
this form acts like a quoted constant of the resulting value.  Used
at top-level, @code{eval-when-compile} is just like @samp{eval-when
(compile eval)}.  In other contexts, @code{eval-when-compile}
allows code to be evaluated once at compile-time for efficiency
or other reasons.

This form is similar to the @samp{#.} syntax of true Common Lisp.
@end defmac

@defmac cl-load-time-value form
The @var{form} is evaluated at load-time; at execution time,
this form acts like a quoted constant of the resulting value.

Early Common Lisp had a @samp{#,} syntax that was similar to
this, but ANSI Common Lisp replaced it with @code{load-time-value}
and gave it more well-defined semantics.

In a compiled file, @code{cl-load-time-value} arranges for @var{form}
to be evaluated when the @file{.elc} file is loaded and then used
as if it were a quoted constant.  In code compiled by
@code{byte-compile} rather than @code{byte-compile-file}, the
effect is identical to @code{eval-when-compile}.  In uncompiled
code, both @code{eval-when-compile} and @code{cl-load-time-value}
act exactly like @code{progn}.

@example
(defun report ()
  (insert "This function was executed on: "
          (current-time-string)
          ", compiled on: "
          (eval-when-compile (current-time-string))
          ;; or '#.(current-time-string) in real Common Lisp
          ", and loaded on: "
          (cl-load-time-value (current-time-string))))
@end example

@noindent
Byte-compiled, the above defun will result in the following code
(or its compiled equivalent, of course) in the @file{.elc} file:

@example
(setq --temp-- (current-time-string))
(defun report ()
  (insert "This function was executed on: "
          (current-time-string)
          ", compiled on: "
          '"Wed Oct 31 16:32:28 2012"
          ", and loaded on: "
          --temp--))
@end example
@end defmac

@node Predicates
@chapter Predicates

@noindent
This section describes functions for testing whether various
facts are true or false.

@menu
* Type Predicates::      @code{cl-typep}, @code{cl-deftype}, and @code{cl-coerce}.
* Equality Predicates::  @code{cl-equalp}.
@end menu

@node Type Predicates
@section Type Predicates

@defun cl-typep object type
Check if @var{object} is of type @var{type}, where @var{type} is a
(quoted) type name of the sort used by Common Lisp.  For example,
@code{(cl-typep foo 'integer)} is equivalent to @code{(integerp foo)}.
@end defun

The @var{type} argument to the above function is either a symbol
or a list beginning with a symbol.

@itemize @bullet
@item
If the type name is a symbol, Emacs appends @samp{-p} to the
symbol name to form the name of a predicate function for testing
the type.  (Built-in predicates whose names end in @samp{p} rather
than @samp{-p} are used when appropriate.)

@item
The type symbol @code{t} stands for the union of all types.
@code{(cl-typep @var{object} t)} is always true.  Likewise, the
type symbol @code{nil} stands for nothing at all, and
@code{(cl-typep @var{object} nil)} is always false.

@item
The type symbol @code{null} represents the symbol @code{nil}.
Thus @code{(cl-typep @var{object} 'null)} is equivalent to
@code{(null @var{object})}.

@item
The type symbol @code{atom} represents all objects that are not cons
cells.  Thus @code{(cl-typep @var{object} 'atom)} is equivalent to
@code{(atom @var{object})}.

@item
The type symbol @code{real} is a synonym for @code{number}, and
@code{fixnum} is a synonym for @code{integer}.

@item
The type symbols @code{character} and @code{string-char} match
integers in the range from 0 to 255.

@item
The type list @code{(integer @var{low} @var{high})} represents all
integers between @var{low} and @var{high}, inclusive.  Either bound
may be a list of a single integer to specify an exclusive limit,
or a @code{*} to specify no limit.  The type @code{(integer * *)}
is thus equivalent to @code{integer}.

@item
Likewise, lists beginning with @code{float}, @code{real}, or
@code{number} represent numbers of that type falling in a particular
range.

@item
Lists beginning with @code{and}, @code{or}, and @code{not} form
combinations of types.  For example, @code{(or integer (float 0 *))}
represents all objects that are integers or non-negative floats.

@item
Lists beginning with @code{member} or @code{cl-member} represent
objects @code{eql} to any of the following values.  For example,
@code{(member 1 2 3 4)} is equivalent to @code{(integer 1 4)},
and @code{(member nil)} is equivalent to @code{null}.

@item
Lists of the form @code{(satisfies @var{predicate})} represent
all objects for which @var{predicate} returns true when called
with that object as an argument.
@end itemize

The following function and macro (not technically predicates) are
related to @code{cl-typep}.

@defun cl-coerce object type
This function attempts to convert @var{object} to the specified
@var{type}.  If @var{object} is already of that type as determined by
@code{cl-typep}, it is simply returned.  Otherwise, certain types of
conversions will be made:  If @var{type} is any sequence type
(@code{string}, @code{list}, etc.)@: then @var{object} will be
converted to that type if possible.  If @var{type} is
@code{character}, then strings of length one and symbols with
one-character names can be coerced.  If @var{type} is @code{float},
then integers can be coerced in versions of Emacs that support
floats.  In all other circumstances, @code{cl-coerce} signals an
error.
@end defun

@defmac cl-deftype name arglist forms@dots{}
This macro defines a new type called @var{name}.  It is similar
to @code{defmacro} in many ways; when @var{name} is encountered
as a type name, the body @var{forms} are evaluated and should
return a type specifier that is equivalent to the type.  The
@var{arglist} is a Common Lisp argument list of the sort accepted
by @code{cl-defmacro}.  The type specifier @samp{(@var{name} @var{args}@dots{})}
is expanded by calling the expander with those arguments; the type
symbol @samp{@var{name}} is expanded by calling the expander with
no arguments.  The @var{arglist} is processed the same as for
@code{cl-defmacro} except that optional arguments without explicit
defaults use @code{*} instead of @code{nil} as the ``default''
default.  Some examples:

@example
(cl-deftype null () '(satisfies null))    ; predefined
(cl-deftype list () '(or null cons))      ; predefined
(cl-deftype unsigned-byte (&optional bits)
  (list 'integer 0 (if (eq bits '*) bits (1- (ash 1 bits)))))
(unsigned-byte 8)  @equiv{}  (integer 0 255)
(unsigned-byte)  @equiv{}  (integer 0 *)
unsigned-byte  @equiv{}  (integer 0 *)
@end example

@noindent
The last example shows how the Common Lisp @code{unsigned-byte}
type specifier could be implemented if desired; this package does
not implement @code{unsigned-byte} by default.
@end defmac

The @code{cl-typecase} (@pxref{Conditionals}) and @code{cl-check-type}
(@pxref{Assertions}) macros also use type names.  The @code{cl-map},
@code{cl-concatenate}, and @code{cl-merge} functions take type-name
arguments to specify the type of sequence to return.  @xref{Sequences}.

@node Equality Predicates
@section Equality Predicates

@noindent
This package defines the Common Lisp predicate @code{cl-equalp}.

@defun cl-equalp a b
This function is a more flexible version of @code{equal}.  In
particular, it compares strings case-insensitively, and it compares
numbers without regard to type (so that @code{(cl-equalp 3 3.0)} is
true).  Vectors and conses are compared recursively.  All other
objects are compared as if by @code{equal}.

This function differs from Common Lisp @code{equalp} in several
respects.  First, Common Lisp's @code{equalp} also compares
@emph{characters} case-insensitively, which would be impractical
in this package since Emacs does not distinguish between integers
and characters.  In keeping with the idea that strings are less
vector-like in Emacs Lisp, this package's @code{cl-equalp} also will
not compare strings against vectors of integers.
@end defun

Also note that the Common Lisp functions @code{member} and @code{assoc}
use @code{eql} to compare elements, whereas Emacs Lisp follows the
MacLisp tradition and uses @code{equal} for these two functions.
The functions @code{cl-member} and @code{cl-assoc} use @code{eql},
as in Common Lisp.  The standard Emacs Lisp functions @code{memq} and
@code{assq} use @code{eq}, and the standard @code{memql} uses @code{eql}.

@node Control Structure
@chapter Control Structure

@noindent
The features described in the following sections implement
various advanced control structures, including extensions to the
standard @code{setf} facility, and a number of looping and conditional
constructs.

@menu
* Assignment::             The @code{cl-psetq} form.
* Generalized Variables::  Extensions to generalized variables.
* Variable Bindings::      @code{cl-progv}, @code{cl-flet}, @code{cl-macrolet}.
* Conditionals::           @code{cl-case}, @code{cl-typecase}.
* Blocks and Exits::       @code{cl-block}, @code{cl-return}, @code{cl-return-from}.
* Iteration::              @code{cl-do}, @code{cl-dotimes}, @code{cl-dolist}, @code{cl-do-symbols}.
* Loop Facility::          The Common Lisp @code{loop} macro.
* Multiple Values::        @code{cl-values}, @code{cl-multiple-value-bind}, etc.
* Macro-Writing Macros::   @code{cl-with-gensyms}, @code{cl-once-only}.
@end menu

@node Assignment
@section Assignment

@noindent
The @code{cl-psetq} form is just like @code{setq}, except that multiple
assignments are done in parallel rather than sequentially.

@defmac cl-psetq [symbol form]@dots{}
This special form (actually a macro) is used to assign to several
variables simultaneously.  Given only one @var{symbol} and @var{form},
it has the same effect as @code{setq}.  Given several @var{symbol}
and @var{form} pairs, it evaluates all the @var{form}s in advance
and then stores the corresponding variables afterwards.

@example
(setq x 2 y 3)
(setq x (+ x y)  y (* x y))
x
     @result{} 5
y                     ; @r{@code{y} was computed after @code{x} was set.}
     @result{} 15
(setq x 2 y 3)
(cl-psetq x (+ x y)  y (* x y))
x
     @result{} 5
y                     ; @r{@code{y} was computed before @code{x} was set.}
     @result{} 6
@end example

The simplest use of @code{cl-psetq} is @code{(cl-psetq x y y x)}, which
exchanges the values of two variables.  (The @code{cl-rotatef} form
provides an even more convenient way to swap two variables;
@pxref{Modify Macros}.)

@code{cl-psetq} always returns @code{nil}.
@end defmac

@node Generalized Variables
@section Generalized Variables
@cindex generalized variable

A @dfn{generalized variable} or @dfn{place form} is one of the many
places in Lisp memory where values can be stored.  The simplest place
form is a regular Lisp variable.  But the @sc{car}s and @sc{cdr}s of lists,
elements of arrays, properties of symbols, and many other locations
are also places where Lisp values are stored.  For basic information,
@pxref{Generalized Variables,,,elisp,GNU Emacs Lisp Reference Manual}.
This package provides several additional features related to
generalized variables.

@menu
* Setf Extensions::    Additional @code{setf} places.
* Modify Macros::      @code{cl-incf}, @code{cl-rotatef}, @code{cl-letf}, @code{cl-callf}, etc.
@end menu

@node Setf Extensions
@subsection Setf Extensions

Several standard (e.g., @code{car}) and Emacs-specific
(e.g., @code{window-point}) Lisp functions are @code{setf}-able by default.
This package defines @code{setf} handlers for several additional functions:

@itemize
@item
Functions from this package:
@example
cl-rest        cl-subseq      cl-get         cl-getf
cl-caaar@dots{}cl-cddddr          cl-first@dots{}cl-tenth
@end example

@noindent
Note that for @code{cl-getf} (as for @code{nthcdr}), the list argument
of the function must itself be a valid @var{place} form.

@c Currently commented out in cl.el.
@ignore
@item
A call of the form @code{(apply '@var{func} @dots{})} or
@code{(apply (function @var{func}) @dots{})}, where @var{func}
is a @code{setf}-able function whose store function is ``suitable''
in the sense described in Steele's book; since none of the standard
Emacs place functions are suitable in this sense, this feature is
only interesting when used with places you define yourself with
@code{define-setf-method} or the long form of @code{defsetf}.
@xref{Obsolete Setf Customization}.
@end ignore

@c FIXME?  Is this still true?
@item
A macro call, in which case the macro is expanded and @code{setf}
is applied to the resulting form.
@end itemize

@c FIXME should this be in lispref?  It seems self-evident.
@c Contrast with the cl-incf example later on.
@c Here it really only serves as a contrast to wrong-order.
The @code{setf} macro takes care to evaluate all subforms in
the proper left-to-right order; for example,

@example
(setf (aref vec (cl-incf i)) i)
@end example

@noindent
looks like it will evaluate @code{(cl-incf i)} exactly once, before the
following access to @code{i}; the @code{setf} expander will insert
temporary variables as necessary to ensure that it does in fact work
this way no matter what setf-method is defined for @code{aref}.
(In this case, @code{aset} would be used and no such steps would
be necessary since @code{aset} takes its arguments in a convenient
order.)

However, if the @var{place} form is a macro which explicitly
evaluates its arguments in an unusual order, this unusual order
will be preserved.  Adapting an example from Steele, given

@example
(defmacro wrong-order (x y) (list 'aref y x))
@end example

@noindent
the form @code{(setf (wrong-order @var{a} @var{b}) 17)} will
evaluate @var{b} first, then @var{a}, just as in an actual call
to @code{wrong-order}.

@node Modify Macros
@subsection Modify Macros

@noindent
This package defines a number of macros that operate on generalized
variables.  Many are interesting and useful even when the @var{place}
is just a variable name.

@defmac cl-psetf [place form]@dots{}
This macro is to @code{setf} what @code{cl-psetq} is to @code{setq}:
When several @var{place}s and @var{form}s are involved, the
assignments take place in parallel rather than sequentially.
Specifically, all subforms are evaluated from left to right, then
all the assignments are done (in an undefined order).
@end defmac

@defmac cl-incf place &optional x
This macro increments the number stored in @var{place} by one, or
by @var{x} if specified.  The incremented value is returned.  For
example, @code{(cl-incf i)} is equivalent to @code{(setq i (1+ i))}, and
@code{(cl-incf (car x) 2)} is equivalent to @code{(setcar x (+ (car x) 2))}.

As with @code{setf}, care is taken to preserve the ``apparent'' order
of evaluation.  For example,

@example
(cl-incf (aref vec (cl-incf i)))
@end example

@noindent
appears to increment @code{i} once, then increment the element of
@code{vec} addressed by @code{i}; this is indeed exactly what it
does, which means the above form is @emph{not} equivalent to the
``obvious'' expansion,

@example
(setf (aref vec (cl-incf i))
      (1+ (aref vec (cl-incf i))))   ; wrong!
@end example

@noindent
but rather to something more like

@example
(let ((temp (cl-incf i)))
  (setf (aref vec temp) (1+ (aref vec temp))))
@end example

@noindent
Again, all of this is taken care of automatically by @code{cl-incf} and
the other generalized-variable macros.

As a more Emacs-specific example of @code{cl-incf}, the expression
@code{(cl-incf (point) @var{n})} is essentially equivalent to
@code{(forward-char @var{n})}.
@end defmac

@defmac cl-decf place &optional x
This macro decrements the number stored in @var{place} by one, or
by @var{x} if specified.
@end defmac

@defmac cl-pushnew x place @t{&key :test :test-not :key}
This macro inserts @var{x} at the front of the list stored in
@var{place}, but only if @var{x} isn't present in the list already.
The optional keyword arguments are interpreted in the same way as for
@code{cl-adjoin}.  @xref{Lists as Sets}.
@end defmac

@defmac cl-shiftf place@dots{} newvalue
This macro shifts the @var{place}s left by one, shifting in the
value of @var{newvalue} (which may be any Lisp expression, not just
a generalized variable), and returning the value shifted out of
the first @var{place}.  Thus, @code{(cl-shiftf @var{a} @var{b} @var{c}
@var{d})} is equivalent to

@example
(prog1
    @var{a}
  (cl-psetf @var{a} @var{b}
            @var{b} @var{c}
            @var{c} @var{d}))
@end example

@noindent
except that the subforms of @var{a}, @var{b}, and @var{c} are actually
evaluated only once each and in the apparent order.
@end defmac

@defmac cl-rotatef place@dots{}
This macro rotates the @var{place}s left by one in circular fashion.
Thus, @code{(cl-rotatef @var{a} @var{b} @var{c} @var{d})} is equivalent to

@example
(cl-psetf @var{a} @var{b}
          @var{b} @var{c}
          @var{c} @var{d}
          @var{d} @var{a})
@end example

@noindent
except for the evaluation of subforms.  @code{cl-rotatef} always
returns @code{nil}.  Note that @code{(cl-rotatef @var{a} @var{b})}
conveniently exchanges @var{a} and @var{b}.
@end defmac

The following macros were invented for this package; they have no
analogues in Common Lisp.

@defmac cl-letf (bindings@dots{}) forms@dots{}
This macro is analogous to @code{let}, but for generalized variables
rather than just symbols.  Each @var{binding} should be of the form
@code{(@var{place} @var{value})}; the original contents of the
@var{place}s are saved, the @var{value}s are stored in them, and
then the body @var{form}s are executed.  Afterwards, the @var{places}
are set back to their original saved contents.  This cleanup happens
even if the @var{form}s exit irregularly due to a @code{throw} or an
error.

For example,

@example
(cl-letf (((point) (point-min))
          (a 17))
     @dots{})
@end example

@noindent
moves point in the current buffer to the beginning of the buffer,
and also binds @code{a} to 17 (as if by a normal @code{let}, since
@code{a} is just a regular variable).  After the body exits, @code{a}
is set back to its original value and point is moved back to its
original position.

Note that @code{cl-letf} on @code{(point)} is not quite like a
@code{save-excursion}, as the latter effectively saves a marker
which tracks insertions and deletions in the buffer.  Actually,
a @code{cl-letf} of @code{(point-marker)} is much closer to this
behavior.  (@code{point} and @code{point-marker} are equivalent
as @code{setf} places; each will accept either an integer or a
marker as the stored value.)

Like in the case of @code{let}, the @var{value} forms are evaluated in
the order they appear, but the order of bindings is unspecified.
Therefore, avoid binding the same @var{place} more than once in a
single @code{cl-letf} form.

Since generalized variables look like lists, @code{let}'s shorthand
of using @samp{foo} for @samp{(foo nil)} as a @var{binding} would
be ambiguous in @code{cl-letf} and is not allowed.

However, a @var{binding} specifier may be a one-element list
@samp{(@var{place})}, which is similar to @samp{(@var{place}
@var{place})}.  In other words, the @var{place} is not disturbed
on entry to the body, and the only effect of the @code{cl-letf} is
to restore the original value of @var{place} afterwards.
@c I suspect this may no longer be true; either way it's
@c implementation detail and so not essential to document.
@ignore
(The redundant access-and-store suggested by the @code{(@var{place}
@var{place})} example does not actually occur.)
@end ignore

Note that in this case, and in fact almost every case, @var{place}
must have a well-defined value outside the @code{cl-letf} body.
There is essentially only one exception to this, which is @var{place}
a plain variable with a specified @var{value} (such as @code{(a 17)}
in the above example).
@c See https://debbugs.gnu.org/12758
@c Some or all of this was true for cl.el, but not for cl-lib.el.
@ignore
The only exceptions are plain variables and calls to
@code{symbol-value} and @code{symbol-function}.  If the symbol is not
bound on entry, it is simply made unbound by @code{makunbound} or
@code{fmakunbound} on exit.
@end ignore
@end defmac

@defmac cl-letf* (bindings@dots{}) forms@dots{}
This macro is to @code{cl-letf} what @code{let*} is to @code{let}:
It does the bindings in sequential rather than parallel order.
@end defmac

@defmac cl-callf @var{function} @var{place} @var{args}@dots{}
This is the ``generic'' modify macro.  It calls @var{function},
which should be an unquoted function name, macro name, or lambda.
It passes @var{place} and @var{args} as arguments, and assigns the
result back to @var{place}.  For example, @code{(cl-incf @var{place}
@var{n})} is the same as @code{(cl-callf + @var{place} @var{n})}.
Some more examples:

@example
(cl-callf abs my-number)
(cl-callf concat (buffer-name) "<" (number-to-string n) ">")
(cl-callf cl-union happy-people (list joe bob) :test 'same-person)
@end example

Note again that @code{cl-callf} is an extension to standard Common Lisp.
@end defmac

@defmac cl-callf2 @var{function} @var{arg1} @var{place} @var{args}@dots{}
This macro is like @code{cl-callf}, except that @var{place} is
the @emph{second} argument of @var{function} rather than the
first.  For example, @code{(push @var{x} @var{place})} is
equivalent to @code{(cl-callf2 cons @var{x} @var{place})}.
@end defmac

The @code{cl-callf} and @code{cl-callf2} macros serve as building
blocks for other macros like @code{cl-incf}, and @code{cl-pushnew}.
The @code{cl-letf} and @code{cl-letf*} macros are used in the processing
of symbol macros; @pxref{Macro Bindings}.

