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1593 lines
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1593 lines
57 KiB
Plaintext
@c -*-texinfo-*-
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@c This is part of the GNU Emacs Lisp Reference Manual.
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@c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc.
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@c See the file elisp.texi for copying conditions.
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@setfilename ../info/objects
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@node Lisp Data Types, Numbers, Introduction, Top
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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 ``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
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types}. Each object belongs to one and only one primitive type. These
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types include @dfn{integer}, @dfn{float}, @dfn{cons}, @dfn{symbol},
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@dfn{string}, @dfn{vector}, @dfn{subr}, @dfn{byte-code function}, and
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several special types, such as @dfn{buffer}, that are related to
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editing. (@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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Note that Lisp is unlike many other languages in that Lisp objects are
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@dfn{self-typing}: the primitive type of the object is implicit in the
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object itself. For example, if an object is a vector, nothing can treat
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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.
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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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* 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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@comment node-name, next, previous, up
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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. The @dfn{read syntax} of an object is the format of the
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input accepted by the Lisp reader (the function @code{read}) for that
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object. Most objects have more than one possible read syntax. Some
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types of object have no read syntax; except for these cases, the printed
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representation of an object is also a read syntax for it.
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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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@cindex hash notation
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Every type has a printed representation. Some types have no read
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syntax, since it may not make sense to enter objects of these types
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directly in a Lisp program. For example, the buffer type does not have
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a read syntax. Objects of these types are printed in @dfn{hash
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notation}: the characters @samp{#<} followed by a descriptive string
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(typically the type name followed by the name of the object), and closed
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with a matching @samp{>}. Hash notation cannot be read at all, so the
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Lisp reader signals the error @code{invalid-read-syntax} whenever it
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encounters @samp{#<}.
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@kindex invalid-read-syntax
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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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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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@comment node-name, next, previous, up
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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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* 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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* Autoload Type:: A type used for automatically loading seldom-used
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functions.
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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 integers in Emacs Lisp is @minus{}134217728 to
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134217727 (28 bits; i.e.,
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@ifinfo
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-2**27
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@end ifinfo
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@tex
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$-2^{27}$
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@end tex
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to
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@ifinfo
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2**27 - 1)
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@end ifinfo
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@tex
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$2^{28}-1$)
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@end tex
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on most machines. (Some machines may provide a wider range.) It is
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important to note that the Emacs Lisp arithmetic functions do not check
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for overflow. Thus @code{(1+ 134217727)} is @minus{}134217728 on most
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machines.
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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 -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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268435457 ; @r{Also the integer 1!}
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; @r{ (on a 28-bit implementation)}
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@end group
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@end example
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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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Emacs version 19 supports floating point numbers (though there is a
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compilation option to disable them). The precise range of floating
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point numbers is machine-specific.
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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{15.0e2},
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@samp{1.5e3}, 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 @sc{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 not often used in programs. It is far more
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common to work with @emph{strings}, which are sequences composed of
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characters. @xref{String Type}.
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Characters in strings, buffers, and files are currently limited to the
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range of 0 to 255---eight bits. If you store a larger integer into a
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string, buffer or file, it is truncated to that range. Characters that
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represent keyboard input have a much wider range.
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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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Since characters are really integers, the printed representation of a
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character is a decimal number. This is also a possible read syntax for
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a character, but writing characters that way in Lisp programs is a very
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bad idea. You should @emph{always} use the special read syntax formats
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that Emacs Lisp provides for characters. These syntax formats start
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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, but it is
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often a good idea to add a @samp{\} so that the Emacs commands for
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editing Lisp code don't get confused. For example, @samp{?\ } is the
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way to write the space character. If the character is @samp{\}, you
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@emph{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
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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
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@cindex @samp{\r}
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@cindex escape
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@cindex @samp{\e}
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You can express the characters Control-g, backspace, tab, newline,
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vertical tab, formfeed, return, and escape as @samp{?\a}, @samp{?\b},
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@samp{?\t}, @samp{?\n}, @samp{?\v}, @samp{?\f}, @samp{?\r}, @samp{?\e},
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respectively. Those values are 7, 8, 9, 10, 11, 12, 13, and 27 in
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decimal. Thus,
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@example
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?\a @result{} 7 ; @r{@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, @key{LFD}, @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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?\\ @result{} 92 ; @r{backslash character, @kbd{\}}
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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 escape
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character; this usage has nothing to do with the character @key{ESC}.
