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The reader now warns about some unescaped character literals, but still allows them for compatibility reasons. Slightly adapt the manual to forbid them officially. * doc/lispref/objects.texi (Basic Char Syntax): Document that backslashes are now required before some characters.
2264 lines
80 KiB
Plaintext
2264 lines
80 KiB
Plaintext
@c -*- mode: texinfo; coding: utf-8 -*-
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@c This is part of the GNU Emacs Lisp Reference Manual.
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@c Copyright (C) 1990-1995, 1998-1999, 2001-2017 Free Software
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@c Foundation, Inc.
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@c See the file elisp.texi for copying conditions.
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@node Lisp Data Types
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@chapter Lisp Data Types
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@cindex object
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@cindex Lisp object
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@cindex type
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@cindex data type
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A Lisp @dfn{object} is a piece of data used and manipulated by Lisp
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programs. For our purposes, a @dfn{type} or @dfn{data type} is a set of
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possible objects.
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Every object belongs to at least one type. Objects of the same type
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have similar structures and may usually be used in the same contexts.
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Types can overlap, and objects can belong to two or more types.
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Consequently, we can ask whether an object belongs to a particular type,
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but not for @emph{the} type of an object.
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@cindex primitive type
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A few fundamental object types are built into Emacs. These, from
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which all other types are constructed, are called @dfn{primitive types}.
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Each object belongs to one and only one primitive type. These types
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include @dfn{integer}, @dfn{float}, @dfn{cons}, @dfn{symbol},
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@dfn{string}, @dfn{vector}, @dfn{hash-table}, @dfn{subr},
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@dfn{byte-code function}, and @dfn{record}, plus several special
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types, such as @dfn{buffer}, that are related to editing.
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(@xref{Editing Types}.)
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Each primitive type has a corresponding Lisp function that checks
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whether an object is a member of that type.
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Lisp is unlike many other languages in that its objects are
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@dfn{self-typing}: the primitive type of each object is implicit in
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the object itself. For example, if an object is a vector, nothing can
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treat it as a number; Lisp knows it is a vector, not a number.
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In most languages, the programmer must declare the data type of each
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variable, and the type is known by the compiler but not represented in
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the data. Such type declarations do not exist in Emacs Lisp. A Lisp
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variable can have any type of value, and it remembers whatever value
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you store in it, type and all. (Actually, a small number of Emacs
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Lisp variables can only take on values of a certain type.
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@xref{Variables with Restricted Values}.)
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This chapter describes the purpose, printed representation, and read
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syntax of each of the standard types in GNU Emacs Lisp. Details on how
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to use these types can be found in later chapters.
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@menu
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* Printed Representation:: How Lisp objects are represented as text.
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* Comments:: Comments and their formatting conventions.
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* Programming Types:: Types found in all Lisp systems.
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* Editing Types:: Types specific to Emacs.
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* Circular Objects:: Read syntax for circular structure.
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* Type Predicates:: Tests related to types.
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* Equality Predicates:: Tests of equality between any two objects.
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@end menu
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@node Printed Representation
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@section Printed Representation and Read Syntax
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@cindex printed representation
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@cindex read syntax
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The @dfn{printed representation} of an object is the format of the
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output generated by the Lisp printer (the function @code{prin1}) for
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that object. Every data type has a unique printed representation.
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The @dfn{read syntax} of an object is the format of the input accepted
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by the Lisp reader (the function @code{read}) for that object. This
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is not necessarily unique; many kinds of object have more than one
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syntax. @xref{Read and Print}.
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@cindex hash notation
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In most cases, an object's printed representation is also a read
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syntax for the object. However, some types have no read syntax, since
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it does not make sense to enter objects of these types as constants in
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a Lisp program. These objects are printed in @dfn{hash notation},
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which consists of the characters @samp{#<}, a descriptive string
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(typically the type name followed by the name of the object), and a
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closing @samp{>}. For example:
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@example
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(current-buffer)
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@result{} #<buffer objects.texi>
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@end example
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@noindent
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Hash notation cannot be read at all, so the Lisp reader signals the
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error @code{invalid-read-syntax} whenever it encounters @samp{#<}.
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@kindex invalid-read-syntax
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In other languages, an expression is text; it has no other form. In
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Lisp, an expression is primarily a Lisp object and only secondarily the
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text that is the object's read syntax. Often there is no need to
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emphasize this distinction, but you must keep it in the back of your
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mind, or you will occasionally be very confused.
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When you evaluate an expression interactively, the Lisp interpreter
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first reads the textual representation of it, producing a Lisp object,
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and then evaluates that object (@pxref{Evaluation}). However,
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evaluation and reading are separate activities. Reading returns the
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Lisp object represented by the text that is read; the object may or may
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not be evaluated later. @xref{Input Functions}, for a description of
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@code{read}, the basic function for reading objects.
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@node Comments
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@section Comments
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@cindex comments
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@cindex @samp{;} in comment
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A @dfn{comment} is text that is written in a program only for the sake
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of humans that read the program, and that has no effect on the meaning
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of the program. In Lisp, a semicolon (@samp{;}) starts a comment if it
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is not within a string or character constant. The comment continues to
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the end of line. The Lisp reader discards comments; they do not become
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part of the Lisp objects which represent the program within the Lisp
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system.
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The @samp{#@@@var{count}} construct, which skips the next @var{count}
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characters, is useful for program-generated comments containing binary
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data. The Emacs Lisp byte compiler uses this in its output files
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(@pxref{Byte Compilation}). It isn't meant for source files, however.
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@xref{Comment Tips}, for conventions for formatting comments.
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@node Programming Types
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@section Programming Types
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@cindex programming types
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There are two general categories of types in Emacs Lisp: those having
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to do with Lisp programming, and those having to do with editing. The
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former exist in many Lisp implementations, in one form or another. The
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latter are unique to Emacs Lisp.
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@menu
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* Integer Type:: Numbers without fractional parts.
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* Floating-Point Type:: Numbers with fractional parts and with a large range.
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* Character Type:: The representation of letters, numbers and
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control characters.
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* Symbol Type:: A multi-use object that refers to a function,
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variable, or property list, and has a unique identity.
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* Sequence Type:: Both lists and arrays are classified as sequences.
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* Cons Cell Type:: Cons cells, and lists (which are made from cons cells).
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* Array Type:: Arrays include strings and vectors.
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* String Type:: An (efficient) array of characters.
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* Vector Type:: One-dimensional arrays.
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* Char-Table Type:: One-dimensional sparse arrays indexed by characters.
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* Bool-Vector Type:: One-dimensional arrays of @code{t} or @code{nil}.
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* Hash Table Type:: Super-fast lookup tables.
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* Function Type:: A piece of executable code you can call from elsewhere.
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* Macro Type:: A method of expanding an expression into another
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expression, more fundamental but less pretty.
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* Primitive Function Type:: A function written in C, callable from Lisp.
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* Byte-Code Type:: A function written in Lisp, then compiled.
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* Record Type:: Compound objects with programmer-defined types.
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* Type Descriptors:: Objects holding information about types.
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* Autoload Type:: A type used for automatically loading seldom-used
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functions.
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* Finalizer Type:: Runs code when no longer reachable.
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@end menu
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@node Integer Type
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@subsection Integer Type
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The range of values for an integer depends on the machine. The
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minimum range is @minus{}536,870,912 to 536,870,911 (30 bits; i.e.,
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@ifnottex
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@minus{}2**29
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@end ifnottex
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@tex
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@math{-2^{29}}
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@end tex
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to
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@ifnottex
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2**29 @minus{} 1)
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@end ifnottex
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@tex
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@math{2^{29}-1})
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@end tex
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but many machines provide a wider range.
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Emacs Lisp arithmetic functions do not check for integer overflow. Thus
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@code{(1+ 536870911)} is @minus{}536,870,912 if Emacs integers are 30 bits.
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The read syntax for integers is a sequence of (base ten) digits with an
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optional sign at the beginning and an optional period at the end. The
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printed representation produced by the Lisp interpreter never has a
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leading @samp{+} or a final @samp{.}.
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@example
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@group
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-1 ; @r{The integer @minus{}1.}
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1 ; @r{The integer 1.}
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1. ; @r{Also the integer 1.}
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+1 ; @r{Also the integer 1.}
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@end group
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@end example
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@noindent
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As a special exception, if a sequence of digits specifies an integer
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too large or too small to be a valid integer object, the Lisp reader
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reads it as a floating-point number (@pxref{Floating-Point Type}).
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For instance, if Emacs integers are 30 bits, @code{536870912} is read
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as the floating-point number @code{536870912.0}.
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@xref{Numbers}, for more information.
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@node Floating-Point Type
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@subsection Floating-Point Type
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Floating-point numbers are the computer equivalent of scientific
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notation; you can think of a floating-point number as a fraction
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together with a power of ten. The precise number of significant
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figures and the range of possible exponents is machine-specific; Emacs
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uses the C data type @code{double} to store the value, and internally
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this records a power of 2 rather than a power of 10.
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The printed representation for floating-point numbers requires either
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a decimal point (with at least one digit following), an exponent, or
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both. For example, @samp{1500.0}, @samp{+15e2}, @samp{15.0e+2},
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@samp{+1500000e-3}, and @samp{.15e4} are five ways of writing a floating-point
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number whose value is 1500. They are all equivalent.
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@xref{Numbers}, for more information.
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@node Character Type
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@subsection Character Type
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@cindex @acronym{ASCII} character codes
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A @dfn{character} in Emacs Lisp is nothing more than an integer. In
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other words, characters are represented by their character codes. For
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example, the character @kbd{A} is represented as the @w{integer 65}.
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Individual characters are used occasionally in programs, but it is
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more common to work with @emph{strings}, which are sequences composed
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of characters. @xref{String Type}.
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Characters in strings and buffers are currently limited to the range
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of 0 to 4194303---twenty two bits (@pxref{Character Codes}). Codes 0
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through 127 are @acronym{ASCII} codes; the rest are
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non-@acronym{ASCII} (@pxref{Non-ASCII Characters}). Characters that
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represent keyboard input have a much wider range, to encode modifier
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keys such as Control, Meta and Shift.
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There are special functions for producing a human-readable textual
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description of a character for the sake of messages. @xref{Describing
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Characters}.
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@menu
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* Basic Char Syntax:: Syntax for regular characters.
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* General Escape Syntax:: How to specify characters by their codes.
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* Ctl-Char Syntax:: Syntax for control characters.
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* Meta-Char Syntax:: Syntax for meta-characters.
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* Other Char Bits:: Syntax for hyper-, super-, and alt-characters.
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@end menu
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@node Basic Char Syntax
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@subsubsection Basic Char Syntax
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@cindex read syntax for characters
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@cindex printed representation for characters
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@cindex syntax for characters
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@cindex @samp{?} in character constant
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@cindex question mark in character constant
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Since characters are really integers, the printed representation of
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a character is a decimal number. This is also a possible read syntax
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for a character, but writing characters that way in Lisp programs is
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not clear programming. You should @emph{always} use the special read
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syntax formats that Emacs Lisp provides for characters. These syntax
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formats start with a question mark.
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The usual read syntax for alphanumeric characters is a question mark
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followed by the character; thus, @samp{?A} for the character
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@kbd{A}, @samp{?B} for the character @kbd{B}, and @samp{?a} for the
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character @kbd{a}.
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For example:
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@example
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?Q @result{} 81 ?q @result{} 113
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@end example
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You can use the same syntax for punctuation characters. However, if
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the punctuation character has a special syntactic meaning in Lisp, you
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must quote it with a @samp{\}. For example, @samp{?\(} is the way to
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write the open-paren character. Likewise, if the character is
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@samp{\}, you must use a second @samp{\} to quote it: @samp{?\\}.