@defmac with-memoization @var{place} @var{code}@dots{}
This macro provides a simple way to do memoization.  @var{code} is
evaluated and then stashed in @var{place}.  If @var{place}'s value is
non-@code{nil}, return that value instead of evaluating @var{code}.
@end defmac


@node Variable Bindings
@section Variable Bindings
@cindex variable binding

@noindent
These Lisp forms make bindings to variables and function names,
analogous to Lisp's built-in @code{let} form.

@xref{Modify Macros}, for the @code{cl-letf} and @code{cl-letf*} forms which
are also related to variable bindings.

@menu
* Dynamic Bindings::     The @code{cl-progv} form.
* Function Bindings::    @code{cl-flet} and @code{cl-labels}.
* Macro Bindings::       @code{cl-macrolet} and @code{cl-symbol-macrolet}.
@end menu

@node Dynamic Bindings
@subsection Dynamic Bindings
@cindex dynamic binding

@noindent
The standard @code{let} form binds variables whose names are known
at compile-time.  The @code{cl-progv} form provides an easy way to
bind variables whose names are computed at run-time.

@defmac cl-progv symbols values forms@dots{}
This form establishes @code{let}-style variable bindings on a
set of variables computed at run-time.  The expressions
@var{symbols} and @var{values} are evaluated, and must return lists
of symbols and values, respectively.  The symbols are bound to the
corresponding values for the duration of the body @var{form}s.
If @var{values} is shorter than @var{symbols}, the last few symbols
are bound to @code{nil}.
If @var{symbols} is shorter than @var{values}, the excess values
are ignored.
@end defmac

@node Function Bindings
@subsection Function Bindings
@cindex function binding

@noindent
These forms make @code{let}-like bindings to functions instead
of variables.

@defmac cl-flet (bindings@dots{}) forms@dots{}
This form establishes @code{let}-style bindings for functions rather
than values.  Each @var{binding} must be a list of one of two forms:
either @w{@code{(@var{name} @var{expr})}} or @w{@code{(@var{name}
@var{arglist} @var{body}@dots{})}}.  The @var{name} is the name of the
function, @var{expr} is an expression which returns the function value
to which the corresponding @var{name} should be bound, and
@var{arglist} and @var{body} are the argument list and the body of the
function to bind to @var{name}.  Within @var{forms}, any reference to
the function @var{name} uses the local definition provided by
@var{bindings} instead of the global one.

A ``reference'' to a function name is either a call to that function,
or a use of its name quoted by @code{function} to be passed on to,
say, @code{mapcar}.

The bindings are lexical in scope.  This means that all references to
the named functions must appear physically within @var{forms}.

Functions defined by @code{cl-flet} may use the full Common Lisp
argument notation supported by @code{cl-defun}; also, the function
body is enclosed in an implicit block as if by @code{cl-defun}.
@xref{Program Structure}.

Note that the @file{cl.el} version of this macro behaves slightly
differently.  In particular, its binding is dynamic rather than
lexical.  @xref{Obsolete Macros}.
@end defmac

@defmac cl-labels (bindings@dots{}) forms@dots{}
The @code{cl-labels} form is like @code{cl-flet}, except that
the function bindings can be recursive.  The scoping is lexical,
but you can only capture functions in closures if
@code{lexical-binding} is @code{t}.
@xref{Closures,,,elisp,GNU Emacs Lisp Reference Manual}, and
@ref{Selecting Lisp Dialect,,,elisp,GNU Emacs Lisp Reference Manual}.

Lexical scoping means that all references to the named
functions must appear physically within the body of the
@code{cl-labels} form.  References may appear both in the body
@var{forms} of @code{cl-labels} itself, and in the bodies of
the functions themselves.  Thus, @code{cl-labels} can define
local recursive functions, or mutually-recursive sets of functions.

Note that the @file{cl.el} version of this macro behaves slightly
differently.  @xref{Obsolete Macros}.
@end defmac

@node Macro Bindings
@subsection Macro Bindings
@cindex macro binding

@noindent
These forms create local macros and ``symbol macros''.

@defmac cl-macrolet (bindings@dots{}) forms@dots{}
This form is analogous to @code{cl-flet}, but for macros instead of
functions.  Each @var{binding} is a list of the same form as the
arguments to @code{cl-defmacro} (i.e., a macro name, argument list,
and macro-expander forms).  The macro is defined accordingly for
use within the body of the @code{cl-macrolet}.

Because of the nature of macros, @code{cl-macrolet} is always lexically
scoped.  The @code{cl-macrolet} binding will
affect only calls that appear physically within the body
@var{forms}, possibly after expansion of other macros in the
body.  Calls of @code{cl-macrolet} bound macros are expanded in the
global environment.
@end defmac

@defmac cl-symbol-macrolet (bindings@dots{}) forms@dots{}
This form creates @dfn{symbol macros}, which are macros that look
like variable references rather than function calls.  Each
@var{binding} is a list @samp{(@var{var} @var{expansion})};
any reference to @var{var} within the body @var{forms} is
replaced by @var{expansion}.

@example
(setq bar '(5 . 9))
(cl-symbol-macrolet ((foo (car bar)))
  (cl-incf foo))
bar
     @result{} (6 . 9)
@end example

A @code{setq} of a symbol macro is treated the same as a @code{setf}.
I.e., @code{(setq foo 4)} in the above would be equivalent to
@code{(setf foo 4)}, which in turn expands to @code{(setf (car bar)
4)}.  A @code{let} (or @code{let*}, @code{lambda}, ...) binding of
the same symbol will locally shadow the symbol macro as is the case in
Common Lisp.

There is no analogue of @code{defmacro} for symbol macros; all symbol
macros are local.  A typical use of @code{cl-symbol-macrolet} is in the
expansion of another macro:

@example
(cl-defmacro my-dolist ((x list) &rest body)
  (let ((var (cl-gensym)))
    (list 'cl-loop 'for var 'on list 'do
          (cl-list* 'cl-symbol-macrolet
                    (list (list x (list 'car var)))
                    body))))

(setq mylist '(1 2 3 4))
(my-dolist (x mylist) (cl-incf x))
mylist
     @result{} (2 3 4 5)
@end example

@noindent
In this example, the @code{my-dolist} macro is similar to @code{dolist}
(@pxref{Iteration}) except that the variable @code{x} becomes a true
reference onto the elements of the list.  The @code{my-dolist} call
shown here expands to

@example
(cl-loop for G1234 on mylist do
      (cl-symbol-macrolet ((x (car G1234)))
        (cl-incf x)))
@end example

@noindent
which in turn expands to

@example
(cl-loop for G1234 on mylist do (cl-incf (car G1234)))
@end example

@xref{Loop Facility}, for a description of the @code{cl-loop} macro.
This package defines a nonstandard @code{in-ref} loop clause that
works much like @code{my-dolist}.
@end defmac

@node Conditionals
@section Conditionals
@cindex conditionals

@noindent
These conditional forms augment Emacs Lisp's simple @code{if},
@code{and}, @code{or}, and @code{cond} forms.

@defmac cl-case keyform clause@dots{}
This macro evaluates @var{keyform}, then compares it with the key
values listed in the various @var{clause}s.  Whichever clause matches
the key is executed; comparison is done by @code{eql}.  If no clause
matches, the @code{cl-case} form returns @code{nil}.  The clauses are
of the form

@example
(@var{keylist} @var{body-forms}@dots{})
@end example

@noindent
where @var{keylist} is a list of key values.  If there is exactly
one value, and it is not a cons cell or the symbol @code{nil} or
@code{t}, then it can be used by itself as a @var{keylist} without
being enclosed in a list.  All key values in the @code{cl-case} form
must be distinct.  The final clauses may use @code{t} in place of
a @var{keylist} to indicate a default clause that should be taken
if none of the other clauses match.  (The symbol @code{otherwise}
is also recognized in place of @code{t}.  To make a clause that
matches the actual symbol @code{t}, @code{nil}, or @code{otherwise},
enclose the symbol in a list.)

For example, this expression reads a keystroke, then does one of
four things depending on whether it is an @samp{a}, a @samp{b},
a @key{RET} or @kbd{C-j}, or anything else.

@example
(cl-case (read-char)
  (?a (do-a-thing))
  (?b (do-b-thing))
  ((?\r ?\n) (do-ret-thing))
  (t (do-other-thing)))
@end example
@end defmac

@defmac cl-ecase keyform clause@dots{}
This macro is just like @code{cl-case}, except that if the key does
not match any of the clauses, an error is signaled rather than
simply returning @code{nil}.
@end defmac

@defmac cl-typecase keyform clause@dots{}
This macro is a version of @code{cl-case} that checks for types
rather than values.  Each @var{clause} is of the form
@samp{(@var{type} @var{body}@dots{})}.  @xref{Type Predicates},
for a description of type specifiers.  For example,

@example
(cl-typecase x
  (integer (munch-integer x))
  (float (munch-float x))
  (string (munch-integer (string-to-number x)))
  (t (munch-anything x)))
@end example

The type specifier @code{t} matches any type of object; the word
@code{otherwise} is also allowed.  To make one clause match any of
several types, use an @code{(or @dots{})} type specifier.
@end defmac

@defmac cl-etypecase keyform clause@dots{}
This macro is just like @code{cl-typecase}, except that if the key does
not match any of the clauses, an error is signaled rather than
simply returning @code{nil}.
@end defmac

@node Blocks and Exits
@section Blocks and Exits
@cindex block
@cindex exit

@noindent
Common Lisp @dfn{blocks} provide a non-local exit mechanism very
similar to @code{catch} and @code{throw}, with lexical scoping.
This package actually implements @code{cl-block}
in terms of @code{catch}; however, the lexical scoping allows the
byte-compiler to omit the costly @code{catch} step if the
body of the block does not actually @code{cl-return-from} the block.

@defmac cl-block name forms@dots{}
The @var{forms} are evaluated as if by a @code{progn}.  However,
if any of the @var{forms} execute @code{(cl-return-from @var{name})},
they will jump out and return directly from the @code{cl-block} form.
The @code{cl-block} returns the result of the last @var{form} unless
a @code{cl-return-from} occurs.

The @code{cl-block}/@code{cl-return-from} mechanism is quite similar to
the @code{catch}/@code{throw} mechanism.  The main differences are
that block @var{name}s are unevaluated symbols, rather than forms
(such as quoted symbols) that evaluate to a tag at run-time; and
also that blocks are always lexically scoped.
In a dynamically scoped @code{catch}, functions called from the
@code{catch} body can also @code{throw} to the @code{catch}.  This
is not an option for @code{cl-block}, where
the @code{cl-return-from} referring to a block name must appear
physically within the @var{forms} that make up the body of the block.
They may not appear within other called functions, although they may
appear within macro expansions or @code{lambda}s in the body.  Block
names and @code{catch} names form independent name-spaces.

In true Common Lisp, @code{defun} and @code{defmacro} surround
the function or expander bodies with implicit blocks with the
same name as the function or macro.  This does not occur in Emacs
Lisp, but this package provides @code{cl-defun} and @code{cl-defmacro}
forms, which do create the implicit block.

The Common Lisp looping constructs defined by this package,
such as @code{cl-loop} and @code{cl-dolist}, also create implicit blocks
just as in Common Lisp.

Because they are implemented in terms of Emacs Lisp's @code{catch}
and @code{throw}, blocks have the same overhead as actual
@code{catch} constructs (roughly two function calls).  However,
the byte compiler will optimize away the @code{catch}
if the block does
not in fact contain any @code{cl-return} or @code{cl-return-from} calls
that jump to it.  This means that @code{cl-do} loops and @code{cl-defun}
functions that don't use @code{cl-return} don't pay the overhead to
support it.
@end defmac

@defmac cl-return-from name [result]
This macro returns from the block named @var{name}, which must be
an (unevaluated) symbol.  If a @var{result} form is specified, it
is evaluated to produce the result returned from the @code{block}.
Otherwise, @code{nil} is returned.
@end defmac

@defmac cl-return [result]
This macro is exactly like @code{(cl-return-from nil @var{result})}.
Common Lisp loops like @code{cl-do} and @code{cl-dolist} implicitly enclose
themselves in @code{nil} blocks.
@end defmac

@c FIXME?  Maybe this should be in a separate section?
@defmac cl-tagbody &rest labels-or-statements
This macro executes statements while allowing for control transfer to
user-defined labels.  Each element of @var{labels-or-statements} can
be either a label (an integer or a symbol), or a cons-cell
(a statement).  This distinction is made before macroexpansion.
Statements are executed in sequence, discarding any return value.
Any statement can transfer control at any time to the statements that follow
one of the labels with the special form @code{(go @var{label})}.
Labels have lexical scope and dynamic extent.
@end defmac


@node Iteration
@section Iteration
@cindex iteration

@noindent
The macros described here provide more sophisticated, high-level
looping constructs to complement Emacs Lisp's basic loop forms
(@pxref{Iteration,,,elisp,GNU Emacs Lisp Reference Manual}).

@defmac cl-loop forms@dots{}
This package supports both the simple, old-style meaning of
@code{loop} and the extremely powerful and flexible feature known as
the @dfn{Loop Facility} or @dfn{Loop Macro}.  This more advanced
facility is discussed in the following section; @pxref{Loop Facility}.
The simple form of @code{loop} is described here.

If @code{cl-loop} is followed by zero or more Lisp expressions,
then @code{(cl-loop @var{exprs}@dots{})} simply creates an infinite
loop executing the expressions over and over.  The loop is
enclosed in an implicit @code{nil} block.  Thus,

@example
(cl-loop (foo)  (if (no-more) (return 72))  (bar))
@end example

@noindent
is exactly equivalent to

@example
(cl-block nil (while t (foo)  (if (no-more) (return 72))  (bar)))
@end example

If any of the expressions are plain symbols, the loop is instead
interpreted as a Loop Macro specification as described later.
(This is not a restriction in practice, since a plain symbol
in the above notation would simply access and throw away the
value of a variable.)
@end defmac

@defmac cl-do (spec@dots{}) (end-test [result@dots{}]) forms@dots{}
This macro creates a general iterative loop.  Each @var{spec} is
of the form

@example
(@var{var} [@var{init} [@var{step}]])
@end example

The loop works as follows:  First, each @var{var} is bound to the
associated @var{init} value as if by a @code{let} form.  Then, in
each iteration of the loop, the @var{end-test} is evaluated; if
true, the loop is finished.  Otherwise, the body @var{forms} are
evaluated, then each @var{var} is set to the associated @var{step}
expression (as if by a @code{cl-psetq} form) and the next iteration
begins.  Once the @var{end-test} becomes true, the @var{result}
forms are evaluated (with the @var{var}s still bound to their
values) to produce the result returned by @code{cl-do}.

The entire @code{cl-do} loop is enclosed in an implicit @code{nil}
block, so that you can use @code{(cl-return)} to break out of the
loop at any time.

If there are no @var{result} forms, the loop returns @code{nil}.
If a given @var{var} has no @var{step} form, it is bound to its
@var{init} value but not otherwise modified during the @code{cl-do}
loop (unless the code explicitly modifies it); this case is just
a shorthand for putting a @code{(let ((@var{var} @var{init})) @dots{})}
around the loop.  If @var{init} is also omitted it defaults to
@code{nil}, and in this case a plain @samp{@var{var}} can be used
in place of @samp{(@var{var})}, again following the analogy with
@code{let}.

This example (from Steele) illustrates a loop that applies the
function @code{f} to successive pairs of values from the lists
@code{foo} and @code{bar}; it is equivalent to the call
@code{(cl-mapcar 'f foo bar)}.  Note that this loop has no body
@var{forms} at all, performing all its work as side effects of
the rest of the loop.

@example
(cl-do ((x foo (cdr x))
        (y bar (cdr y))
        (z nil (cons (f (car x) (car y)) z)))
     ((or (null x) (null y))
      (nreverse z)))
@end example
@end defmac

@defmac cl-do* (spec@dots{}) (end-test [result@dots{}]) forms@dots{}
This is to @code{cl-do} what @code{let*} is to @code{let}.  In
particular, the initial values are bound as if by @code{let*}
rather than @code{let}, and the steps are assigned as if by
@code{setq} rather than @code{cl-psetq}.

Here is another way to write the above loop:

@example
(cl-do* ((xp foo (cdr xp))
         (yp bar (cdr yp))
         (x (car xp) (car xp))
         (y (car yp) (car yp))
         z)
  ((or (null xp) (null yp))
   (nreverse z))
  (push (f x y) z))
@end example
@end defmac

@defmac cl-dolist (var list [result]) forms@dots{}
This is exactly like the standard Emacs Lisp macro @code{dolist},
but surrounds the loop with an implicit @code{nil} block.
@end defmac

@defmac cl-dotimes (var count [result]) forms@dots{}
This is exactly like the standard Emacs Lisp macro @code{dotimes},
but surrounds the loop with an implicit @code{nil} block.
The body is executed with @var{var} bound to the integers
from zero (inclusive) to @var{count} (exclusive), in turn.  Then
@c FIXME lispref does not state this part explicitly, could move this there.
the @var{result} form is evaluated with @var{var} bound to the total
number of iterations that were done (i.e., @code{(max 0 @var{count})})
to get the return value for the loop form.  Use of @var{result} is deprecated.
@end defmac

@defmac cl-do-symbols (var [obarray [result]]) forms@dots{}
This loop iterates over all interned symbols.  If @var{obarray}
is specified and is not @code{nil}, it loops over all symbols in
that obarray.  For each symbol, the body @var{forms} are evaluated
with @var{var} bound to that symbol.  The symbols are visited in
an unspecified order.  Afterward the @var{result} form, if any,
is evaluated (with @var{var} bound to @code{nil}) to get the return
value.  The loop is surrounded by an implicit @code{nil} block.
@end defmac

@defmac cl-do-all-symbols (var [result]) forms@dots{}
This is identical to @code{cl-do-symbols} except that the @var{obarray}
argument is omitted; it always iterates over the default obarray.
@end defmac

@xref{Mapping over Sequences}, for some more functions for
iterating over vectors or lists.

@node Loop Facility
@section Loop Facility
@cindex loop facility

@noindent
A common complaint with Lisp's traditional looping constructs was
that they were either too simple and limited, such as @code{dotimes}
or @code{while}, or too unreadable and obscure, like Common Lisp's
@code{do} loop.

To remedy this, Common Lisp added a construct called the ``Loop
Facility'' or ``@code{loop} macro'', with an easy-to-use but very
powerful and expressive syntax.

@menu
* Loop Basics::           The @code{cl-loop} macro, basic clause structure.
* Loop Examples::         Working examples of the @code{cl-loop} macro.
* For Clauses::           Clauses introduced by @code{for} or @code{as}.
* Iteration Clauses::     @code{repeat}, @code{while}, @code{thereis}, etc.
* Accumulation Clauses::  @code{collect}, @code{sum}, @code{maximize}, etc.
* Other Clauses::         @code{with}, @code{if}, @code{initially}, @code{finally}.
@end menu

@node Loop Basics
@subsection Loop Basics

@noindent
The @code{cl-loop} macro essentially creates a mini-language within
Lisp that is specially tailored for describing loops.  While this
language is a little strange-looking by the standards of regular Lisp,
it turns out to be very easy to learn and well-suited to its purpose.

Since @code{cl-loop} is a macro, all parsing of the loop language
takes place at byte-compile time; compiled @code{cl-loop}s are just
as efficient as the equivalent @code{while} loops written longhand.

@defmac cl-loop clauses@dots{}
A loop construct consists of a series of @var{clause}s, each
introduced by a symbol like @code{for} or @code{do}.  Clauses
are simply strung together in the argument list of @code{cl-loop},
with minimal extra parentheses.  The various types of clauses
specify initializations, such as the binding of temporary
variables, actions to be taken in the loop, stepping actions,
and final cleanup.

Common Lisp specifies a certain general order of clauses in a
loop:

@example
(loop @var{name-clause}
      @var{var-clauses}@dots{}
      @var{action-clauses}@dots{})
@end example

The @var{name-clause} optionally gives a name to the implicit
block that surrounds the loop.  By default, the implicit block
is named @code{nil}.  The @var{var-clauses} specify what
variables should be bound during the loop, and how they should
be modified or iterated throughout the course of the loop.  The
@var{action-clauses} are things to be done during the loop, such
as computing, collecting, and returning values.

The Emacs version of the @code{cl-loop} macro is less restrictive about
the order of clauses, but things will behave most predictably if
you put the variable-binding clauses @code{with}, @code{for}, and
@code{repeat} before the action clauses.  As in Common Lisp,
@code{initially} and @code{finally} clauses can go anywhere.

Loops generally return @code{nil} by default, but you can cause
them to return a value by using an accumulation clause like
@code{collect}, an end-test clause like @code{always}, or an
explicit @code{return} clause to jump out of the implicit block.
(Because the loop body is enclosed in an implicit block, you can
also use regular Lisp @code{cl-return} or @code{cl-return-from} to
break out of the loop.)
@end defmac

The following sections give some examples of the loop macro in
action, and describe the particular loop clauses in great detail.
Consult the second edition of Steele for additional discussion
and examples.