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@cindex control characters
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Control characters may 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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For use in strings and buffers, you are limited to the control
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characters that exist in @sc{ASCII}, but for keyboard input purposes,
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you can turn any character into a control character with @samp{C-}. The
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character codes for these non-@sc{ASCII} control characters include the
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@iftex
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$2^{26}$
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@end iftex
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@ifinfo
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2**26
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@end ifinfo
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bit as well as the code for the corresponding non-control
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character. Ordinary terminals have no way of generating non-@sc{ASCII}
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control characters, but you can generate them straightforwardly using an
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X terminal.
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For historical reasons, Emacs treats the @key{DEL} character as
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the control equivalent of @kbd{?}:
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@example
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?\^? @result{} 127 ?\C-? @result{} 127
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@end example
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@noindent
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As a result, it is currently not possible to represent the character
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@kbd{Control-?}, which is a meaningful input character under X. It is
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not easy to change this as various Lisp files refer to @key{DEL} in this
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way.
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For representing control characters to be found in files or strings,
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we recommend the @samp{^} syntax; for control characters in keyboard
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input, we prefer the @samp{C-} syntax. This does not affect the meaning
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of the program, but may guide the understanding of people who read it.
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@cindex meta characters
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A @dfn{meta character} is a character typed with the @key{META}
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modifier key. The integer that represents such a character has the
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@iftex
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$2^{27}$
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@end iftex
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@ifinfo
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2**27
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@end ifinfo
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bit set (which on most machines makes it a negative number). We
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use high bits for this and other modifiers to make possible a wide range
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of basic character codes.
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In a string, the
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@iftex
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$2^{7}$
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@end iftex
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@ifinfo
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2**7
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@end ifinfo
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bit indicates a meta character, so the meta
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characters that can fit in a string have codes in the range from 128 to
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255, and are the meta versions of the ordinary @sc{ASCII} characters.
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(In Emacs versions 18 and older, this convention was used for characters
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outside of strings as well.)
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The read syntax for meta characters uses @samp{\M-}. For example,
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@samp{?\M-A} stands for @kbd{M-A}. You can use @samp{\M-} together with
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octal character codes (see below), with @samp{\C-}, or with any other
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syntax for a character. Thus, you can write @kbd{M-A} as @samp{?\M-A},
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or as @samp{?\M-\101}. Likewise, you can write @kbd{C-M-b} as
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@samp{?\M-\C-b}, @samp{?\C-\M-b}, or @samp{?\M-\002}.
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The case of an ordinary letter is indicated by its character code as
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part of @sc{ASCII}, but @sc{ASCII} has no way to represent whether a
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control character is upper case or lower case. Emacs uses the
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@iftex
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$2^{25}$
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@end iftex
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@ifinfo
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2**25
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@end ifinfo
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bit to indicate that the shift key was used for typing a control
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character. This distinction is possible only when you use X terminals
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or other special terminals; ordinary terminals do not indicate the
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distinction to the computer in any way.
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@cindex hyper characters
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@cindex super characters
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@cindex alt characters
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The X Window System defines three other modifier bits that can be set
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in a character: @dfn{hyper}, @dfn{super} and @dfn{alt}. The syntaxes
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for these bits are @samp{\H-}, @samp{\s-} and @samp{\A-}. Thus,
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@samp{?\H-\M-\A-x} represents @kbd{Alt-Hyper-Meta-x}.
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@iftex
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Numerically, the
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bit values are $2^{22}$ for alt, $2^{23}$ for super and $2^{24}$ for hyper.
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@end iftex
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@ifinfo
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Numerically, the
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bit values are 2**22 for alt, 2**23 for super and 2**24 for hyper.
