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[gnu-emacs] / lispref / eval.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc. 
@c See the file elisp.texi for copying conditions.
@setfilename ../info/eval
@node Evaluation, Control Structures, Symbols, Top
@chapter Evaluation
@cindex evaluation
@cindex  interpreter
@cindex interpreter
@cindex value of expression

  The @dfn{evaluation} of expressions in Emacs Lisp is performed by the
@dfn{Lisp interpreter}---a program that receives a Lisp object as input
and computes its @dfn{value as an expression}.  How it does this depends
on the data type of the object, according to rules described in this
chapter.  The interpreter runs automatically to evaluate portions of
your program, but can also be called explicitly via the Lisp primitive
function @code{eval}.

@ifinfo
@menu
* Intro Eval::  Evaluation in the scheme of things.
* Eval::        How to invoke the Lisp interpreter explicitly.
* Forms::       How various sorts of objects are evaluated.
* Quoting::     Avoiding evaluation (to put constants in the program).
@end menu

@node Intro Eval
@section Introduction to Evaluation

  The Lisp interpreter, or evaluator, is the program that computes
the value of an expression that is given to it.  When a function 
written in Lisp is called, the evaluator computes the value of the
function by evaluating the expressions in the function body.  Thus,
running any Lisp program really means running the Lisp interpreter.

  How the evaluator handles an object depends primarily on the data
type of the object.
@end ifinfo

@cindex forms
@cindex expression
  A Lisp object that is intended for evaluation is called an
@dfn{expression} or a @dfn{form}.  The fact that expressions are data
objects and not merely text is one of the fundamental differences
between Lisp-like languages and typical programming languages.  Any
object can be evaluated, but in practice only numbers, symbols, lists
and strings are evaluated very often.

  It is very common to read a Lisp expression and then evaluate the
expression, but reading and evaluation are separate activities, and
either can be performed alone.  Reading per se does not evaluate
anything; it converts the printed representation of a Lisp object to the
object itself.  It is up to the caller of @code{read} whether this
object is a form to be evaluated, or serves some entirely different
purpose.  @xref{Input Functions}.

  Do not confuse evaluation with command key interpretation.  The
editor command loop translates keyboard input into a command (an
interactively callable function) using the active keymaps, and then
uses @code{call-interactively} to invoke the command.  The execution of
the command itself involves evaluation if the command is written in
Lisp, but that is not a part of command key interpretation itself.
@xref{Command Loop}.

@cindex recursive evaluation
  Evaluation is a recursive process.  That is, evaluation of a form may
call @code{eval} to evaluate parts of the form.  For example, evaluation
of a function call first evaluates each argument of the function call,
and then evaluates each form in the function body.  Consider evaluation
of the form @code{(car x)}: the subform @code{x} must first be evaluated
recursively, so that its value can be passed as an argument to the
function @code{car}.

  Evaluation of a function call ultimately calls the function specified
in it.  @xref{Functions}.  The execution of the function may itself work
by evaluating the function definition; or the function may be a Lisp
primitive implemented in C, or it may be a byte-compiled function
(@pxref{Byte Compilation}).

@cindex environment
  The evaluation of forms takes place in a context called the
@dfn{environment}, which consists of the current values and bindings of
all Lisp variables.@footnote{This definition of ``environment'' is
specifically not intended to include all the data that can affect the
result of a program.}  Whenever the form refers to a variable without
creating a new binding for it, the value of the binding in the current
environment is used.  @xref{Variables}.

@cindex side effect
  Evaluation of a form may create new environments for recursive
evaluation by binding variables (@pxref{Local Variables}).  These
environments are temporary and vanish by the time evaluation of the form
is complete.  The form may also make changes that persist; these changes
are called @dfn{side effects}.  An example of a form that produces side
effects is @code{(setq foo 1)}.

  The details of what evaluation means for each kind of form are
described below (@pxref{Forms}).

@node Eval
@section Eval
@c ??? Perhaps this should be the last section in the chapter.

  Most often, forms are evaluated automatically, by virtue of their
occurrence in a program being run.  On rare occasions, you may need to
write code that evaluates a form that is computed at run time, such as
after reading a form from text being edited or getting one from a
property list.  On these occasions, use the @code{eval} function.

  @strong{Note:} it is generally cleaner and more flexible to call
functions that are stored in data structures, rather than to evaluate
expressions stored in data structures.  Using functions provides the
ability to pass information to them as arguments.

  The functions and variables described in this section evaluate forms,
specify limits to the evaluation process, or record recently returned
values.  Loading a file also does evaluation (@pxref{Loading}).

@defun eval form
This is the basic function for performing evaluation.  It evaluates
@var{form} in the current environment and returns the result.  How the
evaluation proceeds depends on the type of the object (@pxref{Forms}).

Since @code{eval} is a function, the argument expression that appears
in a call to @code{eval} is evaluated twice: once as preparation before
@code{eval} is called, and again by the @code{eval} function itself.
Here is an example:

@example
@group
(setq foo 'bar)
     @result{} bar
@end group
@group
(setq bar 'baz)
     @result{} baz
;; @r{@code{eval} receives argument @code{bar}, which is the value of @code{foo}}
(eval foo)
     @result{} baz
(eval 'foo)
     @result{} bar
@end group
@end example

The number of currently active calls to @code{eval} is limited to
@code{max-lisp-eval-depth} (see below).
@end defun

@deffn Command eval-region start end &optional stream
This function evaluates the forms in the current buffer in the region
defined by the positions @var{start} and @var{end}.  It reads forms from
the region and calls @code{eval} on them until the end of the region is
reached, or until an error is signaled and not handled.