@node Loop Examples
@subsection Loop Examples

@noindent
Before listing the full set of clauses that are allowed, let's
look at a few example loops just to get a feel for the @code{cl-loop}
language.

@example
(cl-loop for buf in (buffer-list)
         collect (buffer-file-name buf))
@end example

@noindent
This loop iterates over all Emacs buffers, using the list
returned by @code{buffer-list}.  For each buffer @var{buf},
it calls @code{buffer-file-name} and collects the results into
a list, which is then returned from the @code{cl-loop} construct.
The result is a list of the file names of all the buffers in
Emacs's memory.  The words @code{for}, @code{in}, and @code{collect}
are reserved words in the @code{cl-loop} language.

@example
(cl-loop repeat 20 do (insert "Yowsa\n"))
@end example

@noindent
This loop inserts the phrase ``Yowsa'' twenty times in the
current buffer.

@example
(cl-loop until (eobp) do (munch-line) (forward-line 1))
@end example

@noindent
This loop calls @code{munch-line} on every line until the end
of the buffer.  If point is already at the end of the buffer,
the loop exits immediately.

@example
(cl-loop do (munch-line) until (eobp) do (forward-line 1))
@end example

@noindent
This loop is similar to the above one, except that @code{munch-line}
is always called at least once.

@example
(cl-loop for x from 1 to 100
         for y = (* x x)
         until (>= y 729)
         finally return (list x (= y 729)))
@end example

@noindent
This more complicated loop searches for a number @code{x} whose
square is 729.  For safety's sake it only examines @code{x}
values up to 100; dropping the phrase @samp{to 100} would
cause the loop to count upwards with no limit.  The second
@code{for} clause defines @code{y} to be the square of @code{x}
within the loop; the expression after the @code{=} sign is
reevaluated each time through the loop.  The @code{until}
clause gives a condition for terminating the loop, and the
@code{finally} clause says what to do when the loop finishes.
(This particular example was written less concisely than it
could have been, just for the sake of illustration.)

Note that even though this loop contains three clauses (two
@code{for}s and an @code{until}) that would have been enough to
define loops all by themselves, it still creates a single loop
rather than some sort of triple-nested loop.  You must explicitly
nest your @code{cl-loop} constructs if you want nested loops.

@node For Clauses
@subsection For Clauses

@noindent
Most loops are governed by one or more @code{for} clauses.
A @code{for} clause simultaneously describes variables to be
bound, how those variables are to be stepped during the loop,
and usually an end condition based on those variables.

The word @code{as} is a synonym for the word @code{for}.  This
word is followed by a variable name, then a word like @code{from}
or @code{across} that describes the kind of iteration desired.
In Common Lisp, the phrase @code{being the} sometimes precedes
the type of iteration; in this package both @code{being} and
@code{the} are optional.  The word @code{each} is a synonym
for @code{the}, and the word that follows it may be singular
or plural:  @samp{for x being the elements of y} or
@samp{for x being each element of y}.  Which form you use
is purely a matter of style.

The variable is bound around the loop as if by @code{let}:

@example
(setq i 'happy)
(cl-loop for i from 1 to 10 do (do-something-with i))
i
     @result{} happy
@end example

@table @code
@item for @var{var} from @var{expr1} to @var{expr2} by @var{expr3}
This type of @code{for} clause creates a counting loop.  Each of
the three sub-terms is optional, though there must be at least one
term so that the clause is marked as a counting clause.

The three expressions are the starting value, the ending value, and
the step value, respectively, of the variable.  The loop counts
upwards by default (@var{expr3} must be positive), from @var{expr1}
to @var{expr2} inclusively.  If you omit the @code{from} term, the
loop counts from zero; if you omit the @code{to} term, the loop
counts forever without stopping (unless stopped by some other
loop clause, of course); if you omit the @code{by} term, the loop
counts in steps of one.

You can replace the word @code{from} with @code{upfrom} or
@code{downfrom} to indicate the direction of the loop.  Likewise,
you can replace @code{to} with @code{upto} or @code{downto}.
For example, @samp{for x from 5 downto 1} executes five times
with @code{x} taking on the integers from 5 down to 1 in turn.
Also, you can replace @code{to} with @code{below} or @code{above},
which are like @code{upto} and @code{downto} respectively except
that they are exclusive rather than inclusive limits:

@example
(cl-loop for x to 10 collect x)
        @result{} (0 1 2 3 4 5 6 7 8 9 10)
(cl-loop for x below 10 collect x)
        @result{} (0 1 2 3 4 5 6 7 8 9)
@end example

The @code{by} value is always positive, even for downward-counting
loops.  Some sort of @code{from} value is required for downward
loops; @samp{for x downto 5} is not a valid loop clause all by
itself.

@item for @var{var} in @var{list} by @var{function}
This clause iterates @var{var} over all the elements of @var{list},
in turn.  If you specify the @code{by} term, then @var{function}
is used to traverse the list instead of @code{cdr}; it must be a
function taking one argument.  For example:

@example
(cl-loop for x in '(1 2 3 4 5 6) collect (* x x))
        @result{} (1 4 9 16 25 36)
(cl-loop for x in '(1 2 3 4 5 6) by 'cddr collect (* x x))
        @result{} (1 9 25)
@end example

@item for @var{var} on @var{list} by @var{function}
This clause iterates @var{var} over all the cons cells of @var{list}.

@example
(cl-loop for x on '(1 2 3 4) collect x)
        @result{} ((1 2 3 4) (2 3 4) (3 4) (4))
@end example

@item for @var{var} in-ref @var{list} by @var{function}
This is like a regular @code{in} clause, but @var{var} becomes
a @code{setf}-able ``reference'' onto the elements of the list
rather than just a temporary variable.  For example,

@example
(cl-loop for x in-ref my-list do (cl-incf x))
@end example

@noindent
increments every element of @code{my-list} in place.  This clause
is an extension to standard Common Lisp.

@item for @var{var} across @var{array}
This clause iterates @var{var} over all the elements of @var{array},
which may be a vector or a string.

@example
(cl-loop for x across "aeiou"
         do (use-vowel (char-to-string x)))
@end example

@item for @var{var} across-ref @var{array}
This clause iterates over an array, with @var{var} a @code{setf}-able
reference onto the elements; see @code{in-ref} above.

@item for @var{var} being the elements of @var{sequence}
This clause iterates over the elements of @var{sequence}, which may
be a list, vector, or string.  Since the type must be determined
at run-time, this is somewhat less efficient than @code{in} or
@code{across}.  The clause may be followed by the additional term
@samp{using (index @var{var2})} to cause @var{var2} to be bound to
the successive indices (starting at 0) of the elements.

This clause type is taken from older versions of the @code{loop} macro,
and is not present in modern Common Lisp.  The @samp{using (sequence @dots{})}
term of the older macros is not supported.

@item for @var{var} being the elements of-ref @var{sequence}
This clause iterates over a sequence, with @var{var} a @code{setf}-able
reference onto the elements; see @code{in-ref} above.

@item for @var{var} being the symbols [of @var{obarray}]
This clause iterates over symbols, either over all interned symbols
or over all symbols in @var{obarray}.  The loop is executed with
@var{var} bound to each symbol in turn.  The symbols are visited in
an unspecified order.

As an example,

@example
(cl-loop for sym being the symbols
         when (fboundp sym)
         when (string-match "^map" (symbol-name sym))
         collect sym)
@end example

@noindent
returns a list of all the functions whose names begin with @samp{map}.

The Common Lisp words @code{external-symbols} and @code{present-symbols}
are also recognized but are equivalent to @code{symbols} in Emacs Lisp.

Due to a minor implementation restriction, it will not work to have
more than one @code{for} clause iterating over symbols, hash tables,
keymaps, overlays, or intervals in a given @code{cl-loop}.  Fortunately,
it would rarely if ever be useful to do so.  It @emph{is} valid to mix
one of these types of clauses with other clauses like @code{for @dots{} to}
or @code{while}.

@item for @var{var} being the hash-keys of @var{hash-table}
@itemx for @var{var} being the hash-values of @var{hash-table}
This clause iterates over the entries in @var{hash-table} with
@var{var} bound to each key, or value.  A @samp{using} clause can bind
a second variable to the opposite part.

@example
(cl-loop for k being the hash-keys of h
               using (hash-values v)
         do
         (message "key %S -> value %S" k v))
@end example

@item for @var{var} being the key-codes of @var{keymap}
@itemx for @var{var} being the key-bindings of @var{keymap}
This clause iterates over the entries in @var{keymap}.
The iteration does not enter nested keymaps but does enter inherited
(parent) keymaps.
A @code{using} clause can access both the codes and the bindings
together.

@example
(cl-loop for c being the key-codes of (current-local-map)
               using (key-bindings b)
         do
         (message "key %S -> binding %S" c b))
@end example


@item for @var{var} being the key-seqs of @var{keymap}
This clause iterates over all key sequences defined by @var{keymap}
and its nested keymaps, where @var{var} takes on values which are
vectors.  The strings or vectors
are reused for each iteration, so you must copy them if you wish to keep
them permanently.  You can add a @samp{using (key-bindings @dots{})}
clause to get the command bindings as well.

@item for @var{var} being the overlays [of @var{buffer}] @dots{}
This clause iterates over the ``overlays'' of a buffer
(the clause @code{extents} is synonymous
with @code{overlays}).  If the @code{of} term is omitted, the current
buffer is used.
This clause also accepts optional @samp{from @var{pos}} and
@samp{to @var{pos}} terms, limiting the clause to overlays which
overlap the specified region.

@item for @var{var} being the intervals [of @var{object}] @dots{}
This clause iterates over all intervals of a buffer or string with
constant text properties.  The variable @var{var} will be bound to
conses of start and end positions, where one start position is always
equal to the previous end position.  The clause allows @code{of},
@code{from}, @code{to}, and @code{property} terms, where the latter
term restricts the search to just the specified property.  The
@code{of} term may specify either a buffer or a string.  @xref{Text
Properties,,,elisp}.

@item for @var{var} being the frames
This clause iterates over all Emacs frames.  The clause @code{screens} is
a synonym for @code{frames}.  The frames are visited in
@code{next-frame} order starting from @code{selected-frame}.

@item for @var{var} being the windows [of @var{frame}]
This clause iterates over the windows (in the Emacs sense) of
the current frame, or of the specified @var{frame}.  It visits windows
in @code{next-window} order starting from @code{selected-window}
(or @code{frame-selected-window} if you specify @var{frame}).
This clause treats the minibuffer window in the same way as
@code{next-window} does.  For greater flexibility, consider using
@code{walk-windows} instead.

@item for @var{var} being the buffers
This clause iterates over all buffers in Emacs.  It is equivalent
to @samp{for @var{var} in (buffer-list)}.

@item for @var{var} = @var{expr1} then @var{expr2}
This clause does a general iteration.  The first time through
the loop, @var{var} will be bound to @var{expr1}.  On the second
and successive iterations it will be set by evaluating @var{expr2}
(which may refer to the old value of @var{var}).  For example,
these two loops are effectively the same:

@example
(cl-loop for x on my-list by 'cddr do @dots{})
(cl-loop for x = my-list then (cddr x) while x do @dots{})
@end example

Note that this type of @code{for} clause does not imply any sort
of terminating condition; the above example combines it with a
@code{while} clause to tell when to end the loop.

If you omit the @code{then} term, @var{expr1} is used both for
the initial setting and for successive settings:

@example
(cl-loop for x = (random) when (> x 0) return x)
@end example

@noindent
This loop keeps taking random numbers from the @code{(random)}
function until it gets a positive one, which it then returns.
@end table

If you include several @code{for} clauses in a row, they are
treated sequentially (as if by @code{let*} and @code{setq}).
You can instead use the word @code{and} to link the clauses,
in which case they are processed in parallel (as if by @code{let}
and @code{cl-psetq}).

@example
(cl-loop for x below 5 for y = nil then x collect (list x y))
        @result{} ((0 nil) (1 1) (2 2) (3 3) (4 4))
(cl-loop for x below 5 and y = nil then x collect (list x y))
        @result{} ((0 nil) (1 0) (2 1) (3 2) (4 3))
@end example

@noindent
In the first loop, @code{y} is set based on the value of @code{x}
that was just set by the previous clause; in the second loop,
@code{x} and @code{y} are set simultaneously so @code{y} is set
based on the value of @code{x} left over from the previous time
through the loop.

@cindex destructuring, in cl-loop
Another feature of the @code{cl-loop} macro is @emph{destructuring},
similar in concept to the destructuring provided by @code{defmacro}
(@pxref{Argument Lists}).
The @var{var} part of any @code{for} clause can be given as a list
of variables instead of a single variable.  The values produced
during loop execution must be lists; the values in the lists are
stored in the corresponding variables.

@example
(cl-loop for (x y) in '((2 3) (4 5) (6 7)) collect (+ x y))
        @result{} (5 9 13)
@end example

In loop destructuring, if there are more values than variables
the trailing values are ignored, and if there are more variables
than values the trailing variables get the value @code{nil}.
If @code{nil} is used as a variable name, the corresponding
values are ignored.  Destructuring may be nested, and dotted
lists of variables like @code{(x . y)} are allowed, so for example
to process an alist

@example
(cl-loop for (key . value) in '((a . 1) (b . 2))
         collect value)
        @result{} (1 2)
@end example

@node Iteration Clauses
@subsection Iteration Clauses

@noindent
Aside from @code{for} clauses, there are several other loop clauses
that control the way the loop operates.  They might be used by
themselves, or in conjunction with one or more @code{for} clauses.

@table @code
@item repeat @var{integer}
This clause simply counts up to the specified number using an
internal temporary variable.  The loops

@example
(cl-loop repeat (1+ n) do @dots{})
(cl-loop for temp to n do @dots{})
@end example

@noindent
are identical except that the second one forces you to choose
a name for a variable you aren't actually going to use.

@item while @var{condition}
This clause stops the loop when the specified condition (any Lisp
expression) becomes @code{nil}.  For example, the following two
loops are equivalent, except for the implicit @code{nil} block
that surrounds the second one:

@example
(while @var{cond} @var{forms}@dots{})
(cl-loop while @var{cond} do @var{forms}@dots{})
@end example

@item until @var{condition}
This clause stops the loop when the specified condition is true,
i.e., non-@code{nil}.

@item always @var{condition}
This clause stops the loop when the specified condition is @code{nil}.
Unlike @code{while}, it stops the loop using @code{return nil} so that
the @code{finally} clauses are not executed.  If all the conditions
were non-@code{nil}, the loop returns @code{t}:

@example
(if (cl-loop for size in size-list always (> size 10))
    (only-big-sizes)
  (some-small-sizes))
@end example

@item never @var{condition}
This clause is like @code{always}, except that the loop returns
@code{t} if all conditions were false, or @code{nil} otherwise.

@item thereis @var{condition}
This clause stops the loop when the specified form is non-@code{nil};
in this case, it returns that non-@code{nil} value.  If all the
values were @code{nil}, the loop returns @code{nil}.

@item iter-by @var{iterator}
This clause iterates over the values from the specified form, an
iterator object.  See (@pxref{Generators,,,elisp,GNU Emacs Lisp
Reference Manual}).
@end table

@node Accumulation Clauses
@subsection Accumulation Clauses

@noindent
These clauses cause the loop to accumulate information about the
specified Lisp @var{form}.  The accumulated result is returned
from the loop unless overridden, say, by a @code{return} clause.

@table @code
@item collect @var{form}
This clause collects the values of @var{form} into a list.  Several
examples of @code{collect} appear elsewhere in this manual.

The word @code{collecting} is a synonym for @code{collect}, and
likewise for the other accumulation clauses.

@item append @var{form}
This clause collects lists of values into a result list using
@code{append}.

@item nconc @var{form}
This clause collects lists of values into a result list by
destructively modifying the lists rather than copying them.

@item concat @var{form}
This clause concatenates the values of the specified @var{form}
into a string.  (It and the following clause are extensions to
standard Common Lisp.)

@item vconcat @var{form}
This clause concatenates the values of the specified @var{form}
into a vector.

@item count @var{form}
This clause counts the number of times the specified @var{form}
evaluates to a non-@code{nil} value.

@item sum @var{form}
This clause accumulates the sum of the values of the specified
@var{form}, which must evaluate to a number.

@item maximize @var{form}
This clause accumulates the maximum value of the specified @var{form},
which must evaluate to a number.  The return value is undefined if
@code{maximize} is executed zero times.

@item minimize @var{form}
This clause accumulates the minimum value of the specified @var{form}.
@end table

Accumulation clauses can be followed by @samp{into @var{var}} to
cause the data to be collected into variable @var{var} (which is
automatically @code{let}-bound during the loop) rather than an
unnamed temporary variable.  Also, @code{into} accumulations do
not automatically imply a return value.  The loop must use some
explicit mechanism, such as @code{finally return}, to return
the accumulated result.

It is valid for several accumulation clauses of the same type to
accumulate into the same place.  From Steele:

@example
(cl-loop for name in '(fred sue alice joe june)
         for kids in '((bob ken) () () (kris sunshine) ())
         collect name
         append kids)
        @result{} (fred bob ken sue alice joe kris sunshine june)
@end example

@node Other Clauses
@subsection Other Clauses

@noindent
This section describes the remaining loop clauses.

@table @code
@item with @var{var} = @var{value}
This clause binds a variable to a value around the loop, but
otherwise leaves the variable alone during the loop.  The following
loops are basically equivalent:

@example
(cl-loop with x = 17 do @dots{})
(let ((x 17)) (cl-loop do @dots{}))
(cl-loop for x = 17 then x do @dots{})
@end example

Naturally, the variable @var{var} might be used for some purpose
in the rest of the loop.  For example:

@example
(cl-loop for x in my-list  with res = nil  do (push x res)
         finally return res)
@end example

This loop inserts the elements of @code{my-list} at the front of
a new list being accumulated in @code{res}, then returns the
list @code{res} at the end of the loop.  The effect is similar
to that of a @code{collect} clause, but the list gets reversed
by virtue of the fact that elements are being pushed onto the
front of @code{res} rather than the end.

If you omit the @code{=} term, the variable is initialized to
@code{nil}.  (Thus the @samp{= nil} in the above example is
unnecessary.)

Bindings made by @code{with} are sequential by default, as if
by @code{let*}.  Just like @code{for} clauses, @code{with} clauses
can be linked with @code{and} to cause the bindings to be made by
@code{let} instead.

@item if @var{condition} @var{clause}
This clause executes the following loop clause only if the specified
condition is true.  The following @var{clause} should be an accumulation,
@code{do}, @code{return}, @code{if}, or @code{unless} clause.
Several clauses may be linked by separating them with @code{and}.
These clauses may be followed by @code{else} and a clause or clauses
to execute if the condition was false.  The whole construct may
optionally be followed by the word @code{end} (which may be used to
disambiguate an @code{else} or @code{and} in a nested @code{if}).

The actual non-@code{nil} value of the condition form is available
by the name @code{it} in the ``then'' part.  For example:

@example
(setq funny-numbers '(6 13 -1))
     @result{} (6 13 -1)
(cl-loop for x below 10
         if (cl-oddp x)
           collect x into odds
           and if (memq x funny-numbers) return (cdr it) end
         else
           collect x into evens
         finally return (vector odds evens))
        @result{} [(1 3 5 7 9) (0 2 4 6 8)]
(setq funny-numbers '(6 7 13 -1))
     @result{} (6 7 13 -1)
(cl-loop <@r{same thing again}>)
        @result{} (13 -1)
@end example

Note the use of @code{and} to put two clauses into the ``then''
part, one of which is itself an @code{if} clause.  Note also that
@code{end}, while normally optional, was necessary here to make
it clear that the @code{else} refers to the outermost @code{if}
clause.  In the first case, the loop returns a vector of lists
of the odd and even values of @var{x}.  In the second case, the
odd number 7 is one of the @code{funny-numbers} so the loop
returns early; the actual returned value is based on the result
of the @code{memq} call.

@item when @var{condition} @var{clause}
This clause is just a synonym for @code{if}.

@item unless @var{condition} @var{clause}
The @code{unless} clause is just like @code{if} except that the
sense of the condition is reversed.

@item named @var{name}
This clause gives a name other than @code{nil} to the implicit
block surrounding the loop.  The @var{name} is the symbol to be
used as the block name.

@item initially [do] @var{forms}@dots{}
This keyword introduces one or more Lisp forms which will be
executed before the loop itself begins (but after any variables
requested by @code{for} or @code{with} have been bound to their
initial values).  @code{initially} clauses can appear anywhere;
if there are several, they are executed in the order they appear
in the loop.  The keyword @code{do} is optional.

@item finally [do] @var{forms}@dots{}
This introduces Lisp forms which will be executed after the loop
finishes (say, on request of a @code{for} or @code{while}).
@code{initially} and @code{finally} clauses may appear anywhere
in the loop construct, but they are executed (in the specified
order) at the beginning or end, respectively, of the loop.