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@end ifinfo
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@cindex @samp{?} in character constant
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@cindex question mark in character constant
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@cindex @samp{\} in character constant
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@cindex backslash in character constant
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@cindex octal character code
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Finally, the most general read syntax consists of a question mark
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followed by a backslash and the character code in octal (up to three
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octal digits); thus, @samp{?\101} for the character @kbd{A},
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@samp{?\001} for the character @kbd{C-a}, and @code{?\002} for the
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character @kbd{C-b}. Although this syntax can represent any @sc{ASCII}
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character, it is preferred only when the precise octal value is more
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important than the @sc{ASCII} representation.
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@example
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@group
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?\012 @result{} 10 ?\n @result{} 10 ?\C-j @result{} 10
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?\101 @result{} 65 ?A @result{} 65
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@end group
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@end example
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A backslash is allowed, and harmless, preceding any character without
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a special escape meaning; thus, @samp{?\+} is equivalent to @samp{?+}.
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There is no reason to add a backslash before most characters. However,
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you should add a backslash before any of the characters
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@samp{()\|;'`"#.,} to avoid confusing the Emacs commands for editing
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Lisp code. Also add a backslash before whitespace characters such as
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space, tab, newline and formfeed. However, it is cleaner to use one of
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the easily readable escape sequences, such as @samp{\t}, instead of an
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actual whitespace character such as a tab.
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@node Symbol Type
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@subsection Symbol Type
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A @dfn{symbol} in GNU Emacs Lisp is an object with a name. The symbol
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name serves as the printed representation of the symbol. In ordinary
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use, the name is unique---no two symbols have the same name.
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A symbol can serve as a variable, as a function name, or to hold a
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property list. Or it may serve only to be distinct from all other Lisp
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objects, so that its presence in a data structure may be recognized
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reliably. In a given context, usually only one of these uses is
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intended. But you can use one symbol in all of these ways,
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independently.
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@cindex @samp{\} in symbols
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@cindex backslash in symbols
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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 @kbd{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 fifth 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}.}
|
|
char-to-string ; @r{A symbol named @samp{char-to-string}.}
|
|
@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
|
|
|
|
@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, lists and
|
|
arrays. Thus, an object of type list or of type array is also
|
|
considered a sequence.
|
|
|
|
Arrays are further subdivided into strings and vectors. Vectors can
|
|
hold elements of any type, but string elements must be characters in the
|
|
range from 0 to 255. However, the characters in a string can have text
|
|
properties like characters in a buffer (@pxref{Text Properties});
|
|
vectors do not support text properties even when their elements happen
|
|
to be characters.
|
|
|
|
Lists, strings and vectors are different, but they have 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. Also,
|
|
several functions, called sequence functions, accept any kind of
|
|
sequence. For example, the function @code{elt} can be used to extract
|
|
an element of a sequence, given its index. @xref{Sequences Arrays
|
|
Vectors}.
|
|
|
|
It is 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
|
|
|
|
A @dfn{cons cell} is an object comprising two pointers named the
|
|
@sc{car} and the @sc{cdr}. Each of them can point to any Lisp object.
|
|
|
|
A @dfn{list} is a series of cons cells, linked together so that the
|
|
@sc{cdr} of each cons cell points either to another cons cell or to the
|
|
empty list. @xref{Lists}, for functions that work on lists. Because
|
|
most cons cells are used as part of lists, the phrase @dfn{list
|
|
structure} has come to refer to any structure made out of cons cells.
|
|
|
|
The names @sc{car} and @sc{cdr} have only historical meaning now. The
|
|
original Lisp implementation ran on an @w{IBM 704} computer which
|
|
divided words into two parts, called the ``address'' part 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 is named
|
|
for its purpose, the construction of cells.
|
|
|
|
@cindex atom
|
|
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
|
|
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.
|
|
|
|
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} of the cons cell points to the element, and its @sc{cdr} points
|
|
to the next cons cell of the list, which holds the next element in the
|
|
list. The @sc{cdr} of the last cons cell is set to point to @code{nil}.
|
|
|
|
@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. (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.) The following 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 refer to any Lisp
|
|
object. Each pair of boxes represents a cons cell. Each arrow is a
|
|
reference to a Lisp object, either an atom or another cons cell.
