works is to experiment. We will start with the @code{setcar} function.
@need 1200
-@cindex constant lists
-@cindex mutable lists
First, we can make a list and then set the value of a variable to the
list, using the @code{setq} special form. Because we intend to use
@code{setcar} to change the list, this @code{setq} should not use the
tried to change part of the program while running it. Generally
speaking an Emacs Lisp program's components should be constant (or
unchanged) while the program is running. So we instead construct an
-animal list that is @dfn{mutable} (or changeable) by using the
-@code{list} function, as follows:
+animal list by using the @code{list} function, as follows:
@smallexample
(setq animals (list 'antelope 'giraffe 'lion 'tiger))
* Circular Objects:: Read syntax for circular structure.
* Type Predicates:: Tests related to types.
* Equality Predicates:: Tests of equality between any two objects.
-* Constants and Mutability:: Whether an object's value can change.
+* Mutability:: Some objects should not be modified.
Programming Types
@end group
@end example
- A self-evaluating form yields constant conses, vectors and strings, and you
-should not attempt to modify their contents via @code{setcar}, @code{aset} or
+ A self-evaluating form yields a value that becomes part of the program,
+and you should not try to modify it via @code{setcar}, @code{aset} or
similar operations. The Lisp interpreter might unify the constants
yielded by your program's self-evaluating forms, so that these
-constants might share structure. @xref{Constants and Mutability}.
+constants might share structure. @xref{Mutability}.
It is common to write numbers, characters, strings, and even vectors
in Lisp code, taking advantage of the fact that they self-evaluate.
@defspec quote object
This special form returns @var{object}, without evaluating it.
-The returned value is a constant, and should not be modified.
-@xref{Constants and Mutability}.
+The returned value might be shared and should not be modified.
+@xref{Self-Evaluating Forms}.
@end defspec
@cindex @samp{'} for quoting
Although the expressions @code{(list '+ 1 2)} and @code{'(+ 1 2)}
both yield lists equal to @code{(+ 1 2)}, the former yields a
-freshly-minted mutable list whereas the latter yields a constant list
-built from conses that may be shared with other constants.
-@xref{Constants and Mutability}.
+freshly-minted mutable list whereas the latter yields a list
+built from conses that might be shared and should not be modified.
+@xref{Self-Evaluating Forms}.
Other quoting constructs include @code{function} (@pxref{Anonymous
Functions}), which causes an anonymous lambda expression written in Lisp
@end example
If a subexpression of a backquote construct has no substitutions or
-splices, it acts like @code{quote} in that it yields constant conses,
-vectors and strings that should not be modified.
+splices, it acts like @code{quote} in that it yields conses,
+vectors and strings that might be shared and should not be modified.
+@xref{Self-Evaluating Forms}.
@node Eval
@section Eval
operations because they change existing list structure.
Destructive operations should be applied only to mutable lists,
that is, lists constructed via @code{cons}, @code{list} or similar
-operations. Lists created by quoting are constants and should not be
-changed by destructive operations. @xref{Constants and Mutability}.
+operations. Lists created by quoting are part of the program and
+should not be changed by destructive operations. @xref{Mutability}.
@cindex CL note---@code{rplaca} vs @code{setcar}
@quotation
@example
@group
-(setq x (list 1 2)) ; @r{Create a mutable list.}
+(setq x (list 1 2))
@result{} (1 2)
@end group
@group
@example
@group
-;; @r{Create two mutable lists that are partly shared.}
+;; @r{Create two lists that are partly shared.}
(setq x1 (list 'a 'b 'c))
@result{} (a b c)
(setq x2 (cons 'z (cdr x1)))
@example
@group
-(setq x (list 1 2 3)) ; @r{Create a mutable list.}
+(setq x (list 1 2 3))
@result{} (1 2 3)
@end group
@group
-(setcdr x '(4)) ; @r{Modify the list's tail to be a constant list.}
+(setcdr x '(4))
@result{} (4)
@end group
@group
@example
@group
-(setq x (list 1 2 3)) ; @r{Create a mutable list.}
+(setq x (list 1 2 3))
@result{} (1 2 3)
@end group
@group
-(nconc x '(4 5)) ; @r{Modify the list's tail to be a constant list.}
+(nconc x '(4 5))
@result{} (1 2 3 4 5)
@end group
@group
Lisp variables can only take on values of a certain type.