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@cindex whitespace
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@cindex bell character
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@cindex @samp{\a}
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@cindex backspace
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@cindex @samp{\b}
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@cindex tab (ASCII character)
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@cindex @samp{\t}
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@cindex vertical tab
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@cindex @samp{\v}
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@cindex formfeed
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@cindex @samp{\f}
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@cindex newline
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@cindex @samp{\n}
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@cindex return (ASCII character)
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@cindex @samp{\r}
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@cindex escape (ASCII character)
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@cindex @samp{\e}
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@cindex space (ASCII character)
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@cindex @samp{\s}
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You can express the characters control-g, backspace, tab, newline,
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vertical tab, formfeed, space, return, del, and escape as @samp{?\a},
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@samp{?\b}, @samp{?\t}, @samp{?\n}, @samp{?\v}, @samp{?\f},
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@samp{?\s}, @samp{?\r}, @samp{?\d}, and @samp{?\e}, respectively.
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(@samp{?\s} followed by a dash has a different meaning---it applies
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the Super modifier to the following character.) Thus,
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@example
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?\a @result{} 7 ; @r{control-g, @kbd{C-g}}
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?\b @result{} 8 ; @r{backspace, @key{BS}, @kbd{C-h}}
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?\t @result{} 9 ; @r{tab, @key{TAB}, @kbd{C-i}}
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?\n @result{} 10 ; @r{newline, @kbd{C-j}}
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?\v @result{} 11 ; @r{vertical tab, @kbd{C-k}}
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?\f @result{} 12 ; @r{formfeed character, @kbd{C-l}}
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?\r @result{} 13 ; @r{carriage return, @key{RET}, @kbd{C-m}}
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?\e @result{} 27 ; @r{escape character, @key{ESC}, @kbd{C-[}}
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?\s @result{} 32 ; @r{space character, @key{SPC}}
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?\\ @result{} 92 ; @r{backslash character, @kbd{\}}
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?\d @result{} 127 ; @r{delete character, @key{DEL}}
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@end example
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@cindex escape sequence
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These sequences which start with backslash are also known as
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@dfn{escape sequences}, because backslash plays the role of an
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escape character; this has nothing to do with the
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character @key{ESC}. @samp{\s} is meant for use in character
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constants; in string constants, just write the space.
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A backslash is allowed, and harmless, preceding any character
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without a special escape meaning; thus, @samp{?\+} is equivalent to
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@samp{?+}. There is no reason to add a backslash before most
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characters. However, you must add a backslash before any of the
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characters @samp{()[]\;"}, and you should add a backslash before any
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of the characters @samp{|'`#.,} to avoid confusing the Emacs commands
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for editing Lisp code. You can also add a backslash before whitespace
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characters such as space, tab, newline and formfeed. However, it is
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cleaner to use one of the easily readable escape sequences, such as
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@samp{\t} or @samp{\s}, instead of an actual whitespace character such
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as a tab or a space. (If you do write backslash followed by a space,
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you should write an extra space after the character constant to
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separate it from the following text.)
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@node General Escape Syntax
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@subsubsection General Escape Syntax
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In addition to the specific escape sequences for special important
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control characters, Emacs provides several types of escape syntax that
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you can use to specify non-@acronym{ASCII} text characters.
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@enumerate
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@item
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@cindex @samp{\} in character constant
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@cindex backslash in character constants
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@cindex unicode character escape
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You can specify characters by their Unicode names, if any.
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@code{?\N@{@var{NAME}@}} represents the Unicode character named
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@var{NAME}. Thus, @samp{?\N@{LATIN SMALL LETTER A WITH GRAVE@}} is
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equivalent to @code{?à} and denotes the Unicode character U+00E0. To
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simplify entering multi-line strings, you can replace spaces in the
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names by non-empty sequences of whitespace (e.g., newlines).
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@item
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You can specify characters by their Unicode values.
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@code{?\N@{U+@var{X}@}} represents a character with Unicode code point
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@var{X}, where @var{X} is a hexadecimal number. Also,
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@code{?\u@var{xxxx}} and @code{?\U@var{xxxxxxxx}} represent code
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points @var{xxxx} and @var{xxxxxxxx}, respectively, where each @var{x}
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is a single hexadecimal digit. For example, @code{?\N@{U+E0@}},
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@code{?\u00e0} and @code{?\U000000E0} are all equivalent to @code{?à}
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and to @samp{?\N@{LATIN SMALL LETTER A WITH GRAVE@}}. The Unicode
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Standard defines code points only up to @samp{U+@var{10ffff}}, so if
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you specify a code point higher than that, Emacs signals an error.
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@item
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You can specify characters by their hexadecimal character
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codes. A hexadecimal escape sequence consists of a backslash,
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@samp{x}, and the hexadecimal character code. Thus, @samp{?\x41} is
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the character @kbd{A}, @samp{?\x1} is the character @kbd{C-a}, and
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@code{?\xe0} is the character @kbd{à} (@kbd{a} with grave accent).
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You can use any number of hex digits, so you can represent any
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character code in this way.
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@item
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@cindex octal character code
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You can specify characters by their character code in
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octal. An octal escape sequence consists of a backslash followed by
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up to three octal digits; thus, @samp{?\101} for the character
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@kbd{A}, @samp{?\001} for the character @kbd{C-a}, and @code{?\002}
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for the character @kbd{C-b}. Only characters up to octal code 777 can
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be specified this way.
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@end enumerate
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These escape sequences may also be used in strings. @xref{Non-ASCII
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in Strings}.
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@node Ctl-Char Syntax
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@subsubsection Control-Character Syntax
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@cindex control characters
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Control characters can be represented using yet another read syntax.
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This consists of a question mark followed by a backslash, caret, and the
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corresponding non-control character, in either upper or lower case. For
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example, both @samp{?\^I} and @samp{?\^i} are valid read syntax for the
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character @kbd{C-i}, the character whose value is 9.
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Instead of the @samp{^}, you can use @samp{C-}; thus, @samp{?\C-i} is
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equivalent to @samp{?\^I} and to @samp{?\^i}:
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@example
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?\^I @result{} 9 ?\C-I @result{} 9
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@end example
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In strings and buffers, the only control characters allowed are those
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that exist in @acronym{ASCII}; but for keyboard input purposes, you can turn
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any character into a control character with @samp{C-}. The character
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codes for these non-@acronym{ASCII} control characters include the
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@tex
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@math{2^{26}}
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@end tex
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@ifnottex
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2**26
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@end ifnottex
|
|
bit as well as the code for the corresponding non-control character.
|
|
Ordinary text terminals have no way of generating non-@acronym{ASCII}
|
|
control characters, but you can generate them straightforwardly using
|
|
X and other window systems.
|
|
|
|
For historical reasons, Emacs treats the @key{DEL} character as
|
|
the control equivalent of @kbd{?}:
|
|
|
|
@example
|
|
?\^? @result{} 127 ?\C-? @result{} 127
|
|
@end example
|
|
|
|
@noindent
|
|
As a result, it is currently not possible to represent the character
|
|
@kbd{Control-?}, which is a meaningful input character under X, using
|
|
@samp{\C-}. It is not easy to change this, as various Lisp files refer
|
|
to @key{DEL} in this way.
|
|
|
|
For representing control characters to be found in files or strings,
|
|
we recommend the @samp{^} syntax; for control characters in keyboard
|
|
input, we prefer the @samp{C-} syntax. Which one you use does not
|
|
affect the meaning of the program, but may guide the understanding of
|
|
people who read it.
|
|
|
|
@node Meta-Char Syntax
|
|
@subsubsection Meta-Character Syntax
|
|
|
|
@cindex meta characters
|
|
A @dfn{meta character} is a character typed with the @key{META}
|
|
modifier key. The integer that represents such a character has the
|
|
@tex
|
|
@math{2^{27}}
|
|
@end tex
|
|
@ifnottex
|
|
2**27
|
|
@end ifnottex
|
|
bit set. We use high bits for this and other modifiers to make
|
|
possible a wide range of basic character codes.
|
|
|
|
In a string, the
|
|
@tex
|
|
@math{2^{7}}
|
|
@end tex
|
|
@ifnottex
|
|
2**7
|
|
@end ifnottex
|
|
bit attached to an @acronym{ASCII} character indicates a meta
|
|
character; thus, the meta characters that can fit in a string have
|
|
codes in the range from 128 to 255, and are the meta versions of the
|
|
ordinary @acronym{ASCII} characters. @xref{Strings of Events}, for
|
|
details about @key{META}-handling in strings.
|
|
|
|
The read syntax for meta characters uses @samp{\M-}. For example,
|
|
@samp{?\M-A} stands for @kbd{M-A}. You can use @samp{\M-} together with
|
|
octal character codes (see below), with @samp{\C-}, or with any other
|
|
syntax for a character. Thus, you can write @kbd{M-A} as @samp{?\M-A},
|
|
or as @samp{?\M-\101}. Likewise, you can write @kbd{C-M-b} as
|
|
@samp{?\M-\C-b}, @samp{?\C-\M-b}, or @samp{?\M-\002}.
|
|
|
|
@node Other Char Bits
|
|
@subsubsection Other Character Modifier Bits
|
|
|
|
The case of a graphic character is indicated by its character code;
|
|
for example, @acronym{ASCII} distinguishes between the characters @samp{a}
|
|
and @samp{A}. But @acronym{ASCII} has no way to represent whether a control
|
|
character is upper case or lower case. Emacs uses the
|
|
@tex
|
|
@math{2^{25}}
|
|
@end tex
|
|
@ifnottex
|
|
2**25
|
|
@end ifnottex
|
|
bit to indicate that the shift key was used in typing a control
|
|
character. This distinction is possible only when you use X terminals
|
|
or other special terminals; ordinary text terminals do not report the
|
|
distinction. The Lisp syntax for the shift bit is @samp{\S-}; thus,
|
|
@samp{?\C-\S-o} or @samp{?\C-\S-O} represents the shifted-control-o
|
|
character.
|
|
|
|
@cindex hyper characters
|
|
@cindex super characters
|
|
@cindex alt characters
|
|
The X Window System defines three other
|
|
@anchor{modifier bits}modifier bits that can be set
|
|
in a character: @dfn{hyper}, @dfn{super} and @dfn{alt}. The syntaxes
|
|
for these bits are @samp{\H-}, @samp{\s-} and @samp{\A-}. (Case is
|
|
significant in these prefixes.) Thus, @samp{?\H-\M-\A-x} represents
|
|
@kbd{Alt-Hyper-Meta-x}. (Note that @samp{\s} with no following @samp{-}
|
|
represents the space character.)
|
|
@tex
|
|
Numerically, the bit values are @math{2^{22}} for alt, @math{2^{23}}
|
|
for super and @math{2^{24}} for hyper.
|
|
@end tex
|
|
@ifnottex
|
|
Numerically, the
|
|
bit values are 2**22 for alt, 2**23 for super and 2**24 for hyper.
|
|
@end ifnottex
|
|
|
|
@node Symbol Type
|
|
@subsection Symbol Type
|
|
|
|
A @dfn{symbol} in GNU Emacs Lisp is an object with a name. The
|
|
symbol name serves as the printed representation of the symbol. In
|
|
ordinary Lisp use, with one single obarray (@pxref{Creating Symbols}),
|
|
a symbol's name is unique---no two symbols have the same name.
