@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
-@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, 2003
-@c Free Software Foundation, Inc.
+@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, 2002, 2003,
+@c 2004, 2005, 2006 Free Software Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@setfilename ../info/objects
@node Lisp Data Types, Numbers, Introduction, Top
variable, and the type is known by the compiler but not represented in
the data. Such type declarations do not exist in Emacs Lisp. A Lisp
variable can have any type of value, and it remembers whatever value
-you store in it, type and all.
+you store in it, type and all. (Actually, a small number of Emacs
+Lisp variables can only take on values of a certain type.
+@xref{Variables with Restricted Values}.)
This chapter describes the purpose, printed representation, and read
syntax of each of the standard types in GNU Emacs Lisp. Details on how
The @dfn{printed representation} of an object is the format of the
output generated by the Lisp printer (the function @code{prin1}) for
-that object. The @dfn{read syntax} of an object is the format of the
-input accepted by the Lisp reader (the function @code{read}) for that
-object. @xref{Read and Print}.
-
- Most objects have more than one possible read syntax. Some types of
-object have no read syntax, since it may not make sense to enter objects
-of these types directly in a Lisp program. Except for these cases, the
-printed representation of an object is also a read syntax for it.
-
- In other languages, an expression is text; it has no other form. In
-Lisp, an expression is primarily a Lisp object and only secondarily the
-text that is the object's read syntax. Often there is no need to
-emphasize this distinction, but you must keep it in the back of your
-mind, or you will occasionally be very confused.
+that object. Every data type has a unique printed representation.
+The @dfn{read syntax} of an object is the format of the input accepted
+by the Lisp reader (the function @code{read}) for that object. This
+is not necessarily unique; many kinds of object have more than one
+syntax. @xref{Read and Print}.
@cindex hash notation
- Every type has a printed representation. Some types have no read
-syntax---for example, the buffer type has none. Objects of these types
-are printed in @dfn{hash notation}: the characters @samp{#<} followed by
-a descriptive string (typically the type name followed by the name of
-the object), and closed with a matching @samp{>}. Hash notation cannot
-be read at all, so the Lisp reader signals the error
-@code{invalid-read-syntax} whenever it encounters @samp{#<}.
-@kindex invalid-read-syntax
+ In most cases, an object's printed representation is also a read
+syntax for the object. However, some types have no read syntax, since
+it does not make sense to enter objects of these types as constants in
+a Lisp program. These objects are printed in @dfn{hash notation},
+which consists of the characters @samp{#<}, a descriptive string
+(typically the type name followed by the name of the object), and a
+closing @samp{>}. For example:
@example
(current-buffer)
@result{} #<buffer objects.texi>
@end example
+@noindent
+Hash notation cannot be read at all, so the Lisp reader signals the
+error @code{invalid-read-syntax} whenever it encounters @samp{#<}.
+@kindex invalid-read-syntax
+
+ In other languages, an expression is text; it has no other form. In
+Lisp, an expression is primarily a Lisp object and only secondarily the
+text that is the object's read syntax. Often there is no need to
+emphasize this distinction, but you must keep it in the back of your
+mind, or you will occasionally be very confused.
+
When you evaluate an expression interactively, the Lisp interpreter
first reads the textual representation of it, producing a Lisp object,
and then evaluates that object (@pxref{Evaluation}). However,
@subsection Floating Point Type
Floating point numbers are the computer equivalent of scientific
-notation. The precise number of significant figures and the range of
-possible exponents is machine-specific; Emacs always uses the C data
-type @code{double} to store the value.
+notation; you can think of a floating point number as a fraction
+together with a power of ten. The precise number of significant
+figures and the range of possible exponents is machine-specific; Emacs
+uses the C data type @code{double} to store the value, and internally
+this records a power of 2 rather than a power of 10.
The printed representation for floating point numbers requires either
a decimal point (with at least one digit following), an exponent, or
common to work with @emph{strings}, which are sequences composed of
characters. @xref{String Type}.
- Characters in strings, buffers, and files are currently limited to the
-range of 0 to 524287---nineteen bits. But not all values in that range
-are valid character codes. Codes 0 through 127 are @acronym{ASCII} codes; the
-rest are non-@acronym{ASCII} (@pxref{Non-ASCII Characters}). Characters that represent
-keyboard input have a much wider range, to encode modifier keys such as
+ Characters in strings, buffers, and files are currently limited to
+the range of 0 to 524287---nineteen bits. But not all values in that
+range are valid character codes. Codes 0 through 127 are
+@acronym{ASCII} codes; the rest are non-@acronym{ASCII}
+(@pxref{Non-ASCII Characters}). Characters that represent keyboard
+input have a much wider range, to encode modifier keys such as
Control, Meta and Shift.