@item finally return @var{form}
This says that @var{form} should be executed after the loop
is done to obtain a return value.  (Without this, or some other
clause like @code{collect} or @code{return}, the loop will simply
return @code{nil}.)  Variables bound by @code{for}, @code{with},
or @code{into} will still contain their final values when @var{form}
is executed.

@item do @var{forms}@dots{}
The word @code{do} may be followed by any number of Lisp expressions
which are executed as an implicit @code{progn} in the body of the
loop.  Many of the examples in this section illustrate the use of
@code{do}.

@item return @var{form}
This clause causes the loop to return immediately.  The following
Lisp form is evaluated to give the return value of the loop
form.  The @code{finally} clauses, if any, are not executed.
Of course, @code{return} is generally used inside an @code{if} or
@code{unless}, as its use in a top-level loop clause would mean
the loop would never get to ``loop'' more than once.

The clause @samp{return @var{form}} is equivalent to
@samp{do (cl-return @var{form})} (or @code{cl-return-from} if the loop
was named).  The @code{return} clause is implemented a bit more
efficiently, though.
@end table

While there is no high-level way to add user extensions to @code{cl-loop},
this package does offer two properties called @code{cl-loop-handler}
and @code{cl-loop-for-handler} which are functions to be called when a
given symbol is encountered as a top-level loop clause or @code{for}
clause, respectively.  Consult the source code in file
@file{cl-macs.el} for details.

This package's @code{cl-loop} macro is compatible with that of Common
Lisp, except that a few features are not implemented:  @code{loop-finish}
and data-type specifiers.  Naturally, the @code{for} clauses that
iterate over keymaps, overlays, intervals, frames, windows, and
buffers are Emacs-specific extensions.

@node Multiple Values
@section Multiple Values
@cindex multiple values

@noindent
Common Lisp functions can return zero or more results.  Emacs Lisp
functions, by contrast, always return exactly one result.  This
package makes no attempt to emulate Common Lisp multiple return
values; Emacs versions of Common Lisp functions that return more
than one value either return just the first value (as in
@code{cl-compiler-macroexpand}) or return a list of values.
This package @emph{does} define placeholders
for the Common Lisp functions that work with multiple values, but
in Emacs Lisp these functions simply operate on lists instead.
The @code{cl-values} form, for example, is a synonym for @code{list}
in Emacs.

@defmac cl-multiple-value-bind (var@dots{}) values-form forms@dots{}
This form evaluates @var{values-form}, which must return a list of
values.  It then binds the @var{var}s to these respective values,
as if by @code{let}, and then executes the body @var{forms}.
If there are more @var{var}s than values, the extra @var{var}s
are bound to @code{nil}.  If there are fewer @var{var}s than
values, the excess values are ignored.
@end defmac

@defmac cl-multiple-value-setq (var@dots{}) form
This form evaluates @var{form}, which must return a list of values.
It then sets the @var{var}s to these respective values, as if by
@code{setq}.  Extra @var{var}s or values are treated the same as
in @code{cl-multiple-value-bind}.
@end defmac

Since a perfect emulation is not feasible in Emacs Lisp, this
package opts to keep it as simple and predictable as possible.

@node Macro-Writing Macros
@section Macro-Writing Macros

@noindent
This package includes two classic Common Lisp macro-writing macros to
help render complex macrology easier to read.

@defmac cl-with-gensyms names@dots{} body
This macro expands to code that executes @var{body} with each of the
variables in @var{names} bound to a fresh uninterned symbol, or
@dfn{gensym}, in Common Lisp parlance.  For macros requiring more than
one gensym, use of @code{cl-with-gensyms} shortens the code and
renders one's intentions clearer.  Compare:

@example
(defmacro my-macro (foo)
  (let ((bar (gensym "bar"))
        (baz (gensym "baz"))
        (quux (gensym "quux")))
    `(let ((,bar (+ @dots{})))
       @dots{})))

(defmacro my-macro (foo)
  (cl-with-gensyms (bar baz quux)
    `(let ((,bar (+ @dots{})))
       @dots{})))
@end example
@end defmac

@defmac cl-once-only ((variable form)@dots{}) body
This macro is primarily to help the macro programmer ensure that forms
supplied by the user of the macro are evaluated just once by its
expansion even though the result of evaluating the form is to occur
more than once.  Less often, this macro is used to ensure that forms
supplied by the macro programmer are evaluated just once.

Each @var{variable} may be used to refer to the result of evaluating
@var{form} in @var{body}.  @code{cl-once-only} binds each
@var{variable} to a fresh uninterned symbol during the evaluation of
@var{body}.  Then, @code{cl-once-only} wraps the final expansion in
code to evaluate each @var{form} and bind the result to the
corresponding uninterned symbol.  Thus, when the macro writer
substitutes the value for @var{variable} into the expansion they are
effectively referring to the result of evaluating @var{form}, rather
than @var{form} itself.  Another way to put this is that each
@var{variable} is bound to an expression for the (singular) result of
evaluating @var{form}.

The most common case is where @var{variable} is one of the arguments
to the macro being written, so @code{(variable variable)} may be
abbreviated to just @code{variable}.

For example, consider this macro:

@example
(defmacro my-list (x y &rest forms)
  (let ((x-result (gensym))
        (y-result (gensym)))
    `(let ((,x-result ,x)
           (,y-result ,y))
       (list ,x-result ,y-result ,x-result ,y-result
             (progn ,@@forms))))
@end example

In a call like @w{@code{(my-list (pop foo) @dots{})}} the intermediate
binding to @code{x-result} ensures that the @code{pop} is not done
twice.  But as a result the code is rather complex: the reader must
keep track of how @code{x-result} really just means the first
parameter of the call to the macro, and the required use of multiple
gensyms to avoid variable capture by @code{(progn ,@@forms)} obscures
things further.  @code{cl-once-only} takes care of these details:

@example
(defmacro my-list (x y &rest forms)
  (cl-once-only (x y)
    `(list ,x ,y ,x ,y
           (progn ,@@forms))))
@end example
@end defmac

@node Macros
@chapter Macros

@noindent
This package implements the various Common Lisp features of
@code{defmacro}, such as destructuring, @code{&environment},
and @code{&body}.  Top-level @code{&whole} is not implemented
for @code{defmacro} due to technical difficulties.
@xref{Argument Lists}.

Destructuring is made available to the user by way of the
following macro:

@defmac cl-destructuring-bind arglist expr forms@dots{}
This macro expands to code that executes @var{forms}, with
the variables in @var{arglist} bound to the list of values
returned by @var{expr}.  The @var{arglist} can include all
the features allowed for @code{cl-defmacro} argument lists,
including destructuring.  (The @code{&environment} keyword
is not allowed.)  The macro expansion will signal an error
if @var{expr} returns a list of the wrong number of arguments
or with incorrect keyword arguments.
@end defmac

@cindex compiler macros
@cindex define compiler macros
This package also includes the Common Lisp @code{define-compiler-macro}
facility, which allows you to define compile-time expansions and
optimizations for your functions.

@defmac cl-define-compiler-macro name arglist forms@dots{}
This form is similar to @code{defmacro}, except that it only expands
calls to @var{name} at compile-time; calls processed by the Lisp
interpreter are not expanded, nor are they expanded by the
@code{macroexpand} function.

The argument list may begin with a @code{&whole} keyword and a
variable.  This variable is bound to the macro-call form itself,
i.e., to a list of the form @samp{(@var{name} @var{args}@dots{})}.
If the macro expander returns this form unchanged, then the
compiler treats it as a normal function call.  This allows
compiler macros to work as optimizers for special cases of a
function, leaving complicated cases alone.

For example, here is a simplified version of a definition that
appears as a standard part of this package:

@example
(cl-define-compiler-macro cl-member (&whole form a list &rest keys)
     (if (and (null keys)
              (eq (car-safe a) 'quote)
              (not (floatp (cadr a))))
         (list 'memq a list)
       form))
@end example

@noindent
This definition causes @code{(cl-member @var{a} @var{list})} to change
to a call to the faster @code{memq} in the common case where @var{a}
is a non-floating-point constant; if @var{a} is anything else, or
if there are any keyword arguments in the call, then the original
@code{cl-member} call is left intact.  (The actual compiler macro
for @code{cl-member} optimizes a number of other cases, including
common @code{:test} predicates.)
@end defmac

@defun cl-compiler-macroexpand form
This function is analogous to @code{macroexpand}, except that it
expands compiler macros rather than regular macros.  It returns
@var{form} unchanged if it is not a call to a function for which
a compiler macro has been defined, or if that compiler macro
decided to punt by returning its @code{&whole} argument.  Like
@code{macroexpand}, it expands repeatedly until it reaches a form
for which no further expansion is possible.
@end defun

@xref{Macro Bindings}, for descriptions of the @code{cl-macrolet}
and @code{cl-symbol-macrolet} forms for making ``local'' macro
definitions.

@node Declarations
@chapter Declarations

@noindent
Common Lisp includes a complex and powerful ``declaration''
mechanism that allows you to give the compiler special hints
about the types of data that will be stored in particular variables,
and about the ways those variables and functions will be used.  This
package defines versions of all the Common Lisp declaration forms:
@code{declare}, @code{locally}, @code{proclaim}, @code{declaim},
and @code{the}.

Most of the Common Lisp declarations are not currently useful in Emacs
Lisp.  For example, the byte-code system provides little
opportunity to benefit from type information.
@ignore
and @code{special} declarations are redundant in a fully
dynamically-scoped Lisp.
@end ignore
A few declarations are meaningful when byte compiler optimizations
are enabled, as they are by the default.  Otherwise these
declarations will effectively be ignored.

@defun cl-proclaim decl-spec
This function records a ``global'' declaration specified by
@var{decl-spec}.  Since @code{cl-proclaim} is a function, @var{decl-spec}
is evaluated and thus should normally be quoted.
@end defun

@defmac cl-declaim decl-specs@dots{}
This macro is like @code{cl-proclaim}, except that it takes any number
of @var{decl-spec} arguments, and the arguments are unevaluated and
unquoted.  The @code{cl-declaim} macro also puts @code{(cl-eval-when
(compile load eval) @dots{})} around the declarations so that they will
be registered at compile-time as well as at run-time.  (This is vital,
since normally the declarations are meant to influence the way the
compiler treats the rest of the file that contains the @code{cl-declaim}
form.)
@end defmac

@defmac cl-declare decl-specs@dots{}
This macro is used to make declarations within functions and other
code.  Common Lisp allows declarations in various locations, generally
at the beginning of any of the many ``implicit @code{progn}s''
throughout Lisp syntax, such as function bodies, @code{let} bodies,
etc.  Currently the only declaration understood by @code{cl-declare}
is @code{special}.
@end defmac

@defmac cl-locally declarations@dots{} forms@dots{}
In this package, @code{cl-locally} is no different from @code{progn}.
@end defmac

@defmac cl-the type form
@code{cl-the} returns the value of @code{form}, first checking (if
optimization settings permit) that it is of type @code{type}.  Future
byte-compiler optimizations may also make use of this information to
improve runtime efficiency.

For example, @code{mapcar} can map over both lists and arrays.  It is
hard for the compiler to expand @code{mapcar} into an in-line loop
unless it knows whether the sequence will be a list or an array ahead
of time.  With @code{(mapcar 'car (cl-the vector foo))}, a future
compiler would have enough information to expand the loop in-line.
For now, Emacs Lisp will treat the above code as exactly equivalent
to @code{(mapcar 'car foo)}.
@end defmac

Each @var{decl-spec} in a @code{cl-proclaim}, @code{cl-declaim}, or
@code{cl-declare} should be a list beginning with a symbol that says
what kind of declaration it is.  This package currently understands
@code{special}, @code{inline}, @code{notinline}, @code{optimize},
and @code{warn} declarations.  (The @code{warn} declaration is an
extension of standard Common Lisp.)  Other Common Lisp declarations,
such as @code{type} and @code{ftype}, are silently ignored.

@table @code
@item special
@c FIXME ?
Since all variables in Emacs Lisp are ``special'' (in the Common
Lisp sense), @code{special} declarations are only advisory.  They
simply tell the byte compiler that the specified
variables are intentionally being referred to without being
bound in the body of the function.  The compiler normally emits
warnings for such references, since they could be typographical
errors for references to local variables.

The declaration @code{(cl-declare (special @var{var1} @var{var2}))} is
equivalent to @code{(defvar @var{var1}) (defvar @var{var2})}.

In top-level contexts, it is generally better to write
@code{(defvar @var{var})} than @code{(cl-declaim (special @var{var}))},
since @code{defvar} makes your intentions clearer.

@item inline
The @code{inline} @var{decl-spec} lists one or more functions
whose bodies should be expanded ``in-line'' into calling functions
whenever the compiler is able to arrange for it.  For example,
the function @code{cl-acons} is declared @code{inline}
by this package so that the form @code{(cl-acons @var{key} @var{value}
@var{alist})} will
expand directly into @code{(cons (cons @var{key} @var{value}) @var{alist})}
when it is called in user functions, so as to save function calls.

The following declarations are all equivalent.  Note that the
@code{defsubst} form is a convenient way to define a function
and declare it inline all at once.

@example
(cl-declaim (inline foo bar))
(cl-eval-when (compile load eval)
  (cl-proclaim '(inline foo bar)))
(defsubst foo (@dots{}) @dots{})       ; instead of defun
@end example

@strong{Please note:}  this declaration remains in effect after the
containing source file is done.  It is correct to use it to
request that a function you have defined should be inlined,
but it is impolite to use it to request inlining of an external
function.

In Common Lisp, it is possible to use @code{(declare (inline @dots{}))}
before a particular call to a function to cause just that call to
be inlined; the current byte compilers provide no way to implement
this, so @code{(cl-declare (inline @dots{}))} is currently ignored by
this package.

@item notinline
The @code{notinline} declaration lists functions which should
not be inlined after all; it cancels a previous @code{inline}
declaration.

@item optimize
This declaration controls how much optimization is performed by
the compiler.

The word @code{optimize} is followed by any number of lists like
@code{(speed 3)} or @code{(safety 2)}.  Common Lisp defines several
optimization ``qualities''; this package ignores all but @code{speed}
and @code{safety}.  The value of a quality should be an integer from
0 to 3, with 0 meaning ``unimportant'' and 3 meaning ``very important''.
The default level for both qualities is 1.

In this package, the @code{speed} quality is tied to the @code{byte-optimize}
flag, which is set to @code{nil} for @code{(speed 0)} and to
@code{t} for higher settings; and the @code{safety} quality is
tied to the @code{byte-compile-delete-errors} flag, which is
set to @code{nil} for @code{(safety 3)} and to @code{t} for all
lower settings.  (The latter flag controls whether the compiler
is allowed to optimize out code whose only side-effect could
be to signal an error, e.g., rewriting @code{(progn foo bar)} to
@code{bar} when it is not known whether @code{foo} will be bound
at run-time.)

Note that even compiling with @code{(safety 0)}, the Emacs
byte-code system provides sufficient checking to prevent real
harm from being done.  For example, barring serious bugs in
Emacs itself, Emacs will not crash with a segmentation fault
just because of an error in a fully-optimized Lisp program.

The @code{optimize} declaration is normally used in a top-level
@code{cl-proclaim} or @code{cl-declaim} in a file; Common Lisp allows
it to be used with @code{declare} to set the level of optimization
locally for a given form, but this will not work correctly with the
current byte-compiler.  (The @code{cl-declare}
will set the new optimization level, but that level will not
automatically be unset after the enclosing form is done.)

@item warn
This declaration controls what sorts of warnings are generated
by the byte compiler.  The word @code{warn} is followed by any
number of ``warning qualities'', similar in form to optimization
qualities.  The currently supported warning types are
@code{redefine}, @code{callargs}, @code{unresolved}, and
@code{free-vars}; in the current system, a value of 0 will
disable these warnings and any higher value will enable them.
See the documentation of the variable @code{byte-compile-warnings}
for more details.
@end table

@node Symbols
@chapter Symbols

@noindent
This package defines several symbol-related features that were
missing from Emacs Lisp.

@menu
* Property Lists::       @code{cl-get}, @code{cl-remprop}, @code{cl-getf}, @code{cl-remf}.
* Creating Symbols::     @code{cl-gensym}, @code{cl-gentemp}.
@end menu

@node Property Lists
@section Property Lists

@noindent
These functions augment the standard Emacs Lisp functions @code{get}
and @code{put} for operating on properties attached to symbols.
There are also functions for working with property lists as
first-class data structures not attached to particular symbols.

@defun cl-get symbol property &optional default
This function is like @code{get}, except that if the property is
not found, the @var{default} argument provides the return value.
(The Emacs Lisp @code{get} function always uses @code{nil} as
the default; this package's @code{cl-get} is equivalent to Common
Lisp's @code{get}.)

The @code{cl-get} function is @code{setf}-able; when used in this
fashion, the @var{default} argument is allowed but ignored.
@end defun

@defun cl-remprop symbol property
This function removes the entry for @var{property} from the property
list of @var{symbol}.  It returns a true value if the property was
indeed found and removed, or @code{nil} if there was no such property.
(This function was probably omitted from Emacs originally because,
since @code{get} did not allow a @var{default}, it was very difficult
to distinguish between a missing property and a property whose value
was @code{nil}; thus, setting a property to @code{nil} was close
enough to @code{cl-remprop} for most purposes.)
@end defun

@defun cl-getf place property &optional default
This function scans the list @var{place} as if it were a property
list, i.e., a list of alternating property names and values.  If
an even-numbered element of @var{place} is found which is @code{eq}
to @var{property}, the following odd-numbered element is returned.
Otherwise, @var{default} is returned (or @code{nil} if no default
is given).

In particular,

@example
(get sym prop)  @equiv{}  (cl-getf (symbol-plist sym) prop)
@end example

It is valid to use @code{cl-getf} as a @code{setf} place, in which case
its @var{place} argument must itself be a valid @code{setf} place.
The @var{default} argument, if any, is ignored in this context.
The effect is to change (via @code{setcar}) the value cell in the
list that corresponds to @var{property}, or to cons a new property-value
pair onto the list if the property is not yet present.

@example
(put sym prop val) @equiv{} (setf (cl-getf (symbol-plist sym) prop) val)
@end example

The @code{get} and @code{cl-get} functions are also @code{setf}-able.
The fact that @code{default} is ignored can sometimes be useful:

@example
(cl-incf (cl-get 'foo 'usage-count 0))
@end example

Here, symbol @code{foo}'s @code{usage-count} property is incremented
if it exists, or set to 1 (an incremented 0) otherwise.

When not used as a @code{setf} form, @code{cl-getf} is just a regular
function and its @var{place} argument can actually be any Lisp
expression.
@end defun

@defmac cl-remf place property
This macro removes the property-value pair for @var{property} from
the property list stored at @var{place}, which is any @code{setf}-able
place expression.  It returns true if the property was found.  Note
that if @var{property} happens to be first on the list, this will
effectively do a @code{(setf @var{place} (cddr @var{place}))},
whereas if it occurs later, this simply uses @code{setcdr} to splice
out the property and value cells.
@end defmac

@node Creating Symbols
@section Creating Symbols
@cindex gensym

@noindent
These functions create unique symbols, typically for use as
temporary variables.

@defun cl-gensym &optional x
This function creates a new, uninterned symbol (using @code{make-symbol})
with a unique name.  (The name of an uninterned symbol is relevant
only if the symbol is printed.)  By default, the name is generated
from an increasing sequence of numbers, @samp{G1000}, @samp{G1001},
@samp{G1002}, etc.  If the optional argument @var{x} is a string, that
string is used as a prefix instead of @samp{G}.  Uninterned symbols
are used in macro expansions for temporary variables, to ensure that
their names will not conflict with ``real'' variables in the user's
code.

(Internally, the variable @code{cl--gensym-counter} holds the counter
used to generate names.  It is initialized with zero and incremented
after each use.)
@end defun

@defun cl-gentemp &optional x
This function is like @code{cl-gensym}, except that it produces a new
@emph{interned} symbol.  If the symbol that is generated already
exists, the function keeps incrementing the counter and trying
again until a new symbol is generated.
@end defun

This package automatically creates all keywords that are called for by
@code{&key} argument specifiers, and discourages the use of keywords
as data unrelated to keyword arguments, so the related function
@code{defkeyword} (to create self-quoting keyword symbols) is not
provided.

@node Numbers
@chapter Numbers

@noindent
This section defines a few simple Common Lisp operations on numbers
that were left out of Emacs Lisp.

@menu
* Predicates on Numbers::       @code{cl-plusp}, @code{cl-oddp}, etc.
* Numerical Functions::         @code{cl-floor}, @code{cl-ceiling}, etc.
* Random Numbers::              @code{cl-random}, @code{cl-make-random-state}.
* Implementation Parameters::   @code{cl-most-positive-float}, etc.
@end menu

@node Predicates on Numbers
@section Predicates on Numbers

@noindent
These functions return @code{t} if the specified condition is
true of the numerical argument, or @code{nil} otherwise.