|
|
|
|
A symbol can serve as a variable, as a function name, or to hold a
|
|
property list. Or it may serve only to be distinct from all other Lisp
|
|
objects, so that its presence in a data structure may be recognized
|
|
reliably. In a given context, usually only one of these uses is
|
|
intended. But you can use one symbol in all of these ways,
|
|
independently.
|
|
|
|
A symbol whose name starts with a colon (@samp{:}) is called a
|
|
@dfn{keyword symbol}. These symbols automatically act as constants,
|
|
and are normally used only by comparing an unknown symbol with a few
|
|
specific alternatives. @xref{Constant Variables}.
|
|
|
|
@cindex @samp{\} in symbols
|
|
@cindex backslash in symbols
|
|
A symbol name can contain any characters whatever. Most symbol names
|
|
are written with letters, digits, and the punctuation characters
|
|
@samp{-+=*/}. Such names require no special punctuation; the characters
|
|
of the name suffice as long as the name does not look like a number.
|
|
(If it does, write a @samp{\} at the beginning of the name to force
|
|
interpretation as a symbol.) The characters @samp{_~!@@$%^&:<>@{@}?} are
|
|
less often used but also require no special punctuation. Any other
|
|
characters may be included in a symbol's name by escaping them with a
|
|
backslash. In contrast to its use in strings, however, a backslash in
|
|
the name of a symbol simply quotes the single character that follows the
|
|
backslash. For example, in a string, @samp{\t} represents a tab
|
|
character; in the name of a symbol, however, @samp{\t} merely quotes the
|
|
letter @samp{t}. To have a symbol with a tab character in its name, you
|
|
must actually use a tab (preceded with a backslash). But it's rare to
|
|
do such a thing.
|
|
|
|
@cindex CL note---case of letters
|
|
@quotation
|
|
@b{Common Lisp note:} In Common Lisp, lower case letters are always
|
|
folded to upper case, unless they are explicitly escaped. In Emacs
|
|
Lisp, upper case and lower case letters are distinct.
|
|
@end quotation
|
|
|
|
Here are several examples of symbol names. Note that the @samp{+} in
|
|
the fourth example is escaped to prevent it from being read as a number.
|
|
This is not necessary in the sixth example because the rest of the name
|
|
makes it invalid as a number.
|
|
|
|
@example
|
|
@group
|
|
foo ; @r{A symbol named @samp{foo}.}
|
|
FOO ; @r{A symbol named @samp{FOO}, different from @samp{foo}.}
|
|
@end group
|
|
@group
|
|
1+ ; @r{A symbol named @samp{1+}}
|
|
; @r{(not @samp{+1}, which is an integer).}
|
|
@end group
|
|
@group
|
|
\+1 ; @r{A symbol named @samp{+1}}
|
|
; @r{(not a very readable name).}
|
|
@end group
|
|
@group
|
|
\(*\ 1\ 2\) ; @r{A symbol named @samp{(* 1 2)} (a worse name).}
|
|
@c the @'s in this next line use up three characters, hence the
|
|
@c apparent misalignment of the comment.
|
|
+-*/_~!@@$%^&=:<>@{@} ; @r{A symbol named @samp{+-*/_~!@@$%^&=:<>@{@}}.}
|
|
; @r{These characters need not be escaped.}
|
|
@end group
|
|
@end example
|
|
|
|
@cindex @samp{##} read syntax
|
|
@ifinfo
|
|
@c This uses "colon" instead of a literal ':' because Info cannot
|
|
@c cope with a ':' in a menu.
|
|
@cindex @samp{#@var{colon}} read syntax
|
|
@end ifinfo
|
|
@ifnotinfo
|
|
@cindex @samp{#:} read syntax
|
|
@end ifnotinfo
|
|
As an exception to the rule that a symbol's name serves as its
|
|
printed representation, @samp{##} is the printed representation for an
|
|
interned symbol whose name is an empty string. Furthermore,
|
|
@samp{#:@var{foo}} is the printed representation for an uninterned
|
|
symbol whose name is @var{foo}. (Normally, the Lisp reader interns
|
|
all symbols; @pxref{Creating Symbols}.)
|
|
|
|
@node Sequence Type
|
|
@subsection Sequence Types
|
|
|
|
A @dfn{sequence} is a Lisp object that represents an ordered set of
|
|
elements. There are two kinds of sequence in Emacs Lisp: @dfn{lists}
|
|
and @dfn{arrays}.
|
|
|
|
Lists are the most commonly-used sequences. A list can hold
|
|
elements of any type, and its length can be easily changed by adding
|
|
or removing elements. See the next subsection for more about lists.
|
|
|
|
Arrays are fixed-length sequences. They are further subdivided into
|
|
strings, vectors, char-tables and bool-vectors. Vectors can hold
|
|
elements of any type, whereas string elements must be characters, and
|
|
bool-vector elements must be @code{t} or @code{nil}. Char-tables are
|
|
like vectors except that they are indexed by any valid character code.
|
|
The characters in a string can have text properties like characters in
|
|
a buffer (@pxref{Text Properties}), but vectors do not support text
|
|
properties, even when their elements happen to be characters.
|
|
|
|
Lists, strings and the other array types also share important
|
|
similarities. For example, all have a length @var{l}, and all have
|
|
elements which can be indexed from zero to @var{l} minus one. Several
|
|
functions, called sequence functions, accept any kind of sequence.
|
|
For example, the function @code{length} reports the length of any kind
|
|
of sequence. @xref{Sequences Arrays Vectors}.
|
|
|
|
It is generally impossible to read the same sequence twice, since
|
|
sequences are always created anew upon reading. If you read the read
|
|
syntax for a sequence twice, you get two sequences with equal contents.
|
|
There is one exception: the empty list @code{()} always stands for the
|
|
same object, @code{nil}.
|
|
|
|
@node Cons Cell Type
|
|
@subsection Cons Cell and List Types
|
|
@cindex address field of register
|
|
@cindex decrement field of register
|
|
@cindex pointers
|
|
|
|
A @dfn{cons cell} is an object that consists of two slots, called
|
|
the @sc{car} slot and the @sc{cdr} slot. Each slot can @dfn{hold} any
|
|
Lisp object. We also say that the @sc{car} of this cons cell is
|
|
whatever object its @sc{car} slot currently holds, and likewise for
|
|
the @sc{cdr}.
|
|
|
|
@cindex list structure
|
|
A @dfn{list} is a series of cons cells, linked together so that the
|
|
@sc{cdr} slot of each cons cell holds either the next cons cell or the
|
|
empty list. The empty list is actually the symbol @code{nil}.
|
|
@xref{Lists}, for details. Because most cons cells are used as part
|
|
of lists, we refer to any structure made out of cons cells as a
|
|
@dfn{list structure}.
|
|
|
|
@cindex linked list
|
|
@quotation
|
|
A note to C programmers: a Lisp list thus works as a @dfn{linked list}
|
|
built up of cons cells. Because pointers in Lisp are implicit, we do
|
|
not distinguish between a cons cell slot holding a value versus
|
|
pointing to the value.
|
|
@end quotation
|
|
|
|
@cindex atoms
|
|
Because cons cells are so central to Lisp, we also have a word for
|
|
an object which is not a cons cell. These objects are called
|
|
@dfn{atoms}.
|
|
|
|
@cindex parenthesis
|
|
@cindex @samp{(@dots{})} in lists
|
|
The read syntax and printed representation for lists are identical, and
|
|
consist of a left parenthesis, an arbitrary number of elements, and a
|
|
right parenthesis. Here are examples of lists:
|
|
|
|
@example
|
|
(A 2 "A") ; @r{A list of three elements.}
|
|
() ; @r{A list of no elements (the empty list).}
|
|
nil ; @r{A list of no elements (the empty list).}
|
|
("A ()") ; @r{A list of one element: the string @code{"A ()"}.}
|
|
(A ()) ; @r{A list of two elements: @code{A} and the empty list.}
|
|
(A nil) ; @r{Equivalent to the previous.}
|
|
((A B C)) ; @r{A list of one element}
|
|
; @r{(which is a list of three elements).}
|
|
@end example
|
|
|
|
Upon reading, each object inside the parentheses becomes an element
|
|
of the list. That is, a cons cell is made for each element. The
|
|
@sc{car} slot of the cons cell holds the element, and its @sc{cdr}
|
|
slot refers to the next cons cell of the list, which holds the next
|
|
element in the list. The @sc{cdr} slot of the last cons cell is set to
|
|
hold @code{nil}.
|
|
|
|
The names @sc{car} and @sc{cdr} derive from the history of Lisp. The
|
|
original Lisp implementation ran on an @w{IBM 704} computer which
|
|
divided words into two parts, the address and the
|
|
decrement; @sc{car} was an instruction to extract the contents of
|
|
the address part of a register, and @sc{cdr} an instruction to extract
|
|
the contents of the decrement. By contrast, cons cells are named
|
|
for the function @code{cons} that creates them, which in turn was named
|
|
for its purpose, the construction of cells.
|
|
|
|
@menu
|
|
* Box Diagrams:: Drawing pictures of lists.
|
|
* Dotted Pair Notation:: A general syntax for cons cells.
|
|
* Association List Type:: A specially constructed list.
|
|
@end menu
|
|
|
|
@node Box Diagrams
|
|
@subsubsection Drawing Lists as Box Diagrams
|
|
@cindex box diagrams, for lists
|
|
@cindex diagrams, boxed, for lists
|
|
|
|
A list can be illustrated by a diagram in which the cons cells are
|
|
shown as pairs of boxes, like dominoes. (The Lisp reader cannot read
|
|
such an illustration; unlike the textual notation, which can be
|
|
understood by both humans and computers, the box illustrations can be
|
|
understood only by humans.) This picture represents the three-element
|
|
list @code{(rose violet buttercup)}:
|
|
|
|
@example
|
|
@group
|
|
--- --- --- --- --- ---
|
|
| | |--> | | |--> | | |--> nil
|
|
--- --- --- --- --- ---
|
|
| | |
|
|
| | |
|
|
--> rose --> violet --> buttercup
|
|
@end group
|
|
@end example
|
|
|
|
In this diagram, each box represents a slot that can hold or refer to
|
|
any Lisp object. Each pair of boxes represents a cons cell. Each arrow
|
|
represents a reference to a Lisp object, either an atom or another cons
|
|
cell.
|
|
|
|
In this example, the first box, which holds the @sc{car} of the first
|
|
cons cell, refers to or holds @code{rose} (a symbol). The second
|
|
box, holding the @sc{cdr} of the first cons cell, refers to the next
|
|
pair of boxes, the second cons cell. The @sc{car} of the second cons
|
|
cell is @code{violet}, and its @sc{cdr} is the third cons cell. The
|
|
@sc{cdr} of the third (and last) cons cell is @code{nil}.
|
|
|
|
Here is another diagram of the same list, @code{(rose violet
|
|
buttercup)}, sketched in a different manner:
|
|
|
|
@smallexample
|
|
@group
|
|
--------------- ---------------- -------------------
|
|
| car | cdr | | car | cdr | | car | cdr |
|
|
| rose | o-------->| violet | o-------->| buttercup | nil |
|
|
| | | | | | | | |
|
|
--------------- ---------------- -------------------
|
|
@end group
|
|
@end smallexample
|
|
|
|
@cindex @code{nil} as a list
|
|
@cindex empty list
|
|
A list with no elements in it is the @dfn{empty list}; it is identical
|
|
to the symbol @code{nil}. In other words, @code{nil} is both a symbol
|
|
and a list.