@xref{Variables with Restricted Values}.)
- Some Lisp objects are @dfn{constant}: their values should never change.
-Others are @dfn{mutable}: their values can be changed via destructive
-operations that involve side effects.
-
This chapter describes the purpose, printed representation, and read
syntax of each of the standard types in GNU Emacs Lisp. Details on how
to use these types can be found in later chapters.
* Circular Objects:: Read syntax for circular structure.
* Type Predicates:: Tests related to types.
* Equality Predicates:: Tests of equality between any two objects.
-* Constants and Mutability:: Whether an object's value can change.
+* Mutability:: Some objects should not be modified.
@end menu
@node Printed Representation
@end example
@end defun
-@node Constants and Mutability
-@section Constants and Mutability
-@cindex constants
+@node Mutability
+@section Mutability
@cindex mutable objects
- Some Lisp objects are constant: their values should never change
-during a single execution of Emacs running well-behaved Lisp code.
-For example, you can create a new integer by calculating one, but you
-cannot modify the value of an existing integer.
-
- Other Lisp objects are mutable: it is safe to change their values
-via destructive operations involving side effects. For example, an
-existing marker can be changed by moving the marker to point to
-somewhere else.
-
- Although all numbers are constants and all markers are
-mutable, some types contain both constant and mutable members. These
-types include conses, vectors, strings, and symbols. For example, the string
-literal @code{"aaa"} yields a constant string, whereas the function
-call @code{(make-string 3 ?a)} yields a mutable string that can be
-changed via later calls to @code{aset}.
-
- A mutable object can become constant if it is part of an expression
-that is evaluated. The reverse does not occur: constant objects
-should stay constant.
-
- Trying to modify a constant variable signals an error
-(@pxref{Constant Variables}).
-A program should not attempt to modify other types of constants because the
-resulting behavior is undefined: the Lisp interpreter might or might
-not detect the error, and if it does not detect the error the
-interpreter can behave unpredictably thereafter. Another way to put
-this is that although mutable objects are safe to change and constant
-variables reliably prevent attempts to change them, other constants
-are not safely mutable: if a misbehaving program tries to change such a
-constant then the constant's value might actually change, or the
-program might crash or worse. This problem occurs
-with types that have both constant and mutable members, and that have
-mutators like @code{setcar} and @code{aset} that are valid on mutable
-objects but hazardous on constants.
-
- When the same constant occurs multiple times in a program, the Lisp
+ Some Lisp objects should never change. For example, the Lisp
+expression @code{"aaa"} yields a string, but you should not change
+its contents. Indeed, some objects cannot be changed; for example,
+although you can create a new number by calculating one, Lisp provides
+no operation to change the value of an existing number.
+
+ Other Lisp objects are @dfn{mutable}: it is safe to change their
+values via destructive operations involving side effects. For
+example, an existing marker can be changed by moving the marker to
+point to somewhere else.
+
+ Although numbers never change and all markers are mutable,
+some types have members some of which are mutable and others not. These
+types include conses, vectors, and strings. For example,
+although @code{"aaa"} yields a string that should not be changed,
+@code{(make-string 3 ?a)} yields a mutable string that can be
+changed via later calls to @code{aset}. Another example:
+@code{(symbol-name 'cons)} yields a string @code{"cons"} that should
+not be changed.
+
+ A mutable object stops being mutable if it is part of an expression
+that is evaluated. For example:
+
+@example
+(let* ((x (list 0.5))
+ (y (eval (list 'quote x))))
+ (setcar x 1.5) ;; The program should not do this.
+ y)
+@end example
+
+@noindent
+Although the list @code{(0.5)} was mutable when it was created, it should not
+have been changed via @code{setcar} because it given to @code{eval}. The
+reverse does not occur: an object that should not be changed never
+becomes mutable afterwards.
+
+ If a program attempts to change objects that should not be
+changed, the resulting behavior is undefined: the Lisp interpreter
+might signal an error, or it might crash or behave unpredictably in
+other ways.@footnote{This is the behavior specified for languages like
+Common Lisp and C, and it differs from the behavior of languages like
+JavaScript and Python where an interpreter is required to signal an
+error if a program attempts to change a constant. Ideally the Emacs
+Lisp interpreter will evolve in latter direction.}
+
+ When similar constants occur as parts of a program, the Lisp
interpreter might save time or space by reusing existing constants or
-constant components. For example, @code{(eq "abc" "abc")} returns
+their components. For example, @code{(eq "abc" "abc")} returns
@code{t} if the interpreter creates only one instance of the string
-constant @code{"abc"}, and returns @code{nil} if it creates two
+literal @code{"abc"}, and returns @code{nil} if it creates two
instances. Lisp programs should be written so that they work
regardless of whether this optimization is in use.