@defun cl-plusp number
This predicate tests whether @var{number} is positive.  It is an
error if the argument is not a number.
@end defun

@defun cl-minusp number
This predicate tests whether @var{number} is negative.  It is an
error if the argument is not a number.
@end defun

@defun cl-oddp integer
This predicate tests whether @var{integer} is odd.  It is an
error if the argument is not an integer.
@end defun

@defun cl-evenp integer
This predicate tests whether @var{integer} is even.  It is an
error if the argument is not an integer.
@end defun

@defun cl-digit-char-p char radix
Test if @var{char} is a digit in the specified @var{radix} (default is
10).  If it is, return the numerical value of digit @var{char} in
@var{radix}.
@end defun

@node Numerical Functions
@section Numerical Functions

@noindent
These functions perform various arithmetic operations on numbers.

@defun cl-gcd &rest integers
This function returns the Greatest Common Divisor of the arguments.
For one argument, it returns the absolute value of that argument.
For zero arguments, it returns zero.
@end defun

@defun cl-lcm &rest integers
This function returns the Least Common Multiple of the arguments.
For one argument, it returns the absolute value of that argument.
For zero arguments, it returns one.
@end defun

@defun cl-isqrt integer
This function computes the ``integer square root'' of its integer
argument, i.e., the greatest integer less than or equal to the true
square root of the argument.
@end defun

@defun cl-floor number &optional divisor
With one argument, @code{cl-floor} returns a list of two numbers:
The argument rounded down (toward minus infinity) to an integer,
and the ``remainder'' which would have to be added back to the
first return value to yield the argument again.  If the argument
is an integer @var{x}, the result is always the list @code{(@var{x} 0)}.
If the argument is a floating-point number, the first
result is a Lisp integer and the second is a Lisp float between
0 (inclusive) and 1 (exclusive).

With two arguments, @code{cl-floor} divides @var{number} by
@var{divisor}, and returns the floor of the quotient and the
corresponding remainder as a list of two numbers.  If
@code{(cl-floor @var{x} @var{y})} returns @code{(@var{q} @var{r})},
then @code{@var{q}*@var{y} + @var{r} = @var{x}}, with @var{r}
between 0 (inclusive) and @var{r} (exclusive).  Also, note
that @code{(cl-floor @var{x})} is exactly equivalent to
@code{(cl-floor @var{x} 1)}.

This function is entirely compatible with Common Lisp's @code{floor}
function, except that it returns the two results in a list since
Emacs Lisp does not support multiple-valued functions.
@end defun

@defun cl-ceiling number &optional divisor
This function implements the Common Lisp @code{ceiling} function,
which is analogous to @code{floor} except that it rounds the
argument or quotient of the arguments up toward plus infinity.
The remainder will be between 0 and minus @var{r}.
@end defun

@defun cl-truncate number &optional divisor
This function implements the Common Lisp @code{truncate} function,
which is analogous to @code{floor} except that it rounds the
argument or quotient of the arguments toward zero.  Thus it is
equivalent to @code{cl-floor} if the argument or quotient is
positive, or to @code{cl-ceiling} otherwise.  The remainder has
the same sign as @var{number}.
@end defun

@defun cl-round number &optional divisor
This function implements the Common Lisp @code{round} function,
which is analogous to @code{floor} except that it rounds the
argument or quotient of the arguments to the nearest integer.
In the case of a tie (the argument or quotient is exactly
halfway between two integers), it rounds to the even integer.
@end defun

@defun cl-mod number divisor
This function returns the same value as the second return value
of @code{cl-floor}.
@end defun

@defun cl-rem number divisor
This function returns the same value as the second return value
of @code{cl-truncate}.
@end defun

@defun cl-parse-integer string &key start end radix junk-allowed
This function implements the Common Lisp @code{parse-integer}
function.  It parses an integer in the specified @var{radix} from the
substring of @var{string} between @var{start} and @var{end}.  Any
leading and trailing whitespace chars are ignored.  The function
signals an error if the substring between @var{start} and @var{end}
cannot be parsed as an integer, unless @var{junk-allowed} is
non-@code{nil}.
@end defun

@node Random Numbers
@section Random Numbers

@noindent
This package also provides an implementation of the Common Lisp
random number generator.  It uses its own additive-congruential
algorithm, which is much more likely to give statistically clean
@c FIXME?  Still true?
random numbers than the simple generators supplied by many
operating systems.

@defun cl-random number &optional state
This function returns a random nonnegative number less than
@var{number}, and of the same type (either integer or floating-point).
The @var{state} argument should be a @code{random-state} object
that holds the state of the random number generator.  The
function modifies this state object as a side effect.  If
@var{state} is omitted, it defaults to the internal variable
@code{cl--random-state}, which contains a pre-initialized
default @code{random-state} object.  (Since any number of programs in
the Emacs process may be accessing @code{cl--random-state} in
interleaved fashion, the sequence generated from this will be
irreproducible for all intents and purposes.)
@end defun

@defun cl-make-random-state &optional state
This function creates or copies a @code{random-state} object.
If @var{state} is omitted or @code{nil}, it returns a new copy of
@code{cl--random-state}.  This is a copy in the sense that future
sequences of calls to @code{(cl-random @var{n})} and
@code{(cl-random @var{n} @var{s})} (where @var{s} is the new
random-state object) will return identical sequences of random
numbers.

If @var{state} is a @code{random-state} object, this function
returns a copy of that object.  If @var{state} is @code{t}, this
function returns a new @code{random-state} object seeded from the
date and time.  As an extension to Common Lisp, @var{state} may also
be an integer in which case the new object is seeded from that
integer; each different integer seed will result in a completely
different sequence of random numbers.

It is valid to print a @code{random-state} object to a buffer or
file and later read it back with @code{read}.  If a program wishes
to use a sequence of pseudo-random numbers which can be reproduced
later for debugging, it can call @code{(cl-make-random-state t)} to
get a new sequence, then print this sequence to a file.  When the
program is later rerun, it can read the original run's random-state
from the file.
@end defun

@defun cl-random-state-p object
This predicate returns @code{t} if @var{object} is a
@code{random-state} object, or @code{nil} otherwise.
@end defun

@node Implementation Parameters
@section Implementation Parameters

@noindent
This package defines several useful constants having to do with
floating-point numbers.

It determines their values by exercising the computer's
floating-point arithmetic in various ways.  Because this operation
might be slow, the code for initializing them is kept in a separate
function that must be called before the parameters can be used.

@defun cl-float-limits
This function makes sure that the Common Lisp floating-point parameters
like @code{cl-most-positive-float} have been initialized.  Until it is
called, these parameters have unspecified values.
If the parameters have already been initialized, the function returns
immediately.
@end defun

Since true Common Lisp supports up to four different kinds of floating-point
numbers, it has families of constants like
@code{most-positive-single-float}, @code{most-positive-double-float},
@code{most-positive-long-float}, and so on.  This package uses just
one set of constants because Emacs has only one kind of
floating-point number, namely the IEEE binary64 floating-point format.
@xref{Float Basics,,,elisp,GNU Emacs Lisp Reference Manual}.

@defvar cl-most-positive-float
This constant equals the largest finite value a Lisp float can hold.
For IEEE binary64 format, this equals @code{(- (expt 2 1024) (expt 2
971))}, which equals @code{1.7976931348623157e+308}.
@end defvar

@defvar cl-most-negative-float
This constant equals the most negative finite value a Lisp float can hold.
For IEEE binary64 format, this equals @code{(- cl-most-positive-float)}.
@end defvar

@defvar cl-least-positive-normalized-float
This constant equals the smallest positive Lisp float that is
@dfn{normalized}, i.e., that has full precision.
For IEEE binary64 format, this equals @code{(expt 2 -1022)},
which equals @code{2.2250738585072014e-308}.
@end defvar

@defvar cl-least-positive-float
This constant equals the smallest Lisp float value greater than zero.
For IEEE binary64 format, this equals @code{5e-324} (which equals
@code{(expt 2 -1074)}) if subnormal numbers are supported, and
@code{cl-least-positive-normalized-float} otherwise.
@end defvar

@defvar cl-least-negative-float
This constant is the negative counterpart of @code{cl-least-positive-float}.
@end defvar

@defvar cl-least-negative-normalized-float
This constant is the negative counterpart of
@code{cl-least-positive-normalized-float}.
@end defvar

@defvar cl-float-epsilon
This constant is the smallest positive Lisp float that can be added
to 1.0 to produce a distinct value.  Adding a smaller number to 1.0
will yield 1.0 again due to roundoff.  For IEEE binary64 format, this
equals @code{(expt 2 -52)}, which equals @code{2.220446049250313e-16}.
@end defvar

@defvar cl-float-negative-epsilon
This is the smallest positive value that can be subtracted from
1.0 to produce a distinct value.  For IEEE binary64 format, this
equals @code{(expt 2 -53)}, which equals @code{1.1102230246251565e-16}.
@end defvar

@node Sequences
@chapter Sequences

@noindent
Common Lisp defines a number of functions that operate on
@dfn{sequences}, which are either lists, strings, or vectors.
Emacs Lisp includes a few of these, notably @code{elt} and
@code{length}; this package defines most of the rest.

@menu
* Sequence Basics::          Arguments shared by all sequence functions.
* Mapping over Sequences::   @code{cl-mapcar}, @code{cl-map}, @code{cl-maplist}, etc.
* Sequence Functions::       @code{cl-subseq}, @code{cl-remove}, @code{cl-substitute}, etc.
* Searching Sequences::      @code{cl-find}, @code{cl-count}, @code{cl-search}, etc.
* Sorting Sequences::        @code{cl-sort}, @code{cl-stable-sort}, @code{cl-merge}.
@end menu

@node Sequence Basics
@section Sequence Basics

@noindent
Many of the sequence functions take keyword arguments; @pxref{Argument
Lists}.  All keyword arguments are optional and, if specified,
may appear in any order.

The @code{:key} argument should be passed either @code{nil}, or a
function of one argument.  This key function is used as a filter
through which the elements of the sequence are seen; for example,
@code{(cl-find x y :key 'car)} is similar to @code{(cl-assoc x y)}.
It searches for an element of the list whose @sc{car} equals
@code{x}, rather than for an element which equals @code{x} itself.
If @code{:key} is omitted or @code{nil}, the filter is effectively
the identity function.

The @code{:test} and @code{:test-not} arguments should be either
@code{nil}, or functions of two arguments.  The test function is
used to compare two sequence elements, or to compare a search value
with sequence elements.  (The two values are passed to the test
function in the same order as the original sequence function
arguments from which they are derived, or, if they both come from
the same sequence, in the same order as they appear in that sequence.)
The @code{:test} argument specifies a function which must return
true (non-@code{nil}) to indicate a match; instead, you may use
@code{:test-not} to give a function which returns @emph{false} to
indicate a match.  The default test function is @code{eql}.

Many functions that take @var{item} and @code{:test} or @code{:test-not}
arguments also come in @code{-if} and @code{-if-not} varieties,
where a @var{predicate} function is passed instead of @var{item},
and sequence elements match if the predicate returns true on them
(or false in the case of @code{-if-not}).  For example:

@example
(cl-remove 0 seq :test '=)  @equiv{}  (cl-remove-if 'zerop seq)
@end example

@noindent
to remove all zeros from sequence @code{seq}.

Some operations can work on a subsequence of the argument sequence;
these function take @code{:start} and @code{:end} arguments, which
default to zero and the length of the sequence, respectively.
Only elements between @var{start} (inclusive) and @var{end}
(exclusive) are affected by the operation.  The @var{end} argument
may be passed @code{nil} to signify the length of the sequence;
otherwise, both @var{start} and @var{end} must be integers, with
@code{0 <= @var{start} <= @var{end} <= (length @var{seq})}.
If the function takes two sequence arguments, the limits are
defined by keywords @code{:start1} and @code{:end1} for the first,
and @code{:start2} and @code{:end2} for the second.

A few functions accept a @code{:from-end} argument, which, if
non-@code{nil}, causes the operation to go from right-to-left
through the sequence instead of left-to-right, and a @code{:count}
argument, which specifies an integer maximum number of elements
to be removed or otherwise processed.

The sequence functions make no guarantees about the order in
which the @code{:test}, @code{:test-not}, and @code{:key} functions
are called on various elements.  Therefore, it is a bad idea to depend
on side effects of these functions.  For example, @code{:from-end}
may cause the sequence to be scanned actually in reverse, or it may
be scanned forwards but computing a result ``as if'' it were scanned
backwards.  (Some functions, like @code{cl-mapcar} and @code{cl-every},
@emph{do} specify exactly the order in which the function is called
so side effects are perfectly acceptable in those cases.)

Strings may contain ``text properties'' as well
as character data.  Except as noted, it is undefined whether or
not text properties are preserved by sequence functions.  For
example, @code{(cl-remove ?A @var{str})} may or may not preserve
the properties of the characters copied from @var{str} into the
result.

@node Mapping over Sequences
@section Mapping over Sequences

@noindent
These functions ``map'' the function you specify over the elements
of lists or arrays.  They are all variations on the theme of the
built-in function @code{mapcar}.

@defun cl-mapcar function seq &rest more-seqs
This function calls @var{function} on successive parallel sets of
elements from its argument sequences.  Given a single @var{seq}
argument it is equivalent to @code{mapcar}; given @var{n} sequences,
it calls the function with the first elements of each of the sequences
as the @var{n} arguments to yield the first element of the result
list, then with the second elements, and so on.  The mapping stops as
soon as the shortest sequence runs out.  The argument sequences may
be any mixture of lists, strings, and vectors; the return sequence
is always a list.

Common Lisp's @code{mapcar} accepts multiple arguments but works
only on lists; Emacs Lisp's @code{mapcar} accepts a single sequence
argument.  This package's @code{cl-mapcar} works as a compatible
superset of both.
@end defun

@defun cl-map result-type function seq &rest more-seqs
This function maps @var{function} over the argument sequences,
just like @code{cl-mapcar}, but it returns a sequence of type
@var{result-type} rather than a list.  @var{result-type} must
be one of the following symbols: @code{vector}, @code{string},
@code{list} (in which case the effect is the same as for
@code{cl-mapcar}), or @code{nil} (in which case the results are
thrown away and @code{cl-map} returns @code{nil}).
@end defun

@defun cl-maplist function list &rest more-lists
This function calls @var{function} on each of its argument lists,
then on the @sc{cdr}s of those lists, and so on, until the
shortest list runs out.  The results are returned in the form
of a list.  Thus, @code{cl-maplist} is like @code{cl-mapcar} except
that it passes in the list pointers themselves rather than the
@sc{car}s of the advancing pointers.
@end defun

@defun cl-mapc function seq &rest more-seqs
This function is like @code{cl-mapcar}, except that the values returned
by @var{function} are ignored and thrown away rather than being
collected into a list.  The return value of @code{cl-mapc} is @var{seq},
the first sequence.  This function is more general than the Emacs
primitive @code{mapc}.  (Note that this function is called
@code{cl-mapc} even in @file{cl.el}, rather than @code{mapc*} as you
might expect.)
@c https://debbugs.gnu.org/6575
@end defun

@defun cl-mapl function list &rest more-lists
This function is like @code{cl-maplist}, except that it throws away
the values returned by @var{function}.
@end defun

@defun cl-mapcan function seq &rest more-seqs
This function is like @code{cl-mapcar}, except that it concatenates
the return values (which must be lists) using @code{nconc},
rather than simply collecting them into a list.
@end defun

@defun cl-mapcon function list &rest more-lists
This function is like @code{cl-maplist}, except that it concatenates
the return values using @code{nconc}.
@end defun

@defun cl-some predicate seq &rest more-seqs
This function calls @var{predicate} on each element of @var{seq}
in turn; if @var{predicate} returns a non-@code{nil} value,
@code{cl-some} returns that value, otherwise it returns @code{nil}.
Given several sequence arguments, it steps through the sequences
in parallel until the shortest one runs out, just as in
@code{cl-mapcar}.  You can rely on the left-to-right order in which
the elements are visited, and on the fact that mapping stops
immediately as soon as @var{predicate} returns non-@code{nil}.
@end defun

@defun cl-every predicate seq &rest more-seqs
This function calls @var{predicate} on each element of the sequence(s)
in turn; it returns @code{nil} as soon as @var{predicate} returns
@code{nil} for any element, or @code{t} if the predicate was true
for all elements.
@end defun

@defun cl-notany predicate seq &rest more-seqs
This function calls @var{predicate} on each element of the sequence(s)
in turn; it returns @code{nil} as soon as @var{predicate} returns
a non-@code{nil} value for any element, or @code{t} if the predicate
was @code{nil} for all elements.
@end defun

@defun cl-notevery predicate seq &rest more-seqs
This function calls @var{predicate} on each element of the sequence(s)
in turn; it returns a non-@code{nil} value as soon as @var{predicate}
returns @code{nil} for any element, or @code{nil} if the predicate was
true for all elements.
@end defun

@defun cl-reduce function seq @t{&key :from-end :start :end :initial-value :key}
This function returns the result of calling @var{function} on the
first and second elements of @var{seq}, then calling @var{function}
with that result and the third element of @var{seq}, then with that
result and the fourth element of @var{seq}, etc.

Here is an example.  Suppose @var{function} is @code{*} and @var{seq}
is the list @code{(2 3 4 5)}.  The first two elements of the list are
combined with @code{(* 2 3) = 6}; this is combined with the next
element, @code{(* 6 4) = 24}, and that is combined with the final
element: @code{(* 24 5) = 120}.  Note that the @code{*} function happens
to be self-reducing, so that @code{(* 2 3 4 5)} has the same effect as
an explicit call to @code{cl-reduce}.

If @code{:from-end} is true, the reduction is right-associative instead
of left-associative:

@example
(cl-reduce '- '(1 2 3 4))
        @equiv{} (- (- (- 1 2) 3) 4) @result{} -8
(cl-reduce '- '(1 2 3 4) :from-end t)
        @equiv{} (- 1 (- 2 (- 3 4))) @result{} -2
@end example

If @code{:key} is specified, it is a function of one argument, which
is called on each of the sequence elements in turn.

If @code{:initial-value} is specified, it is effectively added to the
front (or rear in the case of @code{:from-end}) of the sequence.
The @code{:key} function is @emph{not} applied to the initial value.

If the sequence, including the initial value, has exactly one element
then that element is returned without ever calling @var{function}.
If the sequence is empty (and there is no initial value), then
@var{function} is called with no arguments to obtain the return value.
@end defun

All of these mapping operations can be expressed conveniently in
terms of the @code{cl-loop} macro.  In compiled code, @code{cl-loop} will
be faster since it generates the loop as in-line code with no
function calls.

@node Sequence Functions
@section Sequence Functions

@noindent
This section describes a number of Common Lisp functions for
operating on sequences.

@defun cl-subseq sequence start &optional end
This function returns a given subsequence of the argument
@var{sequence}, which may be a list, string, or vector.
The indices @var{start} and @var{end} must be in range, and
@var{start} must be no greater than @var{end}.  If @var{end}
is omitted, it defaults to the length of the sequence.  The
return value is always a copy; it does not share structure
with @var{sequence}.

As an extension to Common Lisp, @var{start} and/or @var{end}
may be negative, in which case they represent a distance back
from the end of the sequence.  This is for compatibility with
Emacs's @code{substring} function.  Note that @code{cl-subseq} is
the @emph{only} sequence function that allows negative
@var{start} and @var{end}.

You can use @code{setf} on a @code{cl-subseq} form to replace a
specified range of elements with elements from another sequence.
The replacement is done as if by @code{cl-replace}, described below.
@end defun

@defun cl-concatenate result-type &rest seqs
This function concatenates the argument sequences together to
form a result sequence of type @var{result-type}, one of the
symbols @code{vector}, @code{string}, or @code{list}.  The
arguments are always copied, even in cases such as
@code{(cl-concatenate 'list '(1 2 3))} where the result is
identical to an argument.
@end defun

@defun cl-fill seq item @t{&key :start :end}
This function fills the elements of the sequence (or the specified
part of the sequence) with the value @var{item}.
@end defun

@defun cl-replace seq1 seq2 @t{&key :start1 :end1 :start2 :end2}
This function copies part of @var{seq2} into part of @var{seq1}.
The sequence @var{seq1} is not stretched or resized; the amount
of data copied is simply the shorter of the source and destination
(sub)sequences.  The function returns @var{seq1}.