@cindex read syntax for characters
vertical tab, formfeed, space, return, del, and escape as @samp{?\a},
@samp{?\b}, @samp{?\t}, @samp{?\n}, @samp{?\v}, @samp{?\f},
@samp{?\s}, @samp{?\r}, @samp{?\d}, and @samp{?\e}, respectively.
-Thus,
+(@samp{?\s} followed by a dash has a different meaning---it applies
+the ``super'' modifier to the following character.) Thus,
@example
?\a @result{} 7 ; @r{control-g, @kbd{C-g}}
These sequences which start with backslash are also known as
@dfn{escape sequences}, because backslash plays the role of an
``escape character''; this terminology has nothing to do with the
-character @key{ESC}. @samp{\s} is meant for use only in character
+character @key{ESC}. @samp{\s} is meant for use in character
constants; in string constants, just write the space.
@cindex control characters
@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. (In Emacs versions 18 and older, this convention was used
-for characters outside of strings as well.)
+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. (In Emacs versions 18 and older,
+this convention was used for characters outside of strings as well.)
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
@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
+ 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.
+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
@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
-use, the name is unique---no two symbols have the same name.
+ 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
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 seventh example because the rest of the name
+This is not necessary in the fourth example because the rest of the name
makes it invalid as a number.
@example
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. @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} derive from the history of Lisp. 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 was named
-for its purpose, the construction of cells.
+empty list. The empty list is actually the symbol @code{nil}.
+@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.
@cindex atom
Because cons cells are so central to Lisp, we also have a word for
@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.
+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
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, 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 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
@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:
@end group
@end example
-@menu
-* Dotted Pair Notation:: An alternative syntax for lists.
-* Association List Type:: A specially constructed list.
-@end menu
+ 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
-@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,
+ @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 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, 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.
+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)}:
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 an array of arrays.) Each type of
-array has its own read syntax; see the following sections for details.
+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 contained in the sequence type and
-contains the string type, the vector type, the bool-vector type, and the
-char-table type.
+ 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
constant is just like backslash-newline; it does not contribute any
character to the string, but it does terminate the preceding hex escape.
- Using a multibyte hex escape forces the string to multibyte. You can
-represent a unibyte non-@acronym{ASCII} character with its character code,
-which must be in the range from 128 (0200 octal) to 255 (0377 octal).
-This forces a unibyte string.
+ You can represent a unibyte non-@acronym{ASCII} character with its
+character code, which must be in the range from 128 (0200 octal) to
+255 (0377 octal). If you write all such character codes in octal and
+the string contains no other characters forcing it to be multibyte,
+this produces a unibyte string. However, using any hex escape in a
+string (even for an @acronym{ASCII} character) forces the string to be
+multibyte.
@xref{Text Representations}, for more information about the two
text representations.
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
+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
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. If the length is not a
-multiple of 8, the printed representation shows extra elements, but
-these extras really make no difference.
+correspond to the lowest indices in the bool-vector.
@example
(make-bool-vector 3 t)
- @result{} #&3"\007"
+ @result{} #&3"^G"
(make-bool-vector 3 nil)
- @result{} #&3"\0"
-;; @r{These are equal since only the first 3 bits are used.}
+ @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
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.
-Hash tables are a new feature in Emacs 21; they have no read syntax, and
-print using hash notation. @xref{Hash Tables}.
+Hash tables have no read syntax, and print using hash notation.
+@xref{Hash Tables}, for functions that operate on hash tables.
@example
(make-hash-table)
@cindex @samp{#@var{n}=} read syntax
@cindex @samp{#@var{n}#} read syntax
- In Emacs 21, to represent shared or circular structure within a
-complex of Lisp objects, you can use the reader constructs
-@samp{#@var{n}=} and @samp{#@var{n}#}.
+ 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
@node Type Predicates
@section Type Predicates
-@cindex predicates
@cindex type checking
@kindex wrong-type-argument
@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 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}.
@end table
The most general way to check the type of an object is to call the
@defun eq object1 object2
This function returns @code{t} if @var{object1} and @var{object2} are
-the same object, @code{nil} otherwise. The ``same object'' means that a
-change in one will be reflected by the same change in the other.
+the same object, @code{nil} otherwise.
@code{eq} returns @code{t} if @var{object1} and @var{object2} are
integers with the same value. Also, since symbol names are normally
@code{eq}. 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.
+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
@end group
@end example
+@cindex equality of strings
Comparison of strings is case-sensitive, but does not take account of
-text properties---it compares only the characters in the strings.
-A unibyte string never equals a multibyte string unless the
-contents are entirely @acronym{ASCII} (@pxref{Text Representations}).
+text properties---it compares only the characters in the strings. 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