|
|
|
|
Here is the list @code{(A ())}, or equivalently @code{(A nil)},
|
|
depicted with boxes and arrows:
|
|
|
|
@example
|
|
@group
|
|
--- --- --- ---
|
|
| | |--> | | |--> nil
|
|
--- --- --- ---
|
|
| |
|
|
| |
|
|
--> A --> nil
|
|
@end group
|
|
@end example
|
|
|
|
Here is a more complex illustration, showing the three-element list,
|
|
@code{((pine needles) oak maple)}, the first element of which is a
|
|
two-element list:
|
|
|
|
@example
|
|
@group
|
|
--- --- --- --- --- ---
|
|
| | |--> | | |--> | | |--> nil
|
|
--- --- --- --- --- ---
|
|
| | |
|
|
| | |
|
|
| --> oak --> maple
|
|
|
|
|
| --- --- --- ---
|
|
--> | | |--> | | |--> nil
|
|
--- --- --- ---
|
|
| |
|
|
| |
|
|
--> pine --> needles
|
|
@end group
|
|
@end example
|
|
|
|
The same list represented in the second box notation looks like this:
|
|
|
|
@example
|
|
@group
|
|
-------------- -------------- --------------
|
|
| car | cdr | | car | cdr | | car | cdr |
|
|
| o | o------->| oak | o------->| maple | nil |
|
|
| | | | | | | | | |
|
|
-- | --------- -------------- --------------
|
|
|
|
|
|
|
|
| -------------- ----------------
|
|
| | car | cdr | | car | cdr |
|
|
------>| pine | o------->| needles | nil |
|
|
| | | | | |
|
|
-------------- ----------------
|
|
@end group
|
|
@end example
|
|
|
|
@node Dotted Pair Notation
|
|
@subsubsection Dotted Pair Notation
|
|
@cindex dotted pair notation
|
|
@cindex @samp{.} in lists
|
|
|
|
@dfn{Dotted pair notation} is a general syntax for cons cells that
|
|
represents the @sc{car} and @sc{cdr} explicitly. In this syntax,
|
|
@code{(@var{a} .@: @var{b})} stands for a cons cell whose @sc{car} is
|
|
the object @var{a} and whose @sc{cdr} is the object @var{b}. Dotted
|
|
pair notation is more general than list syntax because the @sc{cdr}
|
|
does not have to be a list. However, it is more cumbersome in cases
|
|
where list syntax would work. In dotted pair notation, the list
|
|
@samp{(1 2 3)} is written as @samp{(1 . (2 . (3 . nil)))}. For
|
|
@code{nil}-terminated lists, you can use either notation, but list
|
|
notation is usually clearer and more convenient. When printing a
|
|
list, the dotted pair notation is only used if the @sc{cdr} of a cons
|
|
cell is not a list.
|
|
|
|
Here's an example using boxes to illustrate dotted pair notation.
|
|
This example shows the pair @code{(rose . violet)}:
|
|
|
|
@example
|
|
@group
|
|
--- ---
|
|
| | |--> violet
|
|
--- ---
|
|
|
|
|
|
|
|
--> rose
|
|
@end group
|
|
@end example
|
|
|
|
You can combine dotted pair notation with list notation to represent
|
|
conveniently a chain of cons cells with a non-@code{nil} final @sc{cdr}.
|
|
You write a dot after the last element of the list, followed by the
|
|
@sc{cdr} of the final cons cell. For example, @code{(rose violet
|
|
. buttercup)} is equivalent to @code{(rose . (violet . buttercup))}.
|
|
The object looks like this:
|
|
|
|
@example
|
|
@group
|
|
--- --- --- ---
|
|
| | |--> | | |--> buttercup
|
|
--- --- --- ---
|
|
| |
|
|
| |
|
|
--> rose --> violet
|
|
@end group
|
|
@end example
|
|
|
|
The syntax @code{(rose .@: violet .@: buttercup)} is invalid because
|
|
there is nothing that it could mean. If anything, it would say to put
|
|
@code{buttercup} in the @sc{cdr} of a cons cell whose @sc{cdr} is already
|
|
used for @code{violet}.
|
|
|
|
The list @code{(rose violet)} is equivalent to @code{(rose . (violet))},
|
|
and looks like this:
|
|
|
|
@example
|
|
@group
|
|
--- --- --- ---
|
|
| | |--> | | |--> nil
|
|
--- --- --- ---
|
|
| |
|
|
| |
|
|
--> rose --> violet
|
|
@end group
|
|
@end example
|
|
|
|
Similarly, the three-element list @code{(rose violet buttercup)}
|
|
is equivalent to @code{(rose . (violet . (buttercup)))}.
|
|
@ifnottex
|
|
It looks like this:
|
|
|
|
@example
|
|
@group
|
|
--- --- --- --- --- ---
|
|
| | |--> | | |--> | | |--> nil
|
|
--- --- --- --- --- ---
|
|
| | |
|
|
| | |
|
|
--> rose --> violet --> buttercup
|
|
@end group
|
|
@end example
|
|
@end ifnottex
|
|
|
|
@node Association List Type
|
|
@subsubsection Association List Type
|
|
|
|
An @dfn{association list} or @dfn{alist} is a specially-constructed
|
|
list whose elements are cons cells. In each element, the @sc{car} is
|
|
considered a @dfn{key}, and the @sc{cdr} is considered an
|
|
@dfn{associated value}. (In some cases, the associated value is stored
|
|
in the @sc{car} of the @sc{cdr}.) Association lists are often used as
|
|
stacks, since it is easy to add or remove associations at the front of
|
|
the list.
|
|
|
|
For example,
|
|
|
|
@example
|
|
(setq alist-of-colors
|
|
'((rose . red) (lily . white) (buttercup . yellow)))
|
|
@end example
|
|
|
|
@noindent
|
|
sets the variable @code{alist-of-colors} to an alist of three elements. In the
|
|
first element, @code{rose} is the key and @code{red} is the value.
|
|
|
|
@xref{Association Lists}, for a further explanation of alists and for
|
|
functions that work on alists. @xref{Hash Tables}, for another kind of
|
|
lookup table, which is much faster for handling a large number of keys.
|
|
|
|
@node Array Type
|
|
@subsection Array Type
|
|
|
|
An @dfn{array} is composed of an arbitrary number of slots for
|
|
holding or referring to other Lisp objects, arranged in a contiguous block of
|
|
memory. Accessing any element of an array takes approximately the same
|
|
amount of time. In contrast, accessing an element of a list requires
|
|
time proportional to the position of the element in the list. (Elements
|
|
at the end of a list take longer to access than elements at the
|
|
beginning of a list.)
|
|
|
|
Emacs defines four types of array: strings, vectors, bool-vectors, and
|
|
char-tables.
|
|
|
|
A string is an array of characters and a vector is an array of
|
|
arbitrary objects. A bool-vector can hold only @code{t} or @code{nil}.
|
|
These kinds of array may have any length up to the largest integer.
|
|
Char-tables are sparse arrays indexed by any valid character code; they
|
|
can hold arbitrary objects.
|
|
|
|
The first element of an array has index zero, the second element has
|
|
index 1, and so on. This is called @dfn{zero-origin} indexing. For
|
|
example, an array of four elements has indices 0, 1, 2, @w{and 3}. The
|
|
largest possible index value is one less than the length of the array.
|
|
Once an array is created, its length is fixed.
|
|
|
|
All Emacs Lisp arrays are one-dimensional. (Most other programming
|
|
languages support multidimensional arrays, but they are not essential;
|
|
you can get the same effect with nested one-dimensional arrays.) Each
|
|
type of array has its own read syntax; see the following sections for
|
|
details.
|
|
|
|
The array type is a subset of the sequence type, and contains the
|
|
string type, the vector type, the bool-vector type, and the char-table
|
|
type.
|
|
|
|
@node String Type
|
|
@subsection String Type
|
|
|
|
A @dfn{string} is an array of characters. Strings are used for many
|
|
purposes in Emacs, as can be expected in a text editor; for example, as
|
|
the names of Lisp symbols, as messages for the user, and to represent
|
|
text extracted from buffers. Strings in Lisp are constants: evaluation
|
|
of a string returns the same string.
|
|
|
|
@xref{Strings and Characters}, for functions that operate on strings.
|
|
|
|
@menu
|
|
* Syntax for Strings:: How to specify Lisp strings.
|
|
* Non-ASCII in Strings:: International characters in strings.
|
|
* Nonprinting Characters:: Literal unprintable characters in strings.
|
|
* Text Props and Strings:: Strings with text properties.
|
|
@end menu
|
|
|
|
@node Syntax for Strings
|
|
@subsubsection Syntax for Strings
|
|
|
|
@cindex @samp{"} in strings
|
|
@cindex double-quote in strings
|
|
@cindex @samp{\} in strings
|
|
@cindex backslash in strings
|
|
The read syntax for a string is a double-quote, an arbitrary number
|
|
of characters, and another double-quote, @code{"like this"}. To
|
|
include a double-quote in a string, precede it with a backslash; thus,
|
|
@code{"\""} is a string containing just one double-quote
|
|
character. Likewise, you can include a backslash by preceding it with
|
|
another backslash, like this: @code{"this \\ is a single embedded
|
|
backslash"}.
|
|
|
|
@cindex newline in strings
|
|
The newline character is not special in the read syntax for strings;
|
|
if you write a new line between the double-quotes, it becomes a
|
|
character in the string. But an escaped newline---one that is preceded
|
|
by @samp{\}---does not become part of the string; i.e., the Lisp reader
|
|
ignores an escaped newline while reading a string. An escaped space
|
|
@w{@samp{\ }} is likewise ignored.
|
|
|
|
@example
|
|
"It is useful to include newlines
|
|
in documentation strings,
|
|
but the newline is \
|
|
ignored if escaped."
|
|
@result{} "It is useful to include newlines
|
|
in documentation strings,
|
|
but the newline is ignored if escaped."
|
|
@end example
|
|
|
|
@node Non-ASCII in Strings
|
|
@subsubsection Non-@acronym{ASCII} Characters in Strings
|
|
|
|
There are two text representations for non-@acronym{ASCII}
|
|
characters in Emacs strings: multibyte and unibyte (@pxref{Text
|
|
Representations}). Roughly speaking, unibyte strings store raw bytes,
|
|
while multibyte strings store human-readable text. Each character in
|
|
a unibyte string is a byte, i.e., its value is between 0 and 255. By
|
|
contrast, each character in a multibyte string may have a value
|
|
between 0 to 4194303 (@pxref{Character Type}). In both cases,
|
|
characters above 127 are non-@acronym{ASCII}.
|
|
|
|
You can include a non-@acronym{ASCII} character in a string constant
|
|
by writing it literally. If the string constant is read from a
|
|
multibyte source, such as a multibyte buffer or string, or a file that
|
|
would be visited as multibyte, then Emacs reads each
|
|
non-@acronym{ASCII} character as a multibyte character and
|
|
automatically makes the string a multibyte string. If the string
|
|
constant is read from a unibyte source, then Emacs reads the
|
|
non-@acronym{ASCII} character as unibyte, and makes the string
|
|
unibyte.
|
|
|
|
Instead of writing a character literally into a multibyte string,
|
|
you can write it as its character code using an escape sequence.
|
|
@xref{General Escape Syntax}, for details about escape sequences.
|
|
|
|
If you use any Unicode-style escape sequence @samp{\uNNNN} or
|
|
@samp{\U00NNNNNN} in a string constant (even for an @acronym{ASCII}
|
|
character), Emacs automatically assumes that it is multibyte.
|
|
|
|
You can also use hexadecimal escape sequences (@samp{\x@var{n}}) and
|
|
octal escape sequences (@samp{\@var{n}}) in string constants.