|
|
|
|
In this example, the first box, the @sc{car} of the first cons cell,
|
|
refers to or ``contains'' @code{rose} (a symbol). The second box, 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 refers to
|
|
@code{violet} and the @sc{cdr} refers to the third cons cell. The
|
|
@sc{cdr} of the third (and last) cons cell refers to @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 @samp{(@dots{})} in lists
|
|
@cindex @code{nil} in lists
|
|
@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 are examples of lists written in Lisp syntax:
|
|
|
|
@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
|
|
|
|
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
|
|
|
|
@menu
|
|
* Dotted Pair Notation:: An alternative syntax for lists.
|
|
* Association List Type:: A specially constructed list.
|
|
@end menu
|
|
|
|
@node Dotted Pair Notation
|
|
@comment node-name, next, previous, up
|
|
@subsubsection Dotted Pair Notation
|
|
@cindex dotted pair notation
|
|
@cindex @samp{.} in lists
|
|
|
|
@dfn{Dotted pair notation} is an alternative 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 therefore more general than list syntax. In the dotted
|
|
pair notation, the list @samp{(1 2 3)} is written as @samp{(1 . (2 . (3
|
|
. nil)))}. For @code{nil}-terminated lists, the two notations produce
|
|
the same result, but list notation is usually clearer and more
|
|
convenient when it is applicable. When printing a list, the dotted pair
|
|
notation is only used if the @sc{cdr} of a cell is not a list.
|
|
|
|
Here's how box notation can illustrate dotted pairs. This example
|
|
shows the pair @code{(rose . violet)}:
|
|
|
|
@example
|
|
@group
|
|
___ ___
|
|
|___|___|--> violet
|
|
|
|
|
|
|
|
--> rose
|
|
@end group
|
|
@end example
|
|
|
|
Dotted pair notation can be combined with list notation to represent a
|
|
chain of cons cells with a non-@code{nil} final @sc{cdr}. 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
|
|
|
|
These diagrams make it evident why @w{@code{(rose .@: violet .@:
|
|
buttercup)}} is invalid syntax; it would require a cons cell that has
|
|
three parts rather than two.
|
|
|
|
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)))}.
|
|
@ifinfo
|
|
It looks like this:
|
|
|
|
@example
|
|
@group
|
|
___ ___ ___ ___ ___ ___
|
|
|___|___|--> |___|___|--> |___|___|--> nil
|
|
| | |
|
|
| | |
|
|
--> rose --> violet --> buttercup
|
|
@end group
|
|
@end example
|
|
@end ifinfo
|
|
|
|
@node Association List Type
|
|
@comment node-name, next, previous, up
|
|
@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.
|
|
|
|
@node Array Type
|
|
@subsection Array Type
|
|
|
|
An @dfn{array} is composed of an arbitrary number of slots for
|
|
referring to other Lisp objects, arranged in a contiguous block of
|
|
memory. Accessing any element of an array takes 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 two types of array, strings and vectors. A string is an
|
|
array of characters and a vector is an array of arbitrary objects. Both
|
|
are one-dimensional. (Most other programming languages support
|
|
multidimensional arrays, but they are not essential; you can get the
|
|
same effect with an array of arrays.) Each type of array has its own
|
|
read syntax; see @ref{String Type}, and @ref{Vector Type}.
|
|
|
|
An array may have any length up to the largest integer; but once
|
|
created, it has a fixed size. 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 array type is contained in the sequence type and contains both the
|
|
string type and the vector 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.
|
|
|
|
@cindex @samp{"} in strings
|
|
@cindex double-quote in strings
|
|
@cindex @samp{\} in strings
|
|
@cindex backslash in strings
|
|
The read syntax for strings is a double-quote, an arbitrary number of
|
|
characters, and another double-quote, @code{"like this"}. The Lisp
|
|
reader accepts the same formats for reading the characters of a string
|
|
as it does for reading single characters (without the question mark that
|
|
begins a character literal). You can enter a nonprinting character such
|
|
as tab, @kbd{C-a} or @kbd{M-C-A} using the convenient escape sequences,
|
|
like this: @code{"\t, \C-a, \M-\C-a"}. You can include a double-quote
|
|
in a string by preceding it with a backslash; thus, @code{"\""} is a
|
|
string containing just a single double-quote character.