If @var{seq1} and @var{seq2} are @code{eq}, then the replacement
will work correctly even if the regions indicated by the start
and end arguments overlap.  However, if @var{seq1} and @var{seq2}
are lists that share storage but are not @code{eq}, and the
start and end arguments specify overlapping regions, the effect
is undefined.
@end defun

@defun cl-remove item seq @t{&key :test :test-not :key :count :start :end :from-end}
This returns a copy of @var{seq} with all elements matching
@var{item} removed.  The result may share storage with or be
@code{eq} to @var{seq} in some circumstances, but the original
@var{seq} will not be modified.  The @code{:test}, @code{:test-not},
and @code{:key} arguments define the matching test that is used;
by default, elements @code{eql} to @var{item} are removed.  The
@code{:count} argument specifies the maximum number of matching
elements that can be removed (only the leftmost @var{count} matches
are removed).  The @code{:start} and @code{:end} arguments specify
a region in @var{seq} in which elements will be removed; elements
outside that region are not matched or removed.  The @code{:from-end}
argument, if true, says that elements should be deleted from the
end of the sequence rather than the beginning (this matters only
if @var{count} was also specified).
@end defun

@defun cl-delete item seq @t{&key :test :test-not :key :count :start :end :from-end}
This deletes all elements of @var{seq} that match @var{item}.
It is a destructive operation.  Since Emacs Lisp does not support
stretchable strings or vectors, this is the same as @code{cl-remove}
for those sequence types.  On lists, @code{cl-remove} will copy the
list if necessary to preserve the original list, whereas
@code{cl-delete} will splice out parts of the argument list.
Compare @code{append} and @code{nconc}, which are analogous
non-destructive and destructive list operations in Emacs Lisp.
@end defun

@findex cl-remove-if
@findex cl-remove-if-not
@findex cl-delete-if
@findex cl-delete-if-not
The predicate-oriented functions @code{cl-remove-if}, @code{cl-remove-if-not},
@code{cl-delete-if}, and @code{cl-delete-if-not} are defined similarly.

@defun cl-remove-duplicates seq @t{&key :test :test-not :key :start :end :from-end}
This function returns a copy of @var{seq} with duplicate elements
removed.  Specifically, if two elements from the sequence match
according to the @code{:test}, @code{:test-not}, and @code{:key}
arguments, only the rightmost one is retained.  If @code{:from-end}
is true, the leftmost one is retained instead.  If @code{:start} or
@code{:end} is specified, only elements within that subsequence are
examined or removed.
@end defun

@defun cl-delete-duplicates seq @t{&key :test :test-not :key :start :end :from-end}
This function deletes duplicate elements from @var{seq}.  It is
a destructive version of @code{cl-remove-duplicates}.
@end defun

@defun cl-substitute new old seq @t{&key :test :test-not :key :count :start :end :from-end}
This function returns a copy of @var{seq}, with all elements
matching @var{old} replaced with @var{new}.  The @code{:count},
@code{:start}, @code{:end}, and @code{:from-end} arguments may be
used to limit the number of substitutions made.
@end defun

@defun cl-nsubstitute new old seq @t{&key :test :test-not :key :count :start :end :from-end}
This is a destructive version of @code{cl-substitute}; it performs
the substitution using @code{setcar} or @code{aset} rather than
by returning a changed copy of the sequence.
@end defun

@findex cl-substitute-if
@findex cl-substitute-if-not
@findex cl-nsubstitute-if
@findex cl-nsubstitute-if-not
The functions @code{cl-substitute-if}, @code{cl-substitute-if-not},
@code{cl-nsubstitute-if}, and @code{cl-nsubstitute-if-not} are defined
similarly.  For these, a @var{predicate} is given in place of the
@var{old} argument.

@node Searching Sequences
@section Searching Sequences

@noindent
These functions search for elements or subsequences in a sequence.
(See also @code{cl-member} and @code{cl-assoc}; @pxref{Lists}.)

@defun cl-find item seq @t{&key :test :test-not :key :start :end :from-end}
This function searches @var{seq} for an element matching @var{item}.
If it finds a match, it returns the matching element.  Otherwise,
it returns @code{nil}.  It returns the leftmost match, unless
@code{:from-end} is true, in which case it returns the rightmost
match.  The @code{:start} and @code{:end} arguments may be used to
limit the range of elements that are searched.
@end defun

@defun cl-position item seq @t{&key :test :test-not :key :start :end :from-end}
This function is like @code{cl-find}, except that it returns the
integer position in the sequence of the matching item rather than
the item itself.  The position is relative to the start of the
sequence as a whole, even if @code{:start} is non-zero.  The function
returns @code{nil} if no matching element was found.
@end defun

@defun cl-count item seq @t{&key :test :test-not :key :start :end}
This function returns the number of elements of @var{seq} which
match @var{item}.  The result is always a nonnegative integer.
@end defun

@findex cl-find-if
@findex cl-find-if-not
@findex cl-position-if
@findex cl-position-if-not
@findex cl-count-if
@findex cl-count-if-not
The @code{cl-find-if}, @code{cl-find-if-not}, @code{cl-position-if},
@code{cl-position-if-not}, @code{cl-count-if}, and @code{cl-count-if-not}
functions are defined similarly.

@defun cl-mismatch seq1 seq2 @t{&key :test :test-not :key :start1 :end1 :start2 :end2 :from-end}
This function compares the specified parts of @var{seq1} and
@var{seq2}.  If they are the same length and the corresponding
elements match (according to @code{:test}, @code{:test-not},
and @code{:key}), the function returns @code{nil}.  If there is
a mismatch, the function returns the index (relative to @var{seq1})
of the first mismatching element.  This will be the leftmost pair of
elements that do not match, or the position at which the shorter of
the two otherwise-matching sequences runs out.

If @code{:from-end} is true, then the elements are compared from right
to left starting at @code{(1- @var{end1})} and @code{(1- @var{end2})}.
If the sequences differ, then one plus the index of the rightmost
difference (relative to @var{seq1}) is returned.

An interesting example is @code{(cl-mismatch str1 str2 :key 'upcase)},
which compares two strings case-insensitively.
@end defun

@defun cl-search seq1 seq2 @t{&key :test :test-not :key :from-end :start1 :end1 :start2 :end2}
This function searches @var{seq2} for a subsequence that matches
@var{seq1} (or part of it specified by @code{:start1} and
@code{:end1}).  Only matches that fall entirely within the region
defined by @code{:start2} and @code{:end2} will be considered.
The return value is the index of the leftmost element of the
leftmost match, relative to the start of @var{seq2}, or @code{nil}
if no matches were found.  If @code{:from-end} is true, the
function finds the @emph{rightmost} matching subsequence.
@end defun

@node Sorting Sequences
@section Sorting Sequences

@defun cl-sort seq predicate @t{&key :key}
This function sorts @var{seq} into increasing order as determined
by using @var{predicate} to compare pairs of elements.  @var{predicate}
should return true (non-@code{nil}) if and only if its first argument
is less than (not equal to) its second argument.  For example,
@code{<} and @code{string-lessp} are suitable predicate functions
for sorting numbers and strings, respectively; @code{>} would sort
numbers into decreasing rather than increasing order.

This function differs from Emacs's built-in @code{sort} in that it
can operate on any type of sequence, not just lists.  Also, it
accepts a @code{:key} argument, which is used to preprocess data
fed to the @var{predicate} function.  For example,

@example
(setq data (cl-sort data 'string-lessp :key 'downcase))
@end example

@noindent
sorts @var{data}, a sequence of strings, into increasing alphabetical
order without regard to case.  A @code{:key} function of @code{car}
would be useful for sorting association lists.  It should only be a
simple accessor though, since it's used heavily in the current
implementation.

The @code{cl-sort} function is destructive; it sorts lists by actually
rearranging the @sc{cdr} pointers in suitable fashion.
@end defun

@defun cl-stable-sort seq predicate @t{&key :key}
This function sorts @var{seq} @dfn{stably}, meaning two elements
which are equal in terms of @var{predicate} are guaranteed not to
be rearranged out of their original order by the sort.

In practice, @code{cl-sort} and @code{cl-stable-sort} are equivalent
in Emacs Lisp because the underlying @code{sort} function is
stable by default.  However, this package reserves the right to
use non-stable methods for @code{cl-sort} in the future.
@end defun

@defun cl-merge type seq1 seq2 predicate @t{&key :key}
This function merges two sequences @var{seq1} and @var{seq2} by
interleaving their elements.  The result sequence, of type @var{type}
(in the sense of @code{cl-concatenate}), has length equal to the sum
of the lengths of the two input sequences.  The sequences may be
modified destructively.  Order of elements within @var{seq1} and
@var{seq2} is preserved in the interleaving; elements of the two
sequences are compared by @var{predicate} (in the sense of
@code{sort}) and the lesser element goes first in the result.
When elements are equal, those from @var{seq1} precede those from
@var{seq2} in the result.  Thus, if @var{seq1} and @var{seq2} are
both sorted according to @var{predicate}, then the result will be
a merged sequence which is (stably) sorted according to
@var{predicate}.
@end defun

@node Lists
@chapter Lists

@noindent
The functions described here operate on lists.

@menu
* List Functions::                @code{cl-first}, @code{cl-list*}, etc.
* Substitution of Expressions::   @code{cl-subst}, @code{cl-sublis}, etc.
* Lists as Sets::                 @code{cl-member}, @code{cl-adjoin}, @code{cl-union}, etc.
* Association Lists::             @code{cl-assoc}, @code{cl-acons}, @code{cl-pairlis}, etc.
@end menu

@node List Functions
@section List Functions

@noindent
This section describes a number of simple operations on lists,
i.e., chains of cons cells.

@defun cl-first x
This function is a synonym for @code{(car @var{x})}.  Likewise,
the functions @code{cl-second}, @code{cl-third}, @dots{}, through
@code{cl-tenth} return the given element of the list @var{x}.
@end defun

@defun cl-rest x
This function is a synonym for @code{(cdr @var{x})}.
@end defun

@defun cl-endp x
This function acts like @code{null}, but signals an error if @code{x}
is neither a @code{nil} nor a cons cell.
@end defun

@defun cl-list-length x
This function returns the length of list @var{x}, exactly like
@code{(length @var{x})}, except that if @var{x} is a circular
list (where the @sc{cdr}-chain forms a loop rather than terminating
with @code{nil}), this function returns @code{nil}.  (The regular
@code{length} function would get stuck if given a circular list.
See also the @code{safe-length} function.)
@end defun

@defun cl-list* arg &rest others
This function constructs a list of its arguments.  The final
argument becomes the @sc{cdr} of the last cell constructed.
Thus, @code{(cl-list* @var{a} @var{b} @var{c})} is equivalent to
@code{(cons @var{a} (cons @var{b} @var{c}))}, and
@code{(cl-list* @var{a} @var{b} nil)} is equivalent to
@code{(list @var{a} @var{b})}.
@end defun

@defun cl-ldiff list sublist
If @var{sublist} is a sublist of @var{list}, i.e., is @code{eq} to
one of the cons cells of @var{list}, then this function returns
a copy of the part of @var{list} up to but not including
@var{sublist}.  For example, @code{(cl-ldiff x (cddr x))} returns
the first two elements of the list @code{x}.  The result is a
copy; the original @var{list} is not modified.  If @var{sublist}
is not a sublist of @var{list}, a copy of the entire @var{list}
is returned.
@end defun

@defun cl-copy-list list
This function returns a copy of the list @var{list}.  It copies
dotted lists like @code{(1 2 . 3)} correctly.
@end defun

@defun cl-tree-equal x y @t{&key :test :test-not :key}
This function compares two trees of cons cells.  If @var{x} and
@var{y} are both cons cells, their @sc{car}s and @sc{cdr}s are
compared recursively.  If neither @var{x} nor @var{y} is a cons
cell, they are compared by @code{eql}, or according to the
specified test.  The @code{:key} function, if specified, is
applied to the elements of both trees.  @xref{Sequences}.
@end defun

@node Substitution of Expressions
@section Substitution of Expressions

@noindent
These functions substitute elements throughout a tree of cons
cells.  (@xref{Sequence Functions}, for the @code{cl-substitute}
function, which works on just the top-level elements of a list.)

@defun cl-subst new old tree @t{&key :test :test-not :key}
This function substitutes occurrences of @var{old} with @var{new}
in @var{tree}, a tree of cons cells.  It returns a substituted
tree, which will be a copy except that it may share storage with
the argument @var{tree} in parts where no substitutions occurred.
The original @var{tree} is not modified.  This function recurses
on, and compares against @var{old}, both @sc{car}s and @sc{cdr}s
of the component cons cells.  If @var{old} is itself a cons cell,
then matching cells in the tree are substituted as usual without
recursively substituting in that cell.  Comparisons with @var{old}
are done according to the specified test (@code{eql} by default).
The @code{:key} function is applied to the elements of the tree
but not to @var{old}.
@end defun

@defun cl-nsubst new old tree @t{&key :test :test-not :key}
This function is like @code{cl-subst}, except that it works by
destructive modification (by @code{setcar} or @code{setcdr})
rather than copying.
@end defun

@findex cl-subst-if
@findex cl-subst-if-not
@findex cl-nsubst-if
@findex cl-nsubst-if-not
The @code{cl-subst-if}, @code{cl-subst-if-not}, @code{cl-nsubst-if}, and
@code{cl-nsubst-if-not} functions are defined similarly.

@defun cl-sublis alist tree @t{&key :test :test-not :key}
This function is like @code{cl-subst}, except that it takes an
association list @var{alist} of @var{old}-@var{new} pairs.
Each element of the tree (after applying the @code{:key}
function, if any), is compared with the @sc{car}s of
@var{alist}; if it matches, it is replaced by the corresponding
@sc{cdr}.
@end defun

@defun cl-nsublis alist tree @t{&key :test :test-not :key}
This is a destructive version of @code{cl-sublis}.
@end defun

@node Lists as Sets
@section Lists as Sets

@noindent
These functions perform operations on lists that represent sets of
elements.  All these functions (unless otherwise specified) default to
using @code{eql} as the test function, but that can be modified by the
@code{:test} parameter.

@defun cl-member item list @t{&key :test :test-not :key}
This function searches @var{list} for an element matching @var{item}.
If a match is found, it returns the cons cell whose @sc{car} was
the matching element.  Otherwise, it returns @code{nil}.  Elements
are compared by @code{eql} by default; you can use the @code{:test},
@code{:test-not}, and @code{:key} arguments to modify this behavior.
@xref{Sequences}.

The standard Emacs lisp function @code{member} uses @code{equal} for
comparisons; it is equivalent to @code{(cl-member @var{item} @var{list}
:test 'equal)}.
@end defun

@findex cl-member-if
@findex cl-member-if-not
The @code{cl-member-if} and @code{cl-member-if-not} functions
analogously search for elements that satisfy a given predicate.

@defun cl-tailp sublist list
This function returns @code{t} if @var{sublist} is a sublist of
@var{list}, i.e., if @var{sublist} is @code{eql} to @var{list} or to
any of its @sc{cdr}s.
@end defun

@defun cl-adjoin item list @t{&key :test :test-not :key}
This function conses @var{item} onto the front of @var{list},
like @code{(cons @var{item} @var{list})}, but only if @var{item}
is not already present on the list (as determined by @code{cl-member}).
If a @code{:key} argument is specified, it is applied to
@var{item} as well as to the elements of @var{list} during
the search, on the reasoning that @var{item} is ``about'' to
become part of the list.
@end defun

@defun cl-union list1 list2 @t{&key :test :test-not :key}
This function combines two lists that represent sets of items,
returning a list that represents the union of those two sets.
The resulting list contains all items that appear in @var{list1}
or @var{list2}, and no others.  If an item appears in both
@var{list1} and @var{list2} it is copied only once.  If
an item is duplicated in @var{list1} or @var{list2}, it is
undefined whether or not that duplication will survive in the
result list.  The order of elements in the result list is also
undefined.
@end defun

@defun cl-nunion list1 list2 @t{&key :test :test-not :key}
This is a destructive version of @code{cl-union}; rather than copying,
it tries to reuse the storage of the argument lists if possible.
@end defun

@defun cl-intersection list1 list2 @t{&key :test :test-not :key}
This function computes the intersection of the sets represented
by @var{list1} and @var{list2}.  It returns the list of items
that appear in both @var{list1} and @var{list2}.
@end defun

@defun cl-nintersection list1 list2 @t{&key :test :test-not :key}
This is a destructive version of @code{cl-intersection}.  It
tries to reuse storage of @var{list1} rather than copying.
It does @emph{not} reuse the storage of @var{list2}.
@end defun

@defun cl-set-difference list1 list2 @t{&key :test :test-not :key}
This function computes the ``set difference'' of @var{list1}
and @var{list2}, i.e., the set of elements that appear in
@var{list1} but @emph{not} in @var{list2}.
@end defun

@defun cl-nset-difference list1 list2 @t{&key :test :test-not :key}
This is a destructive @code{cl-set-difference}, which will try
to reuse @var{list1} if possible.
@end defun

@defun cl-set-exclusive-or list1 list2 @t{&key :test :test-not :key}
This function computes the ``set exclusive or'' of @var{list1}
and @var{list2}, i.e., the set of elements that appear in
exactly one of @var{list1} and @var{list2}.
@end defun

@defun cl-nset-exclusive-or list1 list2 @t{&key :test :test-not :key}
This is a destructive @code{cl-set-exclusive-or}, which will try
to reuse @var{list1} and @var{list2} if possible.
@end defun

@defun cl-subsetp list1 list2 @t{&key :test :test-not :key}
This function checks whether @var{list1} represents a subset
of @var{list2}, i.e., whether every element of @var{list1}
also appears in @var{list2}.
@end defun

@node Association Lists
@section Association Lists

@noindent
An @dfn{association list} is a list representing a mapping from
one set of values to another; any list whose elements are cons
cells is an association list.

@defun cl-assoc item a-list @t{&key :test :test-not :key}
This function searches the association list @var{a-list} for an
element whose @sc{car} matches (in the sense of @code{:test},
@code{:test-not}, and @code{:key}, or by comparison with @code{eql})
a given @var{item}.  It returns the matching element, if any,
otherwise @code{nil}.  It ignores elements of @var{a-list} that
are not cons cells.  (This corresponds to the behavior of
@code{assq} and @code{assoc} in Emacs Lisp; Common Lisp's
@code{assoc} ignores @code{nil}s but considers any other non-cons
elements of @var{a-list} to be an error.)
@end defun

@defun cl-rassoc item a-list @t{&key :test :test-not :key}
This function searches for an element whose @sc{cdr} matches
@var{item}.  If @var{a-list} represents a mapping, this applies
the inverse of the mapping to @var{item}.
@end defun

@findex cl-assoc-if
@findex cl-assoc-if-not
@findex cl-rassoc-if
@findex cl-rassoc-if-not
The @code{cl-assoc-if}, @code{cl-assoc-if-not}, @code{cl-rassoc-if},
and @code{cl-rassoc-if-not} functions are defined similarly.

Two simple functions for constructing association lists are:

@defun cl-acons key value alist
This is equivalent to @code{(cons (cons @var{key} @var{value}) @var{alist})}.
@end defun

@defun cl-pairlis keys values &optional alist
This is equivalent to @code{(nconc (cl-mapcar 'cons @var{keys} @var{values})
@var{alist})}.
@end defun

@node Structures
@chapter Structures

@noindent
The Common Lisp @dfn{structure} mechanism provides a general way
to define data types similar to C's @code{struct} types.  A
structure is a Lisp object containing some number of @dfn{slots},
each of which can hold any Lisp data object.  Functions are
provided for accessing and setting the slots, creating or copying
structure objects, and recognizing objects of a particular structure
type.

In true Common Lisp, each structure type is a new type distinct
from all existing Lisp types.  Since the underlying Emacs Lisp
system provides no way to create new distinct types, this package
implements structures as vectors (or lists upon request) with a
special ``tag'' symbol to identify them.

@defmac cl-defstruct name slots@dots{}
The @code{cl-defstruct} form defines a new structure type called
@var{name}, with the specified @var{slots}.  (The @var{slots}
may begin with a string which documents the structure type.)
In the simplest case, @var{name} and each of the @var{slots}
are symbols.  For example,

@example
(cl-defstruct person first-name age sex)
@end example

@noindent
defines a struct type called @code{person} that contains three slots.
Given a @code{person} object @var{p}, you can access those slots by
calling @code{(person-first-name @var{p})}, @code{(person-age
@var{p})}, and @code{(person-sex @var{p})}.  You can also change these
slots by using @code{setf} on any of these place forms, for example:

@example
(cl-incf (person-age birthday-boy))
@end example

You can create a new @code{person} by calling @code{make-person},
which takes keyword arguments @code{:first-name}, @code{:age}, and
@code{:sex} to specify the initial values of these slots in the
new object.  (Omitting any of these arguments leaves the corresponding
slot ``undefined'', according to the Common Lisp standard; in Emacs
Lisp, such uninitialized slots are filled with @code{nil}.)

Given a @code{person}, @code{(copy-person @var{p})} makes a new
object of the same type whose slots are @code{eq} to those of @var{p}.

Given any Lisp object @var{x}, @code{(person-p @var{x})} returns
true if @var{x} is a @code{person}, and false otherwise.

Accessors like @code{person-first-name} normally check their arguments
(effectively using @code{person-p}) and signal an error if the
argument is the wrong type.  This check is affected by
@code{(optimize (safety @dots{}))} declarations.  Safety level 1,
the default, uses a somewhat optimized check that will detect all
incorrect arguments, but may use an uninformative error message
(e.g., ``expected a vector'' instead of ``expected a @code{person}'').
Safety level 0 omits all checks except as provided by the underlying
@code{aref} call; safety levels 2 and 3 do rigorous checking that will
always print a descriptive error message for incorrect inputs.
@xref{Declarations}.