|
|
@strong{But beware:} If a string constant contains hexadecimal or
|
|
octal escape sequences, and these escape sequences all specify unibyte
|
|
characters (i.e., less than 256), and there are no other literal
|
|
non-@acronym{ASCII} characters or Unicode-style escape sequences in
|
|
the string, then Emacs automatically assumes that it is a unibyte
|
|
string. That is to say, it assumes that all non-@acronym{ASCII}
|
|
characters occurring in the string are 8-bit raw bytes.
|
|
|
|
In hexadecimal and octal escape sequences, the escaped character
|
|
code may contain a variable number of digits, so the first subsequent
|
|
character which is not a valid hexadecimal or octal digit terminates
|
|
the escape sequence. If the next character in a string could be
|
|
interpreted as a hexadecimal or octal digit, write @w{@samp{\ }}
|
|
(backslash and space) to terminate the escape sequence. For example,
|
|
@w{@samp{\xe0\ }} represents one character, @samp{a} with grave
|
|
accent. @w{@samp{\ }} in a string constant is just like
|
|
backslash-newline; it does not contribute any character to the string,
|
|
but it does terminate any preceding hex escape.
|
|
|
|
@node Nonprinting Characters
|
|
@subsubsection Nonprinting Characters in Strings
|
|
|
|
You can use the same backslash escape-sequences in a string constant
|
|
as in character literals (but do not use the question mark that begins a
|
|
character constant). For example, you can write a string containing the
|
|
nonprinting characters tab and @kbd{C-a}, with commas and spaces between
|
|
them, like this: @code{"\t, \C-a"}. @xref{Character Type}, for a
|
|
description of the read syntax for characters.
|
|
|
|
However, not all of the characters you can write with backslash
|
|
escape-sequences are valid in strings. The only control characters that
|
|
a string can hold are the @acronym{ASCII} control characters. Strings do not
|
|
distinguish case in @acronym{ASCII} control characters.
|
|
|
|
Properly speaking, strings cannot hold meta characters; but when a
|
|
string is to be used as a key sequence, there is a special convention
|
|
that provides a way to represent meta versions of @acronym{ASCII}
|
|
characters in a string. If you use the @samp{\M-} syntax to indicate
|
|
a meta character in a string constant, this sets the
|
|
@tex
|
|
@math{2^{7}}
|
|
@end tex
|
|
@ifnottex
|
|
2**7
|
|
@end ifnottex
|
|
bit of the character in the string. If the string is used in
|
|
@code{define-key} or @code{lookup-key}, this numeric code is translated
|
|
into the equivalent meta character. @xref{Character Type}.
|
|
|
|
Strings cannot hold characters that have the hyper, super, or alt
|
|
modifiers.
|
|
|
|
@node Text Props and Strings
|
|
@subsubsection Text Properties in Strings
|
|
|
|
@cindex @samp{#(} read syntax
|
|
@cindex text properties, read syntax
|
|
A string can hold properties for the characters it contains, in
|
|
addition to the characters themselves. This enables programs that copy
|
|
text between strings and buffers to copy the text's properties with no
|
|
special effort. @xref{Text Properties}, for an explanation of what text
|
|
properties mean. Strings with text properties use a special read and
|
|
print syntax:
|
|
|
|
@example
|
|
#("@var{characters}" @var{property-data}...)
|
|
@end example
|
|
|
|
@noindent
|
|
where @var{property-data} consists of zero or more elements, in groups
|
|
of three as follows:
|
|
|
|
@example
|
|
@var{beg} @var{end} @var{plist}
|
|
@end example
|
|
|
|
@noindent
|
|
The elements @var{beg} and @var{end} are integers, and together specify
|
|
a range of indices in the string; @var{plist} is the property list for
|
|
that range. For example,
|
|
|
|
@example
|
|
#("foo bar" 0 3 (face bold) 3 4 nil 4 7 (face italic))
|
|
@end example
|
|
|
|
@noindent
|
|
represents a string whose textual contents are @samp{foo bar}, in which
|
|
the first three characters have a @code{face} property with value
|
|
@code{bold}, and the last three have a @code{face} property with value
|
|
@code{italic}. (The fourth character has no text properties, so its
|
|
property list is @code{nil}. It is not actually necessary to mention
|
|
ranges with @code{nil} as the property list, since any characters not
|
|
mentioned in any range will default to having no properties.)
|
|
|
|
@node Vector Type
|
|
@subsection Vector Type
|
|
|
|
A @dfn{vector} is a one-dimensional array of elements of any type. It
|
|
takes a constant amount of time to access any element of a vector. (In
|
|
a list, the access time of an element is proportional to the distance of
|
|
the element from the beginning of the list.)
|
|
|
|
The printed representation of a vector consists of a left square
|
|
bracket, the elements, and a right square bracket. This is also the
|
|
read syntax. Like numbers and strings, vectors are considered constants
|
|
for evaluation.
|
|
|
|
@example
|
|
[1 "two" (three)] ; @r{A vector of three elements.}
|
|
@result{} [1 "two" (three)]
|
|
@end example
|
|
|
|
@xref{Vectors}, for functions that work with vectors.
|
|
|
|
@node Char-Table Type
|
|
@subsection Char-Table Type
|
|
|
|
A @dfn{char-table} is a one-dimensional array of elements of any type,
|
|
indexed by character codes. Char-tables have certain extra features to
|
|
make them more useful for many jobs that involve assigning information
|
|
to character codes---for example, a char-table can have a parent to
|
|
inherit from, a default value, and a small number of extra slots to use for
|
|
special purposes. A char-table can also specify a single value for
|
|
a whole character set.
|
|
|
|
@cindex @samp{#^} read syntax
|
|
The printed representation of a char-table is like a vector
|
|
except that there is an extra @samp{#^} at the beginning.@footnote{You
|
|
may also encounter @samp{#^^}, used for sub-char-tables.}
|
|
|
|
@xref{Char-Tables}, for special functions to operate on char-tables.
|
|
Uses of char-tables include:
|
|
|
|
@itemize @bullet
|
|
@item
|
|
Case tables (@pxref{Case Tables}).
|
|
|
|
@item
|
|
Character category tables (@pxref{Categories}).
|
|
|
|
@item
|
|
Display tables (@pxref{Display Tables}).
|
|
|
|
@item
|
|
Syntax tables (@pxref{Syntax Tables}).
|
|
@end itemize
|
|
|
|
@node Bool-Vector Type
|
|
@subsection Bool-Vector Type
|
|
|
|
A @dfn{bool-vector} is a one-dimensional array whose elements must
|
|
be @code{t} or @code{nil}.
|
|
|
|
The printed representation of a bool-vector is like a string, except
|
|
that it begins with @samp{#&} followed by the length. The string
|
|
constant that follows actually specifies the contents of the bool-vector
|
|
as a bitmap---each character in the string contains 8 bits, which
|
|
specify the next 8 elements of the bool-vector (1 stands for @code{t},
|
|
and 0 for @code{nil}). The least significant bits of the character
|
|
correspond to the lowest indices in the bool-vector.
|
|
|
|
@example
|
|
(make-bool-vector 3 t)
|
|
@result{} #&3"^G"
|
|
(make-bool-vector 3 nil)
|
|
@result{} #&3"^@@"
|
|
@end example
|
|
|
|
@noindent
|
|
These results make sense, because the binary code for @samp{C-g} is
|
|
111 and @samp{C-@@} is the character with code 0.
|
|
|
|
If the length is not a multiple of 8, the printed representation
|
|
shows extra elements, but these extras really make no difference. For
|
|
instance, in the next example, the two bool-vectors are equal, because
|
|
only the first 3 bits are used:
|
|
|
|
@example
|
|
(equal #&3"\377" #&3"\007")
|
|
@result{} t
|
|
@end example
|
|
|
|
@node Hash Table Type
|
|
@subsection Hash Table Type
|
|
|
|
A hash table is a very fast kind of lookup table, somewhat like an
|
|
alist in that it maps keys to corresponding values, but much faster.
|
|
The printed representation of a hash table specifies its properties
|
|
and contents, like this:
|
|
|
|
@example
|
|
(make-hash-table)
|
|
@result{} #s(hash-table size 65 test eql rehash-size 1.5
|
|
rehash-threshold 0.8125 data ())
|
|
@end example
|
|
|
|
@noindent
|
|
@xref{Hash Tables}, for more information about hash tables.
|
|
|
|
@node Function Type
|
|
@subsection Function Type
|
|
|
|
Lisp functions are executable code, just like functions in other
|
|
programming languages. In Lisp, unlike most languages, functions are
|
|
also Lisp objects. A non-compiled function in Lisp is a lambda
|
|
expression: that is, a list whose first element is the symbol
|
|
@code{lambda} (@pxref{Lambda Expressions}).
|
|
|
|
In most programming languages, it is impossible to have a function
|
|
without a name. In Lisp, a function has no intrinsic name. A lambda
|
|
expression can be called as a function even though it has no name; to
|
|
emphasize this, we also call it an @dfn{anonymous function}
|
|
(@pxref{Anonymous Functions}). A named function in Lisp is just a
|
|
symbol with a valid function in its function cell (@pxref{Defining
|
|
Functions}).
|
|
|
|
Most of the time, functions are called when their names are written in
|
|
Lisp expressions in Lisp programs. However, you can construct or obtain
|
|
a function object at run time and then call it with the primitive
|
|
functions @code{funcall} and @code{apply}. @xref{Calling Functions}.
|
|
|
|
@node Macro Type
|
|
@subsection Macro Type
|
|
|
|
A @dfn{Lisp macro} is a user-defined construct that extends the Lisp
|
|
language. It is represented as an object much like a function, but with
|
|
different argument-passing semantics. A Lisp macro has the form of a
|
|
list whose first element is the symbol @code{macro} and whose @sc{cdr}
|
|
is a Lisp function object, including the @code{lambda} symbol.
|
|
|
|
Lisp macro objects are usually defined with the built-in
|
|
@code{defmacro} macro, but any list that begins with @code{macro} is a
|
|
macro as far as Emacs is concerned. @xref{Macros}, for an explanation
|
|
of how to write a macro.
|
|
|
|
@strong{Warning}: Lisp macros and keyboard macros (@pxref{Keyboard
|
|
Macros}) are entirely different things. When we use the word ``macro''
|
|
without qualification, we mean a Lisp macro, not a keyboard macro.
|
|
|
|
@node Primitive Function Type
|
|
@subsection Primitive Function Type
|
|
@cindex primitive function
|
|
|
|
A @dfn{primitive function} is a function callable from Lisp but
|
|
written in the C programming language. Primitive functions are also
|
|
called @dfn{subrs} or @dfn{built-in functions}. (The word ``subr'' is
|
|
derived from ``subroutine''.) Most primitive functions evaluate all
|
|
their arguments when they are called. A primitive function that does
|
|
not evaluate all its arguments is called a @dfn{special form}
|
|
(@pxref{Special Forms}).
|
|
|
|
It does not matter to the caller of a function whether the function is
|
|
primitive. However, this does matter if you try to redefine a primitive
|
|
with a function written in Lisp. The reason is that the primitive
|
|
function may be called directly from C code. Calls to the redefined
|
|
function from Lisp will use the new definition, but calls from C code
|
|
may still use the built-in definition. Therefore, @strong{we discourage
|
|
redefinition of primitive functions}.
|
|
|
|
The term @dfn{function} refers to all Emacs functions, whether written
|
|
in Lisp or C@. @xref{Function Type}, for information about the
|
|
functions written in Lisp.
|
|
|
|
Primitive functions have no read syntax and print in hash notation
|
|
with the name of the subroutine.