|
|
(@xref{Character Type}, for a description of the read syntax for
|
|
characters.)
|
|
|
|
If you use the @samp{\M-} syntax to indicate a meta character in a
|
|
string constant, this sets the
|
|
@iftex
|
|
$2^{7}$
|
|
@end iftex
|
|
@ifinfo
|
|
2**7
|
|
@end ifinfo
|
|
bit of the character in the string.
|
|
This is not the same representation that the meta modifier has in a
|
|
character on its own (not inside a string). @xref{Character Type}.
|
|
|
|
Strings cannot hold characters that have the hyper, super, or alt
|
|
modifiers; they can hold @sc{ASCII} control characters, but no others.
|
|
They do not distinguish case in @sc{ASCII} control characters.
|
|
|
|
The printed representation of a string consists of a double-quote, the
|
|
characters it contains, and another double-quote. However, you must
|
|
escape any backslash or double-quote characters in the string with a
|
|
backslash, like this: @code{"this \" is an embedded quote"}.
|
|
|
|
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.
|
|
@cindex newline in strings
|
|
|
|
@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
|
|
|
|
A string can hold properties of the text it contains, in addition to
|
|
the characters themselves. This enables programs that copy text between
|
|
strings and buffers to preserve the properties with no special effort.
|
|
@xref{Text Properties}. Strings with text properties have 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.
|
|
|
|
@xref{Strings and Characters}, for functions that work on strings.
|
|
|
|
@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 Function Type
|
|
@subsection Function Type
|
|
|
|
Just as functions in other programming languages are executable,
|
|
@dfn{Lisp function} objects are pieces of executable code. However,
|
|
functions in Lisp are primarily Lisp objects, and only secondarily the
|
|
text which represents them. These Lisp objects are lambda expressions:
|
|
lists 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 is also called an @dfn{anonymous function} (@pxref{Anonymous
|
|
Functions}). A named function in Lisp is actually 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 parameter-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} function, 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.
|
|
|
|
@node Primitive Function Type
|
|
@subsection Primitive Function Type
|
|
@cindex special forms
|
|
|
|
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}).@refill
|
|
|
|
It does not matter to the caller of a function whether the function is
|
|
primitive. However, this does matter if you try to substitute a
|
|
function written in Lisp for a primitive of the same name. 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.
|
|
|
|
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
|
|
|
|
The byte compiler produces @dfn{byte-code function objects}.
|
|
Internally, a byte-code function object is much like a vector; however,
|
|
the evaluator handles this data type specially when it appears as a
|
|
function to be called. @xref{Byte Compilation}, for information about
|
|
the byte compiler.
|
|
|
|
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 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 as
|
|
a placeholder for the real definition; it says that the real definition
|
|
is found in a file of Lisp code that should be loaded when necessary.
|
|
The autoload object 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 Editing Types
|
|
@section Editing Types
|
|
@cindex editing types
|
|
|
|
The types in the previous section are common to many 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.
|
|
* Window Configuration Type:: Recording the way a frame is subdivided.
|
|
* Process Type:: A process running on the underlying OS.
|
|
* Stream Type:: Receive or send characters.
|
|
* Keymap Type:: What function a keystroke invokes.
|
|
* Syntax Table Type:: What a character means.
|
|
* Display Table Type:: How display tables are represented.
|
|
* Overlay Type:: How an overlay is represented.
|
|
@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.
|
|
|
|
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, insertion of text into a buffer is very
|
|
efficient, whereas ``inserting'' text into a string requires
|
|
concatenating substrings, and the result is an entirely new string
|
|
object.
|
|
|
|
Each buffer has a designated position called @dfn{point}
|
|
(@pxref{Positions}). At any time, one buffer is the @dfn{current
|
|
buffer}. Most editing commands act on the contents of the current
|
|
buffer in the neighborhood of point. 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 local variable binding list (@pxref{Buffer-Local Variables}).