@example
(setq dave (make-person :first-name "Dave" :sex 'male))
     @result{} [cl-struct-person "Dave" nil male]
(setq other (copy-person dave))
     @result{} [cl-struct-person "Dave" nil male]
(eq dave other)
     @result{} nil
(eq (person-first-name dave) (person-first-name other))
     @result{} t
(person-p dave)
     @result{} t
(person-p [1 2 3 4])
     @result{} nil
(person-p "Bogus")
     @result{} nil
(person-p '[cl-struct-person counterfeit person object])
     @result{} t
@end example

In general, @var{name} is either a name symbol or a list of a name
symbol followed by any number of @dfn{structure options}; each @var{slot}
is either a slot symbol or a list of the form @samp{(@var{slot-name}
@var{default-value} @var{slot-options}@dots{})}.  The @var{default-value}
is a Lisp form that is evaluated any time an instance of the
structure type is created without specifying that slot's value.

@example
(cl-defstruct person
     (first-name nil :read-only t)
     age
     (sex 'unknown))
@end example

@var{slot-options} is a list of keyword-value pairs, where the
following keywords can be used:

@table @code
@item :read-only
A non-@code{nil} value means the slot should not be @code{setf}-able;
the slot's value is determined when the object is created and does
not change afterward.

@item :type
The expected type of the values held in this slot.

@item :documentation
A documentation string describing the slot.
@end table

Other slot options are currently ignored.

For obscure historical reasons, structure options take a different
form than slot options.  A structure option is either a keyword
symbol, or a list beginning with a keyword symbol possibly followed
by arguments.  (By contrast, slot options are key-value pairs not
enclosed in lists.)

@example
(cl-defstruct (person (:constructor create-person)
                      (:type list)
                      :named)
     first-name age sex)
@end example

The following structure options are recognized.

@table @code
@item :conc-name
The argument is a symbol whose print name is used as the prefix for
the names of slot accessor functions.  The default is the name of
the struct type followed by a hyphen.  The option @code{(:conc-name p-)}
would change this prefix to @code{p-}.  Specifying @code{nil} as an
argument means no prefix, so that the slot names themselves are used
to name the accessor functions.

@item :constructor
In the simple case, this option takes one argument which is an
alternate name to use for the constructor function.  The default
is @code{make-@var{name}}, e.g., @code{make-person}.  The above
example changes this to @code{create-person}.  Specifying @code{nil}
as an argument means that no standard constructor should be
generated at all.

In the full form of this option, the constructor name is followed
by an arbitrary argument list.  @xref{Program Structure}, for a
description of the format of Common Lisp argument lists.  All
options, such as @code{&rest} and @code{&key}, are supported.
The argument names should match the slot names; each slot is
initialized from the corresponding argument.  Slots whose names
do not appear in the argument list are initialized based on the
@var{default-value} in their slot descriptor.  Also, @code{&optional}
and @code{&key} arguments that don't specify defaults take their
defaults from the slot descriptor.  It is valid to include arguments
that don't correspond to slot names; these are useful if they are
referred to in the defaults for optional, keyword, or @code{&aux}
arguments that @emph{do} correspond to slots.

You can specify any number of full-format @code{:constructor}
options on a structure.  The default constructor is still generated
as well unless you disable it with a simple-format @code{:constructor}
option.

@example
(cl-defstruct
    (person
     (:constructor nil)   ; no default constructor
     (:constructor new-person
                   (first-name sex &optional (age 0)))
     (:constructor new-hound (&key (first-name "Rover")
                                   (dog-years 0)
                              &aux (age (* 7 dog-years))
                                   (sex 'canine))))
    first-name age sex)
@end example

The first constructor here takes its arguments positionally rather
than by keyword.  (In official Common Lisp terminology, constructors
that work By Order of Arguments instead of by keyword are called
``BOA constructors''.  No, I'm not making this up.)  For example,
@code{(new-person "Jane" 'female)} generates a person whose slots
are @code{"Jane"}, 0, and @code{female}, respectively.

The second constructor takes two keyword arguments, @code{:name},
which initializes the @code{name} slot and defaults to @code{"Rover"},
and @code{:dog-years}, which does not itself correspond to a slot
but which is used to initialize the @code{age} slot.  The @code{sex}
slot is forced to the symbol @code{canine} with no syntax for
overriding it.

@item :copier
The argument is an alternate name for the copier function for
this type.  The default is @code{copy-@var{name}}.  @code{nil}
means not to generate a copier function.  (In this implementation,
all copier functions are simply synonyms for @code{copy-sequence}.)

@item :predicate
The argument is an alternate name for the predicate that recognizes
objects of this type.  The default is @code{@var{name}-p}.  @code{nil}
means not to generate a predicate function.  (If the @code{:type}
option is used without the @code{:named} option, no predicate is
ever generated.)

In true Common Lisp, @code{typep} is always able to recognize a
structure object even if @code{:predicate} was used.  In this
package, @code{cl-typep} simply looks for a function called
@code{@var{typename}-p}, so it will work for structure types
only if they used the default predicate name.

@item :include
This option implements a very limited form of C@t{++}-style inheritance.
The argument is the name of another structure type previously
created with @code{cl-defstruct}.  The effect is to cause the new
structure type to inherit all of the included structure's slots
(plus, of course, any new slots described by this struct's slot
descriptors).  The new structure is considered a ``specialization''
of the included one.  In fact, the predicate and slot accessors
for the included type will also accept objects of the new type.

If there are extra arguments to the @code{:include} option after
the included-structure name, these options are treated as replacement
slot descriptors for slots in the included structure, possibly with
modified default values.  Borrowing an example from Steele:

@example
(cl-defstruct person first-name (age 0) sex)
        @result{} person
(cl-defstruct (astronaut (:include person (age 45)))
     helmet-size
     (favorite-beverage 'tang))
        @result{} astronaut

(setq joe (make-person :first-name "Joe"))
     @result{} [cl-struct-person "Joe" 0 nil]
(setq buzz (make-astronaut :first-name "Buzz"))
     @result{} [cl-struct-astronaut "Buzz" 45 nil nil tang]

(list (person-p joe) (person-p buzz))
     @result{} (t t)
(list (astronaut-p joe) (astronaut-p buzz))
     @result{} (nil t)

(person-first-name buzz)
     @result{} "Buzz"
(astronaut-first-name joe)
     @result{} error: "astronaut-first-name accessing a non-astronaut"
@end example

Thus, if @code{astronaut} is a specialization of @code{person},
then every @code{astronaut} is also a @code{person} (but not the
other way around).  Every @code{astronaut} includes all the slots
of a @code{person}, plus extra slots that are specific to
astronauts.  Operations that work on people (like @code{person-first-name})
work on astronauts just like other people.

@item :noinline
If this option is present, this structure's functions will not be
inlined, even functions that normally would.

@item :print-function
In full Common Lisp, this option allows you to specify a function
that is called to print an instance of the structure type.  The
Emacs Lisp system offers no hooks into the Lisp printer which would
allow for such a feature, so this package simply ignores
@code{:print-function}.

@item :type
The argument should be one of the symbols @code{vector} or
@code{list}.  This tells which underlying Lisp data type should be
used to implement the new structure type.  Records are used by
default, but @code{(:type vector)} will cause structure objects to be
stored as vectors and @code{(:type list)} lists instead.

The record and vector representations for structure objects have the
advantage that all structure slots can be accessed quickly, although
creating them are a bit slower in Emacs Lisp.  Lists are easier to
create, but take a relatively long time accessing the later slots.

@item :named
This option, which takes no arguments, causes a characteristic ``tag''
symbol to be stored at the front of the structure object.  Using
@code{:type} without also using @code{:named} will result in a
structure type stored as plain vectors or lists with no identifying
features.

The default, if you don't specify @code{:type} explicitly, is to use
records, which are always tagged.  Therefore, @code{:named} is only
useful in conjunction with @code{:type}.

@example
(cl-defstruct (person1) first-name age sex)
(cl-defstruct (person2 (:type list) :named) first-name age sex)
(cl-defstruct (person3 (:type list)) first-name age sex)
(cl-defstruct (person4 (:type vector)) first-name age sex)

(setq p1 (make-person1))
     @result{} #s(person1 nil nil nil)
(setq p2 (make-person2))
     @result{} (person2 nil nil nil)
(setq p3 (make-person3))
     @result{} (nil nil nil)
(setq p4 (make-person4))
     @result{} [nil nil nil]

(person1-p p1)
     @result{} t
(person2-p p2)
     @result{} t
(person3-p p3)
     @result{} error: function person3-p undefined
@end example

Since unnamed structures don't have tags, @code{cl-defstruct} is not
able to make a useful predicate for recognizing them.  Also,
accessors like @code{person3-first-name} will be generated but they
will not be able to do any type checking.  The @code{person3-first-name}
function, for example, will simply be a synonym for @code{car} in
this case.  By contrast, @code{person2-first-name} is able to verify
that its argument is indeed a @code{person2} object before
proceeding.

@item :initial-offset
The argument must be a nonnegative integer.  It specifies a
number of slots to be left ``empty'' at the front of the
structure.  If the structure is named, the tag appears at the
specified position in the list or vector; otherwise, the first
slot appears at that position.  Earlier positions are filled
with @code{nil} by the constructors and ignored otherwise.  If
the type @code{:include}s another type, then @code{:initial-offset}
specifies a number of slots to be skipped between the last slot
of the included type and the first new slot.
@end table
@end defmac

Except as noted, the @code{cl-defstruct} facility of this package is
entirely compatible with that of Common Lisp.

The @code{cl-defstruct} package also provides a few structure
introspection functions.

@defun cl-struct-sequence-type struct-type
This function returns the underlying data structure for
@code{struct-type}, which is a symbol.  It returns @code{record},
@code{vector} or @code{list}, or @code{nil} if @code{struct-type} is
not actually a structure.
@end defun

@defun cl-struct-slot-info struct-type
This function returns a list of slot descriptors for structure
@code{struct-type}.  Each entry in the list is @code{(name . opts)},
where @code{name} is the name of the slot and @code{opts} is the list
of slot options given to @code{defstruct}.  Dummy entries represent
the slots used for the struct name and that are skipped to implement
@code{:initial-offset}.
@end defun

@defun cl-struct-slot-offset struct-type slot-name
Return the offset of slot @code{slot-name} in @code{struct-type}.  The
returned zero-based slot index is relative to the start of the
structure data type and is adjusted for any structure name and
:initial-offset slots.  Signal error if struct @code{struct-type} does
not contain @code{slot-name}.
@end defun

@defun cl-struct-slot-value struct-type slot-name inst
Return the value of slot @code{slot-name} in @code{inst} of
@code{struct-type}.  @code{struct} and @code{slot-name} are symbols.
@code{inst} is a structure instance.  This routine is also a
@code{setf} place.  Can signal the same errors as @code{cl-struct-slot-offset}.
@end defun

@node Assertions
@chapter Assertions and Errors

@noindent
This section describes two macros that test @dfn{assertions}, i.e.,
conditions which must be true if the program is operating correctly.
Assertions never add to the behavior of a Lisp program; they simply
make ``sanity checks'' to make sure everything is as it should be.

If the optimization property @code{speed} has been set to 3, and
@code{safety} is less than 3, then the byte-compiler will optimize
away the following assertions.  Because assertions might be optimized
away, it is a bad idea for them to include side-effects.

@defmac cl-assert test-form [show-args string args@dots{}]
This form verifies that @var{test-form} is true (i.e., evaluates to
a non-@code{nil} value).  If so, it returns @code{nil}.  If the test
is not satisfied, @code{cl-assert} signals an error.

A default error message will be supplied which includes @var{test-form}.
You can specify a different error message by including a @var{string}
argument plus optional extra arguments.  Those arguments are simply
passed to @code{error} to signal the error.

If the optional second argument @var{show-args} is @code{t} instead
of @code{nil}, then the error message (with or without @var{string})
will also include all non-constant arguments of the top-level
@var{form}.  For example:

@example
(cl-assert (> x 10) t "x is too small: %d")
@end example

This usage of @var{show-args} is an extension to Common Lisp.  In
true Common Lisp, the second argument gives a list of @var{places}
which can be @code{setf}'d by the user before continuing from the
error.  Since Emacs Lisp does not support continuable errors, it
makes no sense to specify @var{places}.
@end defmac

@defmac cl-check-type form type [string]
This form verifies that @var{form} evaluates to a value of type
@var{type}.  If so, it returns @code{nil}.  If not, @code{cl-check-type}
signals a @code{wrong-type-argument} error.  The default error message
lists the erroneous value along with @var{type} and @var{form}
themselves.  If @var{string} is specified, it is included in the
error message in place of @var{type}.  For example:

@example
(cl-check-type x (integer 1 *) "a positive integer")
@end example

@xref{Type Predicates}, for a description of the type specifiers
that may be used for @var{type}.

Note that in Common Lisp, the first argument to @code{check-type}
must be a @var{place} suitable for use by @code{setf}, because
@code{check-type} signals a continuable error that allows the
user to modify @var{place}.
@end defmac

@node Efficiency Concerns
@appendix Efficiency Concerns

@appendixsec Macros

@noindent
Many of the advanced features of this package, such as @code{cl-defun},
@code{cl-loop}, etc., are implemented as Lisp macros.  In
byte-compiled code, these complex notations will be expanded into
equivalent Lisp code which is simple and efficient.  For example,
the form

@example
(cl-incf i n)
@end example

@noindent
is expanded at compile-time to the Lisp form

@example
(setq i (+ i n))
@end example

@noindent
which is the most efficient way of doing this operation
in Lisp.  Thus, there is no performance penalty for using the more
readable @code{cl-incf} form in your compiled code.

@emph{Interpreted} code, on the other hand, must expand these macros
every time they are executed.  For this reason it is strongly
recommended that code making heavy use of macros be compiled.
A loop using @code{cl-incf} a hundred times will execute considerably
faster if compiled, and will also garbage-collect less because the
macro expansion will not have to be generated, used, and thrown away a
hundred times.

You can find out how a macro expands by using the
@code{cl-prettyexpand} function.

@defun cl-prettyexpand form &optional full
This function takes a single Lisp form as an argument and inserts
a nicely formatted copy of it in the current buffer (which must be
in Lisp mode so that indentation works properly).  It also expands
all Lisp macros that appear in the form.  The easiest way to use
this function is to go to the @file{*scratch*} buffer and type, say,

@example
(cl-prettyexpand '(cl-loop for x below 10 collect x))
@end example

@noindent
and type @kbd{C-x C-e} immediately after the closing parenthesis;
an expansion similar to:

@example
(cl-block nil
     (let* ((x 0)
            (G1004 nil))
       (while (< x 10)
         (setq G1004 (cons x G1004))
         (setq x (+ x 1)))
       (nreverse G1004)))
@end example

@noindent
will be inserted into the buffer.  (The @code{cl-block} macro is
expanded differently in the interpreter and compiler, so
@code{cl-prettyexpand} just leaves it alone.  The temporary
variable @code{G1004} was created by @code{cl-gensym}.)

If the optional argument @var{full} is true, then @emph{all}
macros are expanded, including @code{cl-block}, @code{cl-eval-when},
and compiler macros.  Expansion is done as if @var{form} were
a top-level form in a file being compiled.

@c FIXME none of these examples are still applicable.
@ignore
For example,

@example
(cl-prettyexpand '(cl-pushnew 'x list))
     @print{} (setq list (cl-adjoin 'x list))
(cl-prettyexpand '(cl-pushnew 'x list) t)
     @print{} (setq list (if (memq 'x list) list (cons 'x list)))
(cl-prettyexpand '(caddr (cl-member 'a list)) t)
     @print{} (car (cdr (cdr (memq 'a list))))
@end example
@end ignore

Note that @code{cl-adjoin} and @code{cl-member} have built-in compiler
macros to optimize them in common cases.
@end defun

@appendixsec Error Checking

@noindent
Common Lisp compliance has in general not been sacrificed for the
sake of efficiency.  A few exceptions have been made for cases
where substantial gains were possible at the expense of marginal
incompatibility.

The Common Lisp standard (as embodied in Steele's book) uses the
phrase ``it is an error if'' to indicate a situation that is not
supposed to arise in complying programs; implementations are strongly
encouraged but not required to signal an error in these situations.
This package sometimes omits such error checking in the interest of
compactness and efficiency.  For example, @code{cl-do} variable
specifiers are supposed to be lists of one, two, or three forms; extra
forms are ignored by this package rather than signaling a syntax
error.  Functions taking keyword arguments will accept an odd number
of arguments, treating the trailing keyword as if it were followed by
the value @code{nil}.

Argument lists (as processed by @code{cl-defun} and friends)
@emph{are} checked rigorously except for the minor point just
mentioned; in particular, keyword arguments are checked for
validity, and @code{&allow-other-keys} and @code{:allow-other-keys}
are fully implemented.  Keyword validity checking is slightly
time consuming (though not too bad in byte-compiled code);
you can use @code{&allow-other-keys} to omit this check.  Functions
defined in this package such as @code{cl-find} and @code{cl-member}
do check their keyword arguments for validity.

@appendixsec Compiler Optimizations

@noindent
Changing the value of @code{byte-optimize} from the default @code{t}
is highly discouraged; many of the Common
Lisp macros emit
code that can be improved by optimization.  In particular,
@code{cl-block}s (whether explicit or implicit in constructs like
@code{cl-defun} and @code{cl-loop}) carry a fair run-time penalty; the
byte-compiler removes @code{cl-block}s that are not actually
referenced by @code{cl-return} or @code{cl-return-from} inside the block.

@node Common Lisp Compatibility
@appendix Common Lisp Compatibility

@noindent
The following is a list of some of the most important
incompatibilities between this package and Common Lisp as documented
in Steele (2nd edition).

The word @code{cl-defun} is required instead of @code{defun} in order
to use extended Common Lisp argument lists in a function.  Likewise,
@code{cl-defmacro} and @code{cl-function} are versions of those forms
which understand full-featured argument lists.  The @code{&whole}
keyword does not work in @code{cl-defmacro} argument lists (except
inside recursive argument lists).

The @code{equal} predicate does not distinguish
between IEEE floating-point plus and minus zero.  The @code{cl-equalp}
predicate has several differences with Common Lisp; @pxref{Predicates}.

The @code{cl-do-all-symbols} form is the same as @code{cl-do-symbols}
with no @var{obarray} argument.  In Common Lisp, this form would
iterate over all symbols in all packages.  Since Emacs obarrays
are not a first-class package mechanism, there is no way for
@code{cl-do-all-symbols} to locate any but the default obarray.

The @code{cl-loop} macro is complete except that @code{loop-finish}
and type specifiers are unimplemented.

The multiple-value return facility treats lists as multiple
values, since Emacs Lisp cannot support multiple return values
directly.  The macros will be compatible with Common Lisp if
@code{cl-values} or @code{cl-values-list} is always used to return to
a @code{cl-multiple-value-bind} or other multiple-value receiver;
if @code{cl-values} is used without @code{cl-multiple-value-@dots{}}
or vice-versa the effect will be different from Common Lisp.

Many Common Lisp declarations are ignored, and others match
the Common Lisp standard in concept but not in detail.  For
example, local @code{special} declarations, which are purely
advisory in Emacs Lisp, do not rigorously obey the scoping rules
set down in Steele's book.

The variable @code{cl--gensym-counter} starts out with zero.

The @code{cl-defstruct} facility is compatible, except that the
@code{:type} slot option is ignored.

The second argument of @code{cl-check-type} is treated differently.

@node Porting Common Lisp
@appendix Porting Common Lisp

@noindent
This package is meant to be used as an extension to Emacs Lisp,
not as an Emacs implementation of true Common Lisp.  Some of the
remaining differences between Emacs Lisp and Common Lisp make it
difficult to port large Common Lisp applications to Emacs.  For
one, some of the features in this package are not fully compliant
with ANSI or Steele; @pxref{Common Lisp Compatibility}.  But there
are also quite a few features that this package does not provide
at all.  Here are some major omissions that you will want to watch out
for when bringing Common Lisp code into Emacs.

@itemize @bullet
@item
Case-insensitivity.  Symbols in Common Lisp are case-insensitive
by default.  Some programs refer to a function or variable as
@code{foo} in one place and @code{Foo} or @code{FOO} in another.
Emacs Lisp will treat these as three distinct symbols.

Some Common Lisp code is written entirely in upper case.  While Emacs
is happy to let the program's own functions and variables use
this convention, calls to Lisp builtins like @code{if} and
@code{defun} will have to be changed to lower case.