|
|
|
|
@example
|
|
@group
|
|
(symbol-function 'car) ; @r{Access the function cell}
|
|
; @r{of the symbol.}
|
|
@result{} #<subr car>
|
|
(subrp (symbol-function 'car)) ; @r{Is this a primitive function?}
|
|
@result{} t ; @r{Yes.}
|
|
@end group
|
|
@end example
|
|
|
|
@node Byte-Code Type
|
|
@subsection Byte-Code Function Type
|
|
|
|
@dfn{Byte-code function objects} are produced by byte-compiling Lisp
|
|
code (@pxref{Byte Compilation}). Internally, a byte-code function
|
|
object is much like a vector; however, the evaluator handles this data
|
|
type specially when it appears in a function call. @xref{Byte-Code
|
|
Objects}.
|
|
|
|
The printed representation and read syntax for a byte-code function
|
|
object is like that for a vector, with an additional @samp{#} before the
|
|
opening @samp{[}.
|
|
|
|
@node Record Type
|
|
@subsection Record Type
|
|
|
|
A @dfn{record} is much like a @code{vector}. However, the first
|
|
element is used to hold its type as returned by @code{type-of}. The
|
|
purpose of records is to allow programmers to create objects with new
|
|
types that are not built into Emacs.
|
|
|
|
@xref{Records}, for functions that work with records.
|
|
|
|
@node Type Descriptors
|
|
@subsection Type Descriptors
|
|
|
|
A @dfn{type descriptor} is a @code{record} which holds information
|
|
about a type. Slot 1 in the record must be a symbol naming the type, and
|
|
@code{type-of} relies on this to return the type of @code{record}
|
|
objects. No other type descriptor slot is used by Emacs; they are
|
|
free for use by Lisp extensions.
|
|
|
|
An example of a type descriptor is any instance of
|
|
@code{cl-structure-class}.
|
|
|
|
@node Autoload Type
|
|
@subsection Autoload Type
|
|
|
|
An @dfn{autoload object} is a list whose first element is the symbol
|
|
@code{autoload}. It is stored as the function definition of a symbol,
|
|
where it serves as a placeholder for the real definition. The autoload
|
|
object says that the real definition is found in a file of Lisp code
|
|
that should be loaded when necessary. It contains the name of the file,
|
|
plus some other information about the real definition.
|
|
|
|
After the file has been loaded, the symbol should have a new function
|
|
definition that is not an autoload object. The new definition is then
|
|
called as if it had been there to begin with. From the user's point of
|
|
view, the function call works as expected, using the function definition
|
|
in the loaded file.
|
|
|
|
An autoload object is usually created with the function
|
|
@code{autoload}, which stores the object in the function cell of a
|
|
symbol. @xref{Autoload}, for more details.
|
|
|
|
@node Finalizer Type
|
|
@subsection Finalizer Type
|
|
|
|
A @dfn{finalizer object} helps Lisp code clean up after objects that
|
|
are no longer needed. A finalizer holds a Lisp function object.
|
|
When a finalizer object becomes unreachable after a garbage collection
|
|
pass, Emacs calls the finalizer's associated function object.
|
|
When deciding whether a finalizer is reachable, Emacs does not count
|
|
references from finalizer objects themselves, allowing you to use
|
|
finalizers without having to worry about accidentally capturing
|
|
references to finalized objects themselves.
|
|
|
|
Errors in finalizers are printed to @code{*Messages*}. Emacs runs
|
|
a given finalizer object's associated function exactly once, even
|
|
if that function fails.
|
|
|
|
@defun make-finalizer function
|
|
Make a finalizer that will run @var{function}. @var{function} will be
|
|
called after garbage collection when the returned finalizer object
|
|
becomes unreachable. If the finalizer object is reachable only
|
|
through references from finalizer objects, it does not count as
|
|
reachable for the purpose of deciding whether to run @var{function}.
|
|
@var{function} will be run once per finalizer object.
|
|
@end defun
|
|
|
|
@node Editing Types
|
|
@section Editing Types
|
|
@cindex editing types
|
|
|
|
The types in the previous section are used for general programming
|
|
purposes, and most of them are common to most Lisp dialects. Emacs Lisp
|
|
provides several additional data types for purposes connected with
|
|
editing.
|
|
|
|
@menu
|
|
* Buffer Type:: The basic object of editing.
|
|
* Marker Type:: A position in a buffer.
|
|
* Window Type:: Buffers are displayed in windows.
|
|
* Frame Type:: Windows subdivide frames.
|
|
* Terminal Type:: A terminal device displays frames.
|
|
* Window Configuration Type:: Recording the way a frame is subdivided.
|
|
* Frame Configuration Type:: Recording the status of all frames.
|
|
* Process Type:: A subprocess of Emacs running on the underlying OS.
|
|
* Thread Type:: A thread of Emacs Lisp execution.
|
|
* Mutex Type:: An exclusive lock for thread synchronization.
|
|
* Condition Variable Type:: Condition variable for thread synchronization.
|
|
* Stream Type:: Receive or send characters.
|
|
* Keymap Type:: What function a keystroke invokes.
|
|
* Overlay Type:: How an overlay is represented.
|
|
* Font Type:: Fonts for displaying text.
|
|
@end menu
|
|
|
|
@node Buffer Type
|
|
@subsection Buffer Type
|
|
|
|
A @dfn{buffer} is an object that holds text that can be edited
|
|
(@pxref{Buffers}). Most buffers hold the contents of a disk file
|
|
(@pxref{Files}) so they can be edited, but some are used for other
|
|
purposes. Most buffers are also meant to be seen by the user, and
|
|
therefore displayed, at some time, in a window (@pxref{Windows}). But
|
|
a buffer need not be displayed in any window. Each buffer has a
|
|
designated position called @dfn{point} (@pxref{Positions}); most
|
|
editing commands act on the contents of the current buffer in the
|
|
neighborhood of point. At any time, one buffer is the @dfn{current
|
|
buffer}.
|
|
|
|
The contents of a buffer are much like a string, but buffers are not
|
|
used like strings in Emacs Lisp, and the available operations are
|
|
different. For example, you can insert text efficiently into an
|
|
existing buffer, altering the buffer's contents, whereas inserting
|
|
text into a string requires concatenating substrings, and the result
|
|
is an entirely new string object.
|
|
|
|
Many of the standard Emacs functions manipulate or test the
|
|
characters in the current buffer; a whole chapter in this manual is
|
|
devoted to describing these functions (@pxref{Text}).
|
|
|
|
Several other data structures are associated with each buffer:
|
|
|
|
@itemize @bullet
|
|
@item
|
|
a local syntax table (@pxref{Syntax Tables});
|
|
|
|
@item
|
|
a local keymap (@pxref{Keymaps}); and,
|
|
|
|
@item
|
|
a list of buffer-local variable bindings (@pxref{Buffer-Local Variables}).
|
|
|
|
@item
|
|
overlays (@pxref{Overlays}).
|
|
|
|
@item
|
|
text properties for the text in the buffer (@pxref{Text Properties}).
|
|
@end itemize
|
|
|
|
@noindent
|
|
The local keymap and variable list contain entries that individually
|
|
override global bindings or values. These are used to customize the
|
|
behavior of programs in different buffers, without actually changing the
|
|
programs.
|
|
|
|
A buffer may be @dfn{indirect}, which means it shares the text
|
|
of another buffer, but presents it differently. @xref{Indirect Buffers}.
|
|
|
|
Buffers have no read syntax. They print in hash notation, showing the
|
|
buffer name.
|
|
|
|
@example
|
|
@group
|
|
(current-buffer)
|
|
@result{} #<buffer objects.texi>
|
|
@end group
|
|
@end example
|
|
|
|
@node Marker Type
|
|
@subsection Marker Type
|
|
|
|
A @dfn{marker} denotes a position in a specific buffer. Markers
|
|
therefore have two components: one for the buffer, and one for the
|
|
position. Changes in the buffer's text automatically relocate the
|
|
position value as necessary to ensure that the marker always points
|
|
between the same two characters in the buffer.
|
|
|
|
Markers have no read syntax. They print in hash notation, giving the
|
|
current character position and the name of the buffer.
|
|
|
|
@example
|
|
@group
|
|
(point-marker)
|
|
@result{} #<marker at 10779 in objects.texi>
|
|
@end group
|
|
@end example
|
|
|
|
@xref{Markers}, for information on how to test, create, copy, and move
|
|
markers.
|
|
|
|
@node Window Type
|
|
@subsection Window Type
|
|
|
|
A @dfn{window} describes the portion of the terminal screen that Emacs
|
|
uses to display a buffer. Every window has one associated buffer, whose
|
|
contents appear in the window. By contrast, a given buffer may appear
|
|
in one window, no window, or several windows.
|
|
|
|
Though many windows may exist simultaneously, at any time one window
|
|
is designated the @dfn{selected window}. This is the window where the
|
|
cursor is (usually) displayed when Emacs is ready for a command. The
|
|
selected window usually displays the current buffer (@pxref{Current
|
|
Buffer}), but this is not necessarily the case.
|
|
|
|
Windows are grouped on the screen into frames; each window belongs to
|
|
one and only one frame. @xref{Frame Type}.
|
|
|
|
Windows have no read syntax. They print in hash notation, giving the
|
|
window number and the name of the buffer being displayed. The window
|
|
numbers exist to identify windows uniquely, since the buffer displayed
|
|
in any given window can change frequently.
|
|
|
|
@example
|
|
@group
|
|
(selected-window)
|
|
@result{} #<window 1 on objects.texi>
|
|
@end group
|
|
@end example
|
|
|
|
@xref{Windows}, for a description of the functions that work on windows.
|
|
|
|
@node Frame Type
|
|
@subsection Frame Type
|
|
|
|
A @dfn{frame} is a screen area that contains one or more Emacs
|
|
windows; we also use the term ``frame'' to refer to the Lisp object
|
|
that Emacs uses to refer to the screen area.
|
|
|
|
Frames have no read syntax. They print in hash notation, giving the
|
|
frame's title, plus its address in core (useful to identify the frame
|
|
uniquely).
|
|
|
|
@example
|
|
@group
|
|
(selected-frame)
|
|
@result{} #<frame emacs@@psilocin.gnu.org 0xdac80>
|
|
@end group
|
|
@end example
|
|
|
|
@xref{Frames}, for a description of the functions that work on frames.
|
|
|
|
@node Terminal Type
|
|
@subsection Terminal Type
|
|
@cindex terminal type
|
|
|
|
A @dfn{terminal} is a device capable of displaying one or more
|
|
Emacs frames (@pxref{Frame Type}).
|
|
|
|
Terminals have no read syntax. They print in hash notation giving
|
|
the terminal's ordinal number and its TTY device file name.
|
|
|
|
@example
|
|
@group
|
|
(get-device-terminal nil)
|
|
@result{} #<terminal 1 on /dev/tty>
|
|
@end group
|
|
@end example
|
|
|
|
@c FIXME: add an xref to where terminal-related primitives are described.
|
|
|
|
@node Window Configuration Type
|
|
@subsection Window Configuration Type
|
|
@cindex window layout in a frame
|
|
|
|
A @dfn{window configuration} stores information about the positions,
|
|
sizes, and contents of the windows in a frame, so you can recreate the
|
|
same arrangement of windows later.
|
|
|
|
Window configurations do not have a read syntax; their print syntax
|
|
looks like @samp{#<window-configuration>}. @xref{Window
|
|
Configurations}, for a description of several functions related to
|
|
window configurations.