|
|
|
|
@item
|
|
a list of 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. @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, 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 @var{frame} is a rectangle on the screen that contains one or more
|
|
Emacs windows. A frame initially contains a single main window (plus
|
|
perhaps a minibuffer window) which you can subdivide vertically or
|
|
horizontally into smaller windows.
|
|
|
|
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 xemacs@@mole.gnu.ai.mit.edu 0xdac80>
|
|
@end group
|
|
@end example
|
|
|
|
@xref{Frames}, for a description of the functions that work on frames.
|
|
|
|
@node Window Configuration Type
|
|
@subsection Window Configuration Type
|
|
@cindex screen layout
|
|
|
|
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 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 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 Syntax Table Type
|
|
@subsection Syntax Table Type
|
|
|
|
A @dfn{syntax table} is a vector of 256 integers. Each element of the
|
|
vector defines how one character is interpreted when it appears in a
|
|
buffer. For example, in C mode (@pxref{Major Modes}), the @samp{+}
|
|
character is punctuation, but in Lisp mode it is a valid character in a
|
|
symbol. These modes specify different interpretations by changing the
|
|
syntax table entry for @samp{+}, at index 43 in the syntax table.
|
|
|
|
Syntax tables are used only for scanning text in buffers, not for
|
|
reading Lisp expressions. The table the Lisp interpreter uses to read
|
|
expressions is built into the Emacs source code and cannot be changed;
|
|
thus, to change the list delimiters to be @samp{@{} and @samp{@}}
|
|
instead of @samp{(} and @samp{)} would be impossible.
|
|
|
|
@xref{Syntax Tables}, for details about syntax classes and how to make
|
|
and modify syntax tables.
|
|
|
|
@node Display Table Type
|
|
@subsection Display Table Type
|
|
|
|
A @dfn{display table} specifies how to display each character code.
|
|
Each buffer and each window can have its own display table. A display
|
|
table is actually a vector of length 262. @xref{Display Tables}.
|
|
|
|
@node Overlay Type
|
|
@subsection Overlay Type
|
|
|
|
An @dfn{overlay} specifies temporary alteration of the display
|
|
appearance of a part of a buffer. It contains markers delimiting a
|
|
range of the buffer, plus a property list (a list whose elements are
|
|
alternating property names and values). Overlays are used to present
|
|
parts of the buffer temporarily in a different display style. They have
|
|
no read syntax, and print in hash notation, giving the buffer name and
|
|
range of positions.
|
|
|
|
@xref{Overlays}, for how to create and use overlays.
|
|
|
|
@node Type Predicates
|
|
@section Type Predicates
|
|
@cindex 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: integer-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)))
|
|
@need 3000
|
|
(t
|
|
;; We only handle 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 bufferp
|
|
@xref{Buffer Basics, bufferp}.
|
|
|
|
@item byte-code-function-p
|
|
@xref{Byte-Code Type, byte-code-function-p}.
|
|
|
|
@item case-table-p
|
|
@xref{Case Table, case-table-p}.
|
|
|
|
@item char-or-string-p
|
|
@xref{Predicates for Strings, char-or-string-p}.
|
|
|
|
@item commandp
|
|
@xref{Interactive Call, commandp}.
|
|
|
|
@item consp
|
|
@xref{List-related Predicates, consp}.
|
|
|
|
@item floatp
|
|
@xref{Predicates on Numbers, floatp}.
|
|
|
|
@item frame-live-p
|
|
@xref{Deleting Frames, frame-live-p}.
|
|
|
|
@item framep
|
|
@xref{Frames, framep}.
|
|
|
|
@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 listp
|
|
@xref{List-related Predicates, listp}.
|
|
|
|
@item markerp
|
|
@xref{Predicates on Markers, markerp}.
|
|
|
|
@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 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 user-variable-p
|
|
@xref{Defining Variables, user-variable-p}.
|
|
|
|
@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}.
|
|
@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{symbol},
|
|
@code{integer}, @code{float}, @code{string}, @code{cons}, @code{vector},
|
|
@code{marker}, @code{overlay}, @code{window}, @code{buffer},
|
|
@code{subr}, @code{compiled-function}, @code{process}, or
|
|
@code{window-configuration}.