@item
Lexical scoping.  In Common Lisp, function arguments and @code{let}
bindings apply only to references physically within their bodies (or
within macro expansions in their bodies).  Traditionally, Emacs Lisp
uses @dfn{dynamic scoping} wherein a binding to a variable is visible
even inside functions called from the body.
@xref{Dynamic Binding,,,elisp,GNU Emacs Lisp Reference Manual}.
Lexical binding is available since Emacs 24.1, so be sure to set
@code{lexical-binding} to @code{t} if you need to emulate this aspect
of Common Lisp.  @xref{Lexical Binding,,,elisp,GNU Emacs Lisp Reference Manual}.

Here is an example of a Common Lisp code fragment that would fail in
Emacs Lisp if @code{lexical-binding} were set to @code{nil}:

@example
(defun map-odd-elements (func list)
  (loop for x in list
        for flag = t then (not flag)
        collect (if flag x (funcall func x))))

(defun add-odd-elements (list x)
  (map-odd-elements (lambda (a) (+ a x)) list))
@end example

@noindent
With lexical binding, the two functions' usages of @code{x} are
completely independent.  With dynamic binding, the binding to @code{x}
made by @code{add-odd-elements} will have been hidden by the binding
in @code{map-odd-elements} by the time the @code{(+ a x)} function is
called.

Internally, this package uses lexical binding so that such problems do
not occur.  @xref{Obsolete Lexical Binding}, for a description of the obsolete
@code{lexical-let} form that emulates a Common Lisp-style lexical
binding when dynamic binding is in use.

@item
Reader macros.  Common Lisp includes a second type of macro that
works at the level of individual characters.  For example, Common
Lisp implements the quote notation by a reader macro called @code{'},
whereas Emacs Lisp's parser just treats quote as a special case.
Some Lisp packages use reader macros to create special syntaxes
for themselves, which the Emacs parser is incapable of reading.

@item
Other syntactic features.  Common Lisp provides a number of
notations beginning with @code{#} that the Emacs Lisp parser
won't understand.  For example, @samp{#| @dots{} |#} is an
alternate comment notation, and @samp{#+lucid (foo)} tells
the parser to ignore the @code{(foo)} except in Lucid Common
Lisp.

@item
Packages.  In Common Lisp, symbols are divided into @dfn{packages}.
Symbols that are Lisp built-ins are typically stored in one package;
symbols that are vendor extensions are put in another, and each
application program would have a package for its own symbols.
Certain symbols are ``exported'' by a package and others are
internal; certain packages ``use'' or import the exported symbols
of other packages.  To access symbols that would not normally be
visible due to this importing and exporting, Common Lisp provides
a syntax like @code{package:symbol} or @code{package::symbol}.

Emacs Lisp has a single namespace for all interned symbols, and
then uses a naming convention of putting a prefix like @code{cl-}
in front of the name.  Some Emacs packages adopt the Common Lisp-like
convention of using @code{cl:} or @code{cl::} as the prefix.
However, the Emacs parser does not understand colons and just
treats them as part of the symbol name.  Thus, while @code{mapcar}
and @code{lisp:mapcar} may refer to the same symbol in Common
Lisp, they are totally distinct in Emacs Lisp.  Common Lisp
programs that refer to a symbol by the full name sometimes
and the short name other times will not port cleanly to Emacs.

Emacs Lisp does have a concept of ``obarrays'', which are
package-like collections of symbols, but this feature is not
strong enough to be used as a true package mechanism.

@item
The @code{format} function is quite different between Common
Lisp and Emacs Lisp.  It takes an additional ``destination''
argument before the format string.  A destination of @code{nil}
means to format to a string as in Emacs Lisp; a destination
of @code{t} means to write to the terminal (similar to
@code{message} in Emacs).  Also, format control strings are
utterly different; @code{~} is used instead of @code{%} to
introduce format codes, and the set of available codes is
much richer.  There are no notations like @code{\n} for
string literals; instead, @code{format} is used with the
``newline'' format code, @code{~%}.  More advanced formatting
codes provide such features as paragraph filling, case
conversion, and even loops and conditionals.

While it would have been possible to implement most of Common
Lisp @code{format} in this package (under the name @code{cl-format},
of course), it was not deemed worthwhile.  It would have required
a huge amount of code to implement even a decent subset of
@code{format}, yet the functionality it would provide over
Emacs Lisp's @code{format} would rarely be useful.

@item
Vector constants use square brackets in Emacs Lisp, but
@code{#(a b c)} notation in Common Lisp.  To further complicate
matters, Emacs has its own @code{#(} notation for
something entirely different---strings with properties.

@item
Characters are distinct from integers in Common Lisp.  The notation
for character constants is also different: @code{#\A} in Common Lisp
where Emacs Lisp uses @code{?A}.  Also, @code{string=} and
@code{string-equal} are synonyms in Emacs Lisp, whereas the latter is
case-insensitive in Common Lisp.

@item
Data types.  Some Common Lisp data types do not exist in Emacs
Lisp.  Rational numbers and complex numbers are not present,
nor are large integers (all integers are ``fixnums'').  All
arrays are one-dimensional.  There are no readtables or pathnames;
streams are a set of existing data types rather than a new data
type of their own.  Hash tables, random-states, and packages
(obarrays) are built from Lisp vectors or lists rather than being
distinct types.

@item
The Common Lisp Object System (CLOS) is not implemented,
nor is the Common Lisp Condition System.  However, the EIEIO package
(@pxref{Top, , Introduction, eieio, EIEIO}) does implement some
CLOS functionality.

@item
Common Lisp features that are completely redundant with Emacs
Lisp features of a different name generally have not been
implemented.  For example, Common Lisp writes @code{defconstant}
where Emacs Lisp uses @code{defconst}.  Similarly, @code{make-list}
takes its arguments in different ways in the two Lisps but does
exactly the same thing, so this package has not bothered to
implement a Common Lisp-style @code{make-list}.

@item
A few more notable Common Lisp features not included in this package:
@code{compiler-let}, @code{prog}, @code{ldb/dpb}, @code{cerror}.

@item
Recursion.  While recursion works in Emacs Lisp just like it
does in Common Lisp, various details of the Emacs Lisp system
and compiler make recursion much less efficient than it is in
most Lisps.  Some schools of thought prefer to use recursion
in Lisp over other techniques; they would sum a list of
numbers using something like

@example
(defun sum-list (list)
  (if list
      (+ (car list) (sum-list (cdr list)))
    0))
@end example

@noindent
where a more iteratively-minded programmer might write one of
these forms:

@example
(let ((total 0)) (dolist (x my-list) (incf total x)) total)
(loop for x in my-list sum x)
@end example

While this would be mainly a stylistic choice in most Common Lisps,
in Emacs Lisp you should be aware that the iterative forms are
much faster than recursion.  Also, Lisp programmers will want to
note that the current Emacs Lisp compiler does not optimize tail
recursion.
@end itemize

@node Obsolete Features
@appendix Obsolete Features

This section describes some features of the package that are obsolete
and should not be used in new code.  They are either only provided by
the old @file{cl.el} entry point, not by the newer @file{cl-lib.el};
or where versions with a @samp{cl-} prefix do exist they do not behave
in exactly the same way.

@menu
* Obsolete Lexical Binding::    An approximation of lexical binding.
* Obsolete Macros::             Obsolete macros.
* Obsolete Setf Customization:: Obsolete ways to customize setf.
@end menu

@node Obsolete Lexical Binding
@appendixsec Obsolete Lexical Binding

The following macros are extensions to Common Lisp, where all bindings
are lexical unless declared otherwise.  These features are likewise
obsolete since the introduction of true lexical binding in Emacs 24.1.

@defmac lexical-let (bindings@dots{}) forms@dots{}
This form is exactly like @code{let} except that the bindings it
establishes are purely lexical.
@end defmac

@c FIXME remove this and refer to elisp manual.
@c Maybe merge some stuff from here to there?
@noindent
Lexical bindings are similar to local variables in a language like C:
Only the code physically within the body of the @code{lexical-let}
(after macro expansion) may refer to the bound variables.

@example
(setq a 5)
(defun foo (b) (+ a b))
(let ((a 2)) (foo a))
     @result{} 4
(lexical-let ((a 2)) (foo a))
     @result{} 7
@end example

@noindent
In this example, a regular @code{let} binding of @code{a} actually
makes a temporary change to the global variable @code{a}, so @code{foo}
is able to see the binding of @code{a} to 2.  But @code{lexical-let}
actually creates a distinct local variable @code{a} for use within its
body, without any effect on the global variable of the same name.

The most important use of lexical bindings is to create @dfn{closures}.
A closure is a function object that refers to an outside lexical
variable (@pxref{Closures,,,elisp,GNU Emacs Lisp Reference Manual}).
For example:

@example
(defun make-adder (n)
  (lexical-let ((n n))
    (lambda (m) (+ n m))))
(setq add17 (make-adder 17))
(funcall add17 4)
     @result{} 21
@end example

@noindent
The call @code{(make-adder 17)} returns a function object which adds
17 to its argument.  If @code{let} had been used instead of
@code{lexical-let}, the function object would have referred to the
global @code{n}, which would have been bound to 17 only during the
call to @code{make-adder} itself.

@example
(defun make-counter ()
  (lexical-let ((n 0))
    (cl-function (lambda (&optional (m 1)) (cl-incf n m)))))
(setq count-1 (make-counter))
(funcall count-1 3)
     @result{} 3
(funcall count-1 14)
     @result{} 17
(setq count-2 (make-counter))
(funcall count-2 5)
     @result{} 5
(funcall count-1 2)
     @result{} 19
(funcall count-2)
     @result{} 6
@end example

@noindent
Here we see that each call to @code{make-counter} creates a distinct
local variable @code{n}, which serves as a private counter for the
function object that is returned.

Closed-over lexical variables persist until the last reference to
them goes away, just like all other Lisp objects.  For example,
@code{count-2} refers to a function object which refers to an
instance of the variable @code{n}; this is the only reference
to that variable, so after @code{(setq count-2 nil)} the garbage
collector would be able to delete this instance of @code{n}.
Of course, if a @code{lexical-let} does not actually create any
closures, then the lexical variables are free as soon as the
@code{lexical-let} returns.

Many closures are used only during the extent of the bindings they
refer to; these are known as ``downward funargs'' in Lisp parlance.
When a closure is used in this way, regular Emacs Lisp dynamic
bindings suffice and will be more efficient than @code{lexical-let}
closures:

@example
(defun add-to-list (x list)
  (mapcar (lambda (y) (+ x y))) list)
(add-to-list 7 '(1 2 5))
     @result{} (8 9 12)
@end example

@noindent
Since this lambda is only used while @code{x} is still bound,
it is not necessary to make a true closure out of it.

You can use @code{defun} or @code{flet} inside a @code{lexical-let}
to create a named closure.  If several closures are created in the
body of a single @code{lexical-let}, they all close over the same
instance of the lexical variable.

@defmac lexical-let* (bindings@dots{}) forms@dots{}
This form is just like @code{lexical-let}, except that the bindings
are made sequentially in the manner of @code{let*}.
@end defmac

@node Obsolete Macros
@appendixsec Obsolete Macros

The following macros are obsolete, and are replaced by versions with
a @samp{cl-} prefix that do not behave in exactly the same way.
Consequently, the @file{cl.el} versions are not simply aliases to the
@file{cl-lib.el} versions.

@defmac flet (bindings@dots{}) forms@dots{}
This macro is replaced by @code{cl-flet} (@pxref{Function Bindings}),
which behaves the same way as Common Lisp's @code{flet}.
This @code{flet} takes the same arguments as @code{cl-flet}, but does
not behave in precisely the same way.

While @code{flet} in Common Lisp establishes a lexical function
binding, this @code{flet} makes a dynamic binding (it dates from a
time before Emacs had lexical binding).  The result is
that @code{flet} affects indirect calls to a function as well as calls
directly inside the @code{flet} form itself.

This will even work on Emacs primitives, although note that some calls
to primitive functions internal to Emacs are made without going
through the symbol's function cell, and so will not be affected by
@code{flet}.  For example,

@example
(flet ((message (&rest args) (push args saved-msgs)))
  (do-something))
@end example

This code attempts to replace the built-in function @code{message}
with a function that simply saves the messages in a list rather
than displaying them.  The original definition of @code{message}
will be restored after @code{do-something} exits.  This code will
work fine on messages generated by other Lisp code, but messages
generated directly inside Emacs will not be caught since they make
direct C-language calls to the message routines rather than going
through the Lisp @code{message} function.

For those cases where the dynamic scoping of @code{flet} is desired,
@code{cl-flet} is clearly not a substitute.  The most direct replacement would
be instead to use @code{cl-letf} to temporarily rebind @code{(symbol-function
'@var{fun})}.  But in most cases, a better substitute is to use advice, such
as:

@example
(defvar my-fun-advice-enable nil)
(add-advice '@var{fun} :around
            (lambda (orig &rest args)
              (if my-fun-advice-enable (do-something)
                (apply orig args))))
@end example

so that you can then replace the @code{flet} with a simple dynamically scoped
binding of @code{my-fun-advice-enable}.

@c Bug#411.
Note that many primitives (e.g., @code{+}) have special byte-compile handling.
Attempts to redefine such functions using @code{flet}, @code{cl-letf}, or
advice will fail when byte-compiled.
@c Or cl-flet.
@c In such cases, use @code{labels} instead.
@end defmac

@defmac labels (bindings@dots{}) forms@dots{}
This macro is replaced by @code{cl-labels} (@pxref{Function Bindings}),
which behaves the same way as Common Lisp's @code{labels}.
This @code{labels} takes the same arguments as @code{cl-labels}, but
does not behave in precisely the same way.

This version of @code{labels} uses the obsolete @code{lexical-let}
form (@pxref{Obsolete Lexical Binding}), rather than the true
lexical binding that @code{cl-labels} uses.
@end defmac

@node Obsolete Setf Customization
@appendixsec Obsolete Ways to Customize Setf

Common Lisp defines three macros, @code{define-modify-macro},
@code{defsetf}, and @code{define-setf-method}, that allow the
user to extend generalized variables in various ways.
In Emacs, these are obsolete, replaced by various features of
@file{gv.el} in Emacs 24.3.
@xref{Adding Generalized Variables,,,elisp,GNU Emacs Lisp Reference Manual}.


@defmac define-modify-macro name arglist function [doc-string]
This macro defines a ``read-modify-write'' macro similar to
@code{cl-incf} and @code{cl-decf}.  You can replace this macro
with @code{gv-letplace}.

The macro @var{name} is defined to take a @var{place} argument
followed by additional arguments described by @var{arglist}.  The call

@example
(@var{name} @var{place} @var{args}@dots{})
@end example

@noindent
will be expanded to

@example
(cl-callf @var{func} @var{place} @var{args}@dots{})
@end example

@noindent
which in turn is roughly equivalent to

@example
(setf @var{place} (@var{func} @var{place} @var{args}@dots{}))
@end example

For example:

@example
(define-modify-macro incf (&optional (n 1)) +)
(define-modify-macro concatf (&rest args) concat)
@end example

Note that @code{&key} is not allowed in @var{arglist}, but
@code{&rest} is sufficient to pass keywords on to the function.

Most of the modify macros defined by Common Lisp do not exactly
follow the pattern of @code{define-modify-macro}.  For example,
@code{push} takes its arguments in the wrong order, and @code{pop}
is completely irregular.

The above @code{incf} example could be written using
@code{gv-letplace} as:
@example
(defmacro incf (place &optional n)
  (gv-letplace (getter setter) place
    (cl-once-only ((v (or n 1)))
      (funcall setter `(+ ,v ,getter)))))
@end example
@ignore
(defmacro concatf (place &rest args)
  (gv-letplace (getter setter) place
    (cl-once-only ((v `(mapconcat 'identity ',args)))
      (funcall setter `(concat ,getter ,v)))))
@end ignore
@end defmac

@defmac defsetf access-fn update-fn
This is the simpler of two @code{defsetf} forms, and is
replaced by @code{gv-define-simple-setter}.

With @var{access-fn} the name of a function that accesses a place,
this declares @var{update-fn} to be the corresponding store function.
From now on,

@example
(setf (@var{access-fn} @var{arg1} @var{arg2} @var{arg3}) @var{value})
@end example

@noindent
will be expanded to

@example
(@var{update-fn} @var{arg1} @var{arg2} @var{arg3} @var{value})
@end example

@noindent
The @var{update-fn} is required to be either a true function, or
a macro that evaluates its arguments in a function-like way.  Also,
the @var{update-fn} is expected to return @var{value} as its result.
Otherwise, the above expansion would not obey the rules for the way
@code{setf} is supposed to behave.

As a special (non-Common-Lisp) extension, a third argument of @code{t}
to @code{defsetf} says that the return value of @code{update-fn} is
not suitable, so that the above @code{setf} should be expanded to
something more like

@example
(let ((temp @var{value}))
  (@var{update-fn} @var{arg1} @var{arg2} @var{arg3} temp)
  temp)
@end example

Some examples are:

@example
(defsetf car setcar)
(defsetf buffer-name rename-buffer t)
@end example

These translate directly to @code{gv-define-simple-setter}:

@example
(gv-define-simple-setter car setcar)
(gv-define-simple-setter buffer-name rename-buffer t)
@end example
@end defmac

@defmac defsetf access-fn arglist (store-var) forms@dots{}
This is the second, more complex, form of @code{defsetf}.
It can be replaced by @code{gv-define-setter}.

This form of @code{defsetf} is rather like @code{defmacro} except for
the additional @var{store-var} argument.  The @var{forms} should
return a Lisp form that stores the value of @var{store-var} into the
generalized variable formed by a call to @var{access-fn} with
arguments described by @var{arglist}.  The @var{forms} may begin with
a string which documents the @code{setf} method (analogous to the doc
string that appears at the front of a function).

For example, the simple form of @code{defsetf} is shorthand for

@example
(defsetf @var{access-fn} (&rest args) (store)
  (append '(@var{update-fn}) args (list store)))
@end example

The Lisp form that is returned can access the arguments from
@var{arglist} and @var{store-var} in an unrestricted fashion;
macros like @code{cl-incf} that invoke this
setf-method will insert temporary variables as needed to make
sure the apparent order of evaluation is preserved.

Another standard example:

@example
(defsetf nth (n x) (store)
  `(setcar (nthcdr ,n ,x) ,store))
@end example

You could write this using @code{gv-define-setter} as:

@example
(gv-define-setter nth (store n x)
  `(setcar (nthcdr ,n ,x) ,store))
@end example
@end defmac

@defmac define-setf-method access-fn arglist forms@dots{}
This is the most general way to create new place forms.  You can
replace this by @code{gv-define-setter} or @code{gv-define-expander}.

When a @code{setf} to @var{access-fn} with arguments described by
@var{arglist} is expanded, the @var{forms} are evaluated and must
return a list of five items:

@enumerate
@item
A list of @dfn{temporary variables}.

@item
A list of @dfn{value forms} corresponding to the temporary variables
above.  The temporary variables will be bound to these value forms
as the first step of any operation on the generalized variable.

@item
A list of exactly one @dfn{store variable} (generally obtained
from a call to @code{gensym}).

@item
A Lisp form that stores the contents of the store variable into
the generalized variable, assuming the temporaries have been
bound as described above.

@item
A Lisp form that accesses the contents of the generalized variable,
assuming the temporaries have been bound.
@end enumerate

This is exactly like the Common Lisp macro of the same name,
except that the method returns a list of five values rather
than the five values themselves, since Emacs Lisp does not
support Common Lisp's notion of multiple return values.
(Note that the @code{setf} implementation provided by @file{gv.el}
does not use this five item format.  Its use here is only for
backwards compatibility.)

Once again, the @var{forms} may begin with a documentation string.

A setf-method should be maximally conservative with regard to
temporary variables.  In the setf-methods generated by
@code{defsetf}, the second return value is simply the list of
arguments in the place form, and the first return value is a
list of a corresponding number of temporary variables generated
@c FIXME I don't think this is true anymore.
by @code{cl-gensym}.  Macros like @code{cl-incf} that
use this setf-method will optimize away most temporaries that
turn out to be unnecessary, so there is little reason for the
setf-method itself to optimize.
@end defmac

@c Removed in Emacs 24.3, not possible to make a compatible replacement.
@ignore
@defun get-setf-method place &optional env
This function returns the setf-method for @var{place}, by
invoking the definition previously recorded by @code{defsetf}
or @code{define-setf-method}.  The result is a list of five
values as described above.  You can use this function to build
your own @code{cl-incf}-like modify macros.

The argument @var{env} specifies the ``environment'' to be
passed on to @code{macroexpand} if @code{get-setf-method} should
need to expand a macro in @var{place}.  It should come from
an @code{&environment} argument to the macro or setf-method
that called @code{get-setf-method}.
@end defun
@end ignore


@node GNU Free Documentation License
@appendix GNU Free Documentation License
@include doclicense.texi

@node Function Index
@unnumbered Function Index
@printindex fn

@node Variable Index
@unnumbered Variable Index
@printindex vr

@node Concept Index
@unnumbered Concept Index
@printindex cp

@bye