|
|
|
|
@node Frame Configuration Type
|
|
@subsection Frame Configuration Type
|
|
@cindex screen layout
|
|
@cindex window layout, all frames
|
|
|
|
A @dfn{frame configuration} stores information about the positions,
|
|
sizes, and contents of the windows in all frames. It is not a
|
|
primitive type---it is actually a list whose @sc{car} is
|
|
@code{frame-configuration} and whose @sc{cdr} is an alist. Each alist
|
|
element describes one frame, which appears as the @sc{car} of that
|
|
element.
|
|
|
|
@xref{Frame Configurations}, for a description of several functions
|
|
related to frame configurations.
|
|
|
|
@node Process Type
|
|
@subsection Process Type
|
|
|
|
The word @dfn{process} usually means a running program. Emacs itself
|
|
runs in a process of this sort. However, in Emacs Lisp, a process is a
|
|
Lisp object that designates a subprocess created by the Emacs process.
|
|
Programs such as shells, GDB, ftp, and compilers, running in
|
|
subprocesses of Emacs, extend the capabilities of Emacs.
|
|
An Emacs subprocess takes textual input from Emacs and returns textual
|
|
output to Emacs for further manipulation. Emacs can also send signals
|
|
to the subprocess.
|
|
|
|
Process objects have no read syntax. They print in hash notation,
|
|
giving the name of the process:
|
|
|
|
@example
|
|
@group
|
|
(process-list)
|
|
@result{} (#<process shell>)
|
|
@end group
|
|
@end example
|
|
|
|
@xref{Processes}, for information about functions that create, delete,
|
|
return information about, send input or signals to, and receive output
|
|
from processes.
|
|
|
|
@node Thread Type
|
|
@subsection Thread Type
|
|
|
|
A @dfn{thread} in Emacs represents a separate thread of Emacs Lisp
|
|
execution. It runs its own Lisp program, has its own current buffer,
|
|
and can have subprocesses locked to it, i.e.@: subprocesses whose
|
|
output only this thread can accept. @xref{Threads}.
|
|
|
|
Thread objects have no read syntax. They print in hash notation,
|
|
giving the name of the thread (if it has been given a name) or its
|
|
address in core:
|
|
|
|
@example
|
|
@group
|
|
(all-threads)
|
|
@result{} (#<thread 0176fc40>)
|
|
@end group
|
|
@end example
|
|
|
|
@node Mutex Type
|
|
@subsection Mutex Type
|
|
|
|
A @dfn{mutex} is an exclusive lock that threads can own and disown,
|
|
in order to synchronize between them. @xref{Mutexes}.
|
|
|
|
Mutex objects have no read syntax. They print in hash notation,
|
|
giving the name of the mutex (if it has been given a name) or its
|
|
address in core:
|
|
|
|
@example
|
|
@group
|
|
(make-mutex "my-mutex")
|
|
@result{} #<mutex my-mutex>
|
|
(make-mutex)
|
|
@result{} #<mutex 01c7e4e0>
|
|
@end group
|
|
@end example
|
|
|
|
@node Condition Variable Type
|
|
@subsection Condition Variable Type
|
|
|
|
A @dfn{condition variable} is a device for a more complex thread
|
|
synchronization than the one supported by a mutex. A thread can wait
|
|
on a condition variable, to be woken up when some other thread
|
|
notifies the condition.
|
|
|
|
Condition variable objects have no read syntax. They print in hash
|
|
notation, giving the name of the condition variable (if it has been
|
|
given a name) or its address in core:
|
|
|
|
@example
|
|
@group
|
|
(make-condition-variable (make-mutex))
|
|
@result{} #<condvar 01c45ae8>
|
|
@end group
|
|
@end example
|
|
|
|
@node Stream Type
|
|
@subsection Stream Type
|
|
|
|
A @dfn{stream} is an object that can be used as a source or sink for
|
|
characters---either to supply characters for input or to accept them as
|
|
output. Many different types can be used this way: markers, buffers,
|
|
strings, and functions. Most often, input streams (character sources)
|
|
obtain characters from the keyboard, a buffer, or a file, and output
|
|
streams (character sinks) send characters to a buffer, such as a
|
|
@file{*Help*} buffer, or to the echo area.
|
|
|
|
The object @code{nil}, in addition to its other meanings, may be used
|
|
as a stream. It stands for the value of the variable
|
|
@code{standard-input} or @code{standard-output}. Also, the object
|
|
@code{t} as a stream specifies input using the minibuffer
|
|
(@pxref{Minibuffers}) or output in the echo area (@pxref{The Echo
|
|
Area}).
|
|
|
|
Streams have no special printed representation or read syntax, and
|
|
print as whatever primitive type they are.
|
|
|
|
@xref{Read and Print}, for a description of functions
|
|
related to streams, including parsing and printing functions.
|
|
|
|
@node Keymap Type
|
|
@subsection Keymap Type
|
|
|
|
A @dfn{keymap} maps keys typed by the user to commands. This mapping
|
|
controls how the user's command input is executed. A keymap is actually
|
|
a list whose @sc{car} is the symbol @code{keymap}.
|
|
|
|
@xref{Keymaps}, for information about creating keymaps, handling prefix
|
|
keys, local as well as global keymaps, and changing key bindings.
|
|
|
|
@node Overlay Type
|
|
@subsection Overlay Type
|
|
|
|
An @dfn{overlay} specifies properties that apply to a part of a
|
|
buffer. Each overlay applies to a specified range of the buffer, and
|
|
contains a property list (a list whose elements are alternating property
|
|
names and values). Overlay properties are used to present parts of the
|
|
buffer temporarily in a different display style. Overlays have no read
|
|
syntax, and print in hash notation, giving the buffer name and range of
|
|
positions.
|
|
|
|
@xref{Overlays}, for information on how you can create and use overlays.
|
|
|
|
@node Font Type
|
|
@subsection Font Type
|
|
|
|
A @dfn{font} specifies how to display text on a graphical terminal.
|
|
There are actually three separate font types---@dfn{font objects},
|
|
@dfn{font specs}, and @dfn{font entities}---each of which has slightly
|
|
different properties. None of them have a read syntax; their print
|
|
syntax looks like @samp{#<font-object>}, @samp{#<font-spec>}, and
|
|
@samp{#<font-entity>} respectively. @xref{Low-Level Font}, for a
|
|
description of these Lisp objects.
|
|
|
|
@node Circular Objects
|
|
@section Read Syntax for Circular Objects
|
|
@cindex circular structure, read syntax
|
|
@cindex shared structure, read syntax
|
|
@cindex @samp{#@var{n}=} read syntax
|
|
@cindex @samp{#@var{n}#} read syntax
|
|
|
|
To represent shared or circular structures within a complex of Lisp
|
|
objects, you can use the reader constructs @samp{#@var{n}=} and
|
|
@samp{#@var{n}#}.
|
|
|
|
Use @code{#@var{n}=} before an object to label it for later reference;
|
|
subsequently, you can use @code{#@var{n}#} to refer the same object in
|
|
another place. Here, @var{n} is some integer. For example, here is how
|
|
to make a list in which the first element recurs as the third element:
|
|
|
|
@example
|
|
(#1=(a) b #1#)
|
|
@end example
|
|
|
|
@noindent
|
|
This differs from ordinary syntax such as this
|
|
|
|
@example
|
|
((a) b (a))
|
|
@end example
|
|
|
|
@noindent
|
|
which would result in a list whose first and third elements
|
|
look alike but are not the same Lisp object. This shows the difference:
|
|
|
|
@example
|
|
(prog1 nil
|
|
(setq x '(#1=(a) b #1#)))
|
|
(eq (nth 0 x) (nth 2 x))
|
|
@result{} t
|
|
(setq x '((a) b (a)))
|
|
(eq (nth 0 x) (nth 2 x))
|
|
@result{} nil
|
|
@end example
|
|
|
|
You can also use the same syntax to make a circular structure, which
|
|
appears as an element within itself. Here is an example:
|
|
|
|
@example
|
|
#1=(a #1#)
|
|
@end example
|
|
|
|
@noindent
|
|
This makes a list whose second element is the list itself.
|
|
Here's how you can see that it really works:
|
|
|
|
@example
|
|
(prog1 nil
|
|
(setq x '#1=(a #1#)))
|
|
(eq x (cadr x))
|
|
@result{} t
|
|
@end example
|
|
|
|
The Lisp printer can produce this syntax to record circular and shared
|
|
structure in a Lisp object, if you bind the variable @code{print-circle}
|
|
to a non-@code{nil} value. @xref{Output Variables}.
|
|
|
|
@node Type Predicates
|
|
@section Type Predicates
|
|
@cindex type checking
|
|
@kindex wrong-type-argument
|
|
|
|
The Emacs Lisp interpreter itself does not perform type checking on
|
|
the actual arguments passed to functions when they are called. It could
|
|
not do so, since function arguments in Lisp do not have declared data
|
|
types, as they do in other programming languages. It is therefore up to
|
|
the individual function to test whether each actual argument belongs to
|
|
a type that the function can use.
|
|
|
|
All built-in functions do check the types of their actual arguments
|
|
when appropriate, and signal a @code{wrong-type-argument} error if an
|
|
argument is of the wrong type. For example, here is what happens if you
|
|
pass an argument to @code{+} that it cannot handle:
|
|
|
|
@example
|
|
@group
|
|
(+ 2 'a)
|
|
@error{} Wrong type argument: number-or-marker-p, a
|
|
@end group
|
|
@end example
|
|
|
|
@cindex type predicates
|
|
@cindex testing types
|
|
If you want your program to handle different types differently, you
|
|
must do explicit type checking. The most common way to check the type
|
|
of an object is to call a @dfn{type predicate} function. Emacs has a
|
|
type predicate for each type, as well as some predicates for
|
|
combinations of types.
|
|
|
|
A type predicate function takes one argument; it returns @code{t} if
|
|
the argument belongs to the appropriate type, and @code{nil} otherwise.
|
|
Following a general Lisp convention for predicate functions, most type
|
|
predicates' names end with @samp{p}.
|
|
|
|
Here is an example which uses the predicates @code{listp} to check for
|
|
a list and @code{symbolp} to check for a symbol.
|
|
|
|
@example
|
|
(defun add-on (x)
|
|
(cond ((symbolp x)
|
|
;; If X is a symbol, put it on LIST.
|
|
(setq list (cons x list)))
|
|
((listp x)
|
|
;; If X is a list, add its elements to LIST.
|
|
(setq list (append x list)))
|
|
(t
|
|
;; We handle only symbols and lists.
|
|
(error "Invalid argument %s in add-on" x))))
|
|
@end example
|
|
|
|
Here is a table of predefined type predicates, in alphabetical order,
|
|
with references to further information.
|
|
|
|
@table @code
|
|
@item atom
|
|
@xref{List-related Predicates, atom}.
|
|
|
|
@item arrayp
|
|
@xref{Array Functions, arrayp}.
|
|
|
|
@item bool-vector-p
|
|
@xref{Bool-Vectors, bool-vector-p}.
|
|
|
|
@item bufferp
|
|
@xref{Buffer Basics, bufferp}.
|
|
|
|
@item byte-code-function-p
|
|
@xref{Byte-Code Type, byte-code-function-p}.
|
|
|
|
@item case-table-p
|
|
@xref{Case Tables, case-table-p}.
|
|
|
|
@item char-or-string-p
|
|
@xref{Predicates for Strings, char-or-string-p}.
|
|
|
|
@item char-table-p
|
|
@xref{Char-Tables, char-table-p}.
|
|
|
|
@item commandp
|
|
@xref{Interactive Call, commandp}.
|
|
|
|
@item condition-variable-p
|
|
@xref{Condition Variables, condition-variable-p}.
|
|
|
|
@item consp
|
|
@xref{List-related Predicates, consp}.