|
|
|
|
@example
|
|
(type-of 1)
|
|
@result{} integer
|
|
(type-of 'nil)
|
|
@result{} symbol
|
|
(type-of '()) ; @r{@code{()} is @code{nil}.}
|
|
@result{} symbol
|
|
(type-of '(x))
|
|
@result{} cons
|
|
@end example
|
|
@end defun
|
|
|
|
@node Equality Predicates
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@section Equality Predicates
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@cindex equality
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Here we describe two functions that test for equality between any two
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objects. Other functions test equality between objects of specific
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types, e.g., strings. For these predicates, see the appropriate chapter
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describing the data type.
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@defun eq object1 object2
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This function returns @code{t} if @var{object1} and @var{object2} are
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the same object, @code{nil} otherwise. The ``same object'' means that a
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change in one will be reflected by the same change in the other.
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@code{eq} returns @code{t} if @var{object1} and @var{object2} are
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integers with the same value. Also, since symbol names are normally
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unique, if the arguments are symbols with the same name, they are
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@code{eq}. For other types (e.g., lists, vectors, strings), two
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arguments with the same contents or elements are not necessarily
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@code{eq} to each other: they are @code{eq} only if they are the same
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object.
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(The @code{make-symbol} function returns an uninterned symbol that is
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not interned in the standard @code{obarray}. When uninterned symbols
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are in use, symbol names are no longer unique. Distinct symbols with
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the same name are not @code{eq}. @xref{Creating Symbols}.)
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@example
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@group
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(eq 'foo 'foo)
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@result{} t
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@end group
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@group
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(eq 456 456)
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@result{} t
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@end group
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@group
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(eq "asdf" "asdf")
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@result{} nil
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@end group
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@group
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(eq '(1 (2 (3))) '(1 (2 (3))))
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@result{} nil
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@end group
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@group
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(setq foo '(1 (2 (3))))
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@result{} (1 (2 (3)))
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(eq foo foo)
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@result{} t
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(eq foo '(1 (2 (3))))
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@result{} nil
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@end group
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@group
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(eq [(1 2) 3] [(1 2) 3])
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@result{} nil
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@end group
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@group
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(eq (point-marker) (point-marker))
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@result{} nil
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@end group
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@end example
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@end defun
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@defun equal object1 object2
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This function returns @code{t} if @var{object1} and @var{object2} have
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equal components, @code{nil} otherwise. Whereas @code{eq} tests if its
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arguments are the same object, @code{equal} looks inside nonidentical
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arguments to see if their elements are the same. So, if two objects are
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@code{eq}, they are @code{equal}, but the converse is not always true.
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@example
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@group
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(equal 'foo 'foo)
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@result{} t
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@end group
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@group
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(equal 456 456)
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@result{} t
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@end group
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@group
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(equal "asdf" "asdf")
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@result{} t
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@end group
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@group
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(eq "asdf" "asdf")
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@result{} nil
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@end group
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@group
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(equal '(1 (2 (3))) '(1 (2 (3))))
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@result{} t
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@end group
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@group
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(eq '(1 (2 (3))) '(1 (2 (3))))
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@result{} nil
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@end group
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@group
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(equal [(1 2) 3] [(1 2) 3])
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@result{} t
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@end group
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@group
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(eq [(1 2) 3] [(1 2) 3])
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@result{} nil
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@end group
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@group
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(equal (point-marker) (point-marker))
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@result{} t
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@end group
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@group
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(eq (point-marker) (point-marker))
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@result{} nil
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@end group
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@end example
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Comparison of strings is case-sensitive and takes account of text
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properties as well as the characters in the strings. To compare
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two strings' characters without comparing their text properties,
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use @code{string=} (@pxref{Text Comparison}).
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@example
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@group
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(equal "asdf" "ASDF")
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@result{} nil
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@end group
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@end example
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Two distinct buffers are never @code{equal}, even if their contents
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are the same.
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@end defun
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The test for equality is implemented recursively, and circular lists may
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therefore cause infinite recursion (leading to an error).
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