@example
@group
-(setq bar (list 1 2)) ; @r{Create a mutable list.}
+(setq bar (list 1 2))
@result{} (1 2)
@end group
@group
-(setq x (vector 'foo bar)) ; @r{Create a mutable vector.}
+(setq x (vector 'foo bar))
@result{} [foo (1 2)]
@end group
@group
@example
@group
-(setq x (list 'a 'b 'c)) ; @r{Create a mutable list.}
+(setq x (list 'a 'b 'c))
@result{} (a b c)
@end group
@group
For the vector, it is even simpler because you don't need setq:
@example
-(setq x (copy-sequence [1 2 3 4])) ; @r{Create a mutable vector.}
+(setq x (copy-sequence [1 2 3 4]))
@result{} [1 2 3 4]
(nreverse x)
@result{} [4 3 2 1]
Note that unlike @code{reverse}, this function doesn't work with strings.
Although you can alter string data by using @code{aset}, it is strongly
encouraged to treat strings as immutable even when they are mutable.
+@xref{Mutability}.
@end defun
@example
@group
-(setq nums (list 1 3 2 6 5 4 0)) ; @r{Create a mutable list.}
+(setq nums (list 1 3 2 6 5 4 0))
@result{} (1 3 2 6 5 4 0)
@end group
@group
@example
@group
-(setq w (vector 'foo 'bar 'baz)) ; @r{Create a mutable vector.}
+(setq w (vector 'foo 'bar 'baz))
@result{} [foo bar baz]
(aset w 0 'fu)
@result{} fu
@end group
@group
-;; @r{@code{copy-sequence} creates a mutable string.}
+;; @r{@code{copy-sequence} copies the string to be modified later.}
(setq x (copy-sequence "asdfasfd"))
@result{} "asdfasfd"
(aset x 3 ?Z)
@end group
@end example
-The @var{array} should be mutable; that is, it should not be a constant,
-such as the constants created via quoting or via self-evaluating forms.
-@xref{Constants and Mutability}.
+The @var{array} should be mutable. @xref{Mutability}.
If @var{array} is a string and @var{object} is not a character, a
@code{wrong-type-argument} error results. The function converts a
@example
@group
-;; @r{Create a mutable vector and then fill it with zeros.}
(setq a (copy-sequence [a b c d e f g]))
@result{} [a b c d e f g]
(fillarray a 0)
@result{} [0 0 0 0 0 0 0]
@end group
@group
-;; @r{Create a mutable string and then fill it with "-".}
(setq s (copy-sequence "When in the course"))
@result{} "When in the course"
(fillarray s ?-)
evaluation: the result of evaluating it is the same vector. This does
not evaluate or even examine the elements of the vector.
@xref{Self-Evaluating Forms}. Vectors written with square brackets
-are constants and should not be modified via @code{aset} or other
-destructive operations. @xref{Constants and Mutability}.
+should not be modified via @code{aset} or other destructive
+operations. @xref{Mutability}.
Here are examples illustrating these principles:
Since strings are arrays, and therefore sequences as well, you can
operate on them with the general array and sequence functions documented
-in @ref{Sequences Arrays Vectors}. For example, you can access or
-change individual characters in a string using the functions @code{aref}
-and @code{aset} (@pxref{Array Functions}). However, you should not
-try to change the contents of constant strings (@pxref{Modifying Strings}).
+in @ref{Sequences Arrays Vectors}. For example, you can access
+individual characters in a string using the function @code{aref}
+(@pxref{Array Functions}).
There are two text representations for non-@acronym{ASCII}
characters in Emacs strings (and in buffers): unibyte and multibyte.
@cindex string modification
You can alter the contents of a mutable string via operations
-described in this section. However, you should not try to use these
-operations to alter the contents of a constant string.
-@xref{Constants and Mutability}.
+described in this section. @xref{Mutability}.
The most basic way to alter the contents of an existing string is with
@code{aset} (@pxref{Array Functions}). @code{(aset @var{string}
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