|
|
|
|
@item custom-variable-p
|
|
@xref{Variable Definitions, custom-variable-p}.
|
|
|
|
@item floatp
|
|
@xref{Predicates on Numbers, floatp}.
|
|
|
|
@item fontp
|
|
@xref{Low-Level Font}.
|
|
|
|
@item frame-configuration-p
|
|
@xref{Frame Configurations, frame-configuration-p}.
|
|
|
|
@item frame-live-p
|
|
@xref{Deleting Frames, frame-live-p}.
|
|
|
|
@item framep
|
|
@xref{Frames, framep}.
|
|
|
|
@item functionp
|
|
@xref{Functions, functionp}.
|
|
|
|
@item hash-table-p
|
|
@xref{Other Hash, hash-table-p}.
|
|
|
|
@item integer-or-marker-p
|
|
@xref{Predicates on Markers, integer-or-marker-p}.
|
|
|
|
@item integerp
|
|
@xref{Predicates on Numbers, integerp}.
|
|
|
|
@item keymapp
|
|
@xref{Creating Keymaps, keymapp}.
|
|
|
|
@item keywordp
|
|
@xref{Constant Variables}.
|
|
|
|
@item listp
|
|
@xref{List-related Predicates, listp}.
|
|
|
|
@item markerp
|
|
@xref{Predicates on Markers, markerp}.
|
|
|
|
@item mutexp
|
|
@xref{Mutexes, mutexp}.
|
|
|
|
@item wholenump
|
|
@xref{Predicates on Numbers, wholenump}.
|
|
|
|
@item nlistp
|
|
@xref{List-related Predicates, nlistp}.
|
|
|
|
@item numberp
|
|
@xref{Predicates on Numbers, numberp}.
|
|
|
|
@item number-or-marker-p
|
|
@xref{Predicates on Markers, number-or-marker-p}.
|
|
|
|
@item overlayp
|
|
@xref{Overlays, overlayp}.
|
|
|
|
@item processp
|
|
@xref{Processes, processp}.
|
|
|
|
@item recordp
|
|
@xref{Record Type, recordp}.
|
|
|
|
@item sequencep
|
|
@xref{Sequence Functions, sequencep}.
|
|
|
|
@item stringp
|
|
@xref{Predicates for Strings, stringp}.
|
|
|
|
@item subrp
|
|
@xref{Function Cells, subrp}.
|
|
|
|
@item symbolp
|
|
@xref{Symbols, symbolp}.
|
|
|
|
@item syntax-table-p
|
|
@xref{Syntax Tables, syntax-table-p}.
|
|
|
|
@item threadp
|
|
@xref{Basic Thread Functions, threadp}.
|
|
|
|
@item vectorp
|
|
@xref{Vectors, vectorp}.
|
|
|
|
@item window-configuration-p
|
|
@xref{Window Configurations, window-configuration-p}.
|
|
|
|
@item window-live-p
|
|
@xref{Deleting Windows, window-live-p}.
|
|
|
|
@item windowp
|
|
@xref{Basic Windows, windowp}.
|
|
|
|
@item booleanp
|
|
@xref{nil and t, booleanp}.
|
|
|
|
@item string-or-null-p
|
|
@xref{Predicates for Strings, string-or-null-p}.
|
|
|
|
@item threadp
|
|
@xref{Basic Thread Functions, threadp}.
|
|
|
|
@item mutexp
|
|
@xref{Mutexes, mutexp}.
|
|
|
|
@item condition-variable-p
|
|
@xref{Condition Variables, condition-variable-p}.
|
|
@end table
|
|
|
|
The most general way to check the type of an object is to call the
|
|
function @code{type-of}. Recall that each object belongs to one and
|
|
only one primitive type; @code{type-of} tells you which one (@pxref{Lisp
|
|
Data Types}). But @code{type-of} knows nothing about non-primitive
|
|
types. In most cases, it is more convenient to use type predicates than
|
|
@code{type-of}.
|
|
|
|
@defun type-of object
|
|
This function returns a symbol naming the primitive type of
|
|
@var{object}. The value is one of the symbols @code{bool-vector},
|
|
@code{buffer}, @code{char-table}, @code{compiled-function},
|
|
@code{condition-variable}, @code{cons}, @code{finalizer},
|
|
@code{float}, @code{font-entity}, @code{font-object},
|
|
@code{font-spec}, @code{frame}, @code{hash-table}, @code{integer},
|
|
@code{marker}, @code{mutex}, @code{overlay}, @code{process},
|
|
@code{string}, @code{subr}, @code{symbol}, @code{thread},
|
|
@code{vector}, @code{window}, or @code{window-configuration}.
|
|
However, if @var{object} is a record, the type specified by its first
|
|
slot is returned; @ref{Records}.
|
|
|
|
@example
|
|
(type-of 1)
|
|
@result{} integer
|
|
@group
|
|
(type-of 'nil)
|
|
@result{} symbol
|
|
(type-of '()) ; @r{@code{()} is @code{nil}.}
|
|
@result{} symbol
|
|
(type-of '(x))
|
|
@result{} cons
|
|
(type-of (record 'foo))
|
|
@result{} foo
|
|
@end group
|
|
@end example
|
|
@end defun
|
|
|
|
@node Equality Predicates
|
|
@section Equality Predicates
|
|
@cindex equality
|
|
|
|
Here we describe functions that test for equality between two
|
|
objects. Other functions test equality of contents between objects of
|
|
specific types, e.g., strings. For these predicates, see the
|
|
appropriate chapter describing the data type.
|
|
|
|
@defun eq object1 object2
|
|
This function returns @code{t} if @var{object1} and @var{object2} are
|
|
the same object, and @code{nil} otherwise.
|
|
|
|
If @var{object1} and @var{object2} are integers with the same value,
|
|
they are considered to be the same object (i.e., @code{eq} returns
|
|
@code{t}). If @var{object1} and @var{object2} are symbols with the
|
|
same name, they are normally the same object---but see @ref{Creating
|
|
Symbols} for exceptions. For other types (e.g., lists, vectors,
|
|
strings), two arguments with the same contents or elements are not
|
|
necessarily @code{eq} to each other: they are @code{eq} only if they
|
|
are the same object, meaning that a change in the contents of one will
|
|
be reflected by the same change in the contents of the other.
|
|
|
|
@example
|
|
@group
|
|
(eq 'foo 'foo)
|
|
@result{} t
|
|
@end group
|
|
|
|
@group
|
|
(eq 456 456)
|
|
@result{} t
|
|
@end group
|
|
|
|
@group
|
|
(eq "asdf" "asdf")
|
|
@result{} nil
|
|
@end group
|
|
|
|
@group
|
|
(eq "" "")
|
|
@result{} t
|
|
;; @r{This exception occurs because Emacs Lisp}
|
|
;; @r{makes just one multibyte empty string, to save space.}
|
|
@end group
|
|
|
|
@group
|
|
(eq '(1 (2 (3))) '(1 (2 (3))))
|
|
@result{} nil
|
|
@end group
|
|
|
|
@group
|
|
(setq foo '(1 (2 (3))))
|
|
@result{} (1 (2 (3)))
|
|
(eq foo foo)
|
|
@result{} t
|
|
(eq foo '(1 (2 (3))))
|
|
@result{} nil
|
|
@end group
|
|
|
|
@group
|
|
(eq [(1 2) 3] [(1 2) 3])
|
|
@result{} nil
|
|
@end group
|
|
|
|
@group
|
|
(eq (point-marker) (point-marker))
|
|
@result{} nil
|
|
@end group
|
|
@end example
|
|
|
|
@noindent
|
|
The @code{make-symbol} function returns an uninterned symbol, distinct
|
|
from the symbol that is used if you write the name in a Lisp expression.
|
|
Distinct symbols with the same name are not @code{eq}. @xref{Creating
|
|
Symbols}.
|
|
|
|
@example
|
|
@group
|
|
(eq (make-symbol "foo") 'foo)
|
|
@result{} nil
|
|
@end group
|
|
@end example
|
|
@end defun
|
|
|
|
@defun equal object1 object2
|
|
This function returns @code{t} if @var{object1} and @var{object2} have
|
|
equal components, and @code{nil} otherwise. Whereas @code{eq} tests
|
|
if its arguments are the same object, @code{equal} looks inside
|
|
nonidentical arguments to see if their elements or contents are the
|
|
same. So, if two objects are @code{eq}, they are @code{equal}, but
|
|
the converse is not always true.
|
|
|
|
@example
|
|
@group
|
|
(equal 'foo 'foo)
|
|
@result{} t
|
|
@end group
|
|
|
|
@group
|
|
(equal 456 456)
|
|
@result{} t
|
|
@end group
|
|
|
|
@group
|
|
(equal "asdf" "asdf")
|
|
@result{} t
|
|
@end group
|
|
@group
|
|
(eq "asdf" "asdf")
|
|
@result{} nil
|
|
@end group
|
|
|
|
@group
|
|
(equal '(1 (2 (3))) '(1 (2 (3))))
|
|
@result{} t
|
|
@end group
|
|
@group
|
|
(eq '(1 (2 (3))) '(1 (2 (3))))
|
|
@result{} nil
|
|
@end group
|
|
|
|
@group
|
|
(equal [(1 2) 3] [(1 2) 3])
|
|
@result{} t
|
|
@end group
|
|
@group
|
|
(eq [(1 2) 3] [(1 2) 3])
|
|
@result{} nil
|
|
@end group
|
|
|
|
@group
|
|
(equal (point-marker) (point-marker))
|
|
@result{} t
|
|
@end group
|
|
|
|
@group
|
|
(eq (point-marker) (point-marker))
|
|
@result{} nil
|
|
@end group
|
|
@end example
|
|
|
|
Comparison of strings is case-sensitive, but does not take account of
|
|
text properties---it compares only the characters in the strings.
|
|
@xref{Text Properties}. Use @code{equal-including-properties} to also
|
|
compare text properties. For technical reasons, a unibyte string and
|
|
a multibyte string are @code{equal} if and only if they contain the
|
|
same sequence of character codes and all these codes are either in the
|
|
range 0 through 127 (@acronym{ASCII}) or 160 through 255
|
|
(@code{eight-bit-graphic}). (@pxref{Text Representations}).
|
|
|
|
@example
|
|
@group
|
|
(equal "asdf" "ASDF")
|
|
@result{} nil
|
|
@end group
|
|
@end example
|
|
|
|
However, two distinct buffers are never considered @code{equal}, even if
|
|
their textual contents are the same.
|
|
@end defun
|
|
|
|
The test for equality is implemented recursively; for example, given
|
|
two cons cells @var{x} and @var{y}, @code{(equal @var{x} @var{y})}
|
|
returns @code{t} if and only if both the expressions below return
|
|
@code{t}:
|
|
|
|
@example
|
|
(equal (car @var{x}) (car @var{y}))
|
|
(equal (cdr @var{x}) (cdr @var{y}))
|
|
@end example
|
|
|
|
Because of this recursive method, circular lists may therefore cause
|
|
infinite recursion (leading to an error).
|
|
|
|
@defun equal-including-properties object1 object2
|
|
This function behaves like @code{equal} in all cases but also requires
|
|
that for two strings to be equal, they have the same text properties.
|
|
|
|
@example
|
|
@group
|
|
(equal "asdf" (propertize "asdf" 'asdf t))
|
|
@result{} t
|
|
@end group
|
|
@group
|
|
(equal-including-properties "asdf"
|
|
(propertize "asdf" 'asdf t))
|
|
@result{} nil
|
|
@end group
|
|
@end example
|
|
@end defun
|