If @var{stream} is supplied, @code{standard-output} is bound to it
during the evaluation.

You can use the variable @code{load-read-function} to specify a function
for @code{eval-region} to use instead of @code{read} for reading
expressions.  @xref{How Programs Do Loading}.

@code{eval-region} always returns @code{nil}.
@end deffn

@cindex evaluation of buffer contents
@deffn Command eval-current-buffer &optional stream
This is like @code{eval-region} except that it operates on the whole
buffer.
@end deffn

@defvar max-lisp-eval-depth
This variable defines the maximum depth allowed in calls to @code{eval},
@code{apply}, and @code{funcall} before an error is signaled (with error
message @code{"Lisp nesting exceeds max-lisp-eval-depth"}).  This counts
internal uses of those functions, such as for calling the functions
mentioned in Lisp expressions, and recursive evaluation of function call
arguments and function body forms.

This limit, with the associated error when it is exceeded, is one way
that Lisp avoids infinite recursion on an ill-defined function.
@cindex Lisp nesting error

The default value of this variable is 200.  If you set it to a value
less than 100, Lisp will reset it to 100 if the given value is reached.

@code{max-specpdl-size} provides another limit on nesting.
@xref{Local Variables}.
@end defvar

@defvar values
The value of this variable is a list of the values returned by all the
expressions that were read from buffers (including the minibuffer),
evaluated, and printed.  The elements are ordered most recent first.

@example
@group
(setq x 1)
     @result{} 1
@end group
@group
(list 'A (1+ 2) auto-save-default)
     @result{} (A 3 t)
@end group
@group
values
     @result{} ((A 3 t) 1 @dots{})
@end group
@end example

This variable is useful for referring back to values of forms recently
evaluated.  It is generally a bad idea to print the value of
@code{values} itself, since this may be very long.  Instead, examine
particular elements, like this:

@example
@group
;; @r{Refer to the most recent evaluation result.}
(nth 0 values)
     @result{} (A 3 t)
@end group
@group
;; @r{That put a new element on,}
;;   @r{so all elements move back one.}
(nth 1 values)
     @result{} (A 3 t)
@end group
@group
;; @r{This gets the element that was next-to-most-recent}
;;   @r{before this example.}
(nth 3 values)
     @result{} 1
@end group
@end example
@end defvar

@node Forms
@section Kinds of Forms

  A Lisp object that is intended to be evaluated is called a @dfn{form}.
How Emacs evaluates a form depends on its data type.  Emacs has three
different kinds of form that are evaluated differently: symbols, lists,
and ``all other types''.  This section describes all three kinds,
starting with ``all other types'' which are self-evaluating forms.

@menu
* Self-Evaluating Forms::   Forms that evaluate to themselves.
* Symbol Forms::            Symbols evaluate as variables.
* Classifying Lists::       How to distinguish various sorts of list forms.
* Function Indirection::    When a symbol appears as the car of a list,
                              we find the real function via the symbol.
* Function Forms::          Forms that call functions.
* Macro Forms::             Forms that call macros.
* Special Forms::           ``Special forms'' are idiosyncratic primitives,
                              most of them extremely important.
* Autoloading::             Functions set up to load files
                              containing their real definitions.
@end menu

@node Self-Evaluating Forms
@subsection Self-Evaluating Forms
@cindex vector evaluation
@cindex literal evaluation
@cindex self-evaluating form

  A @dfn{self-evaluating form} is any form that is not a list or symbol.
Self-evaluating forms evaluate to themselves: the result of evaluation
is the same object that was evaluated.  Thus, the number 25 evaluates to
25, and the string @code{"foo"} evaluates to the string @code{"foo"}.
Likewise, evaluation of a vector does not cause evaluation of the
elements of the vector---it returns the same vector with its contents
unchanged.

@example
@group
'123               ; @r{An object, shown without evaluation.}
     @result{} 123
@end group
@group
123                ; @r{Evaluated as usual---result is the same.}
     @result{} 123
@end group
@group
(eval '123)        ; @r{Evaluated ``by hand''---result is the same.}
     @result{} 123
@end group
@group
(eval (eval '123)) ; @r{Evaluating twice changes nothing.}
     @result{} 123
@end group
@end example

  It is common to write numbers, characters, strings, and even vectors
in Lisp code, taking advantage of the fact that they self-evaluate.
However, it is quite unusual to do this for types that lack a read
syntax, because there's no way to write them textually.  It is possible
to construct Lisp expressions containing these types by means of a Lisp
program.  Here is an example:

@example
@group
;; @r{Build an expression containing a buffer object.}
(setq buffer (list 'print (current-buffer)))
     @result{} (print #<buffer eval.texi>)
@end group
@group
;; @r{Evaluate it.}
(eval buffer)
     @print{} #<buffer eval.texi>
     @result{} #<buffer eval.texi>
@end group
@end example

@node Symbol Forms
@subsection Symbol Forms
@cindex symbol evaluation

  When a symbol is evaluated, it is treated as a variable.  The result
is the variable's value, if it has one.  If it has none (if its value
cell is void), an error is signaled.  For more information on the use of
variables, see @ref{Variables}.

  In the following example, we set the value of a symbol with
@code{setq}.  Then we evaluate the symbol, and get back the value that
@code{setq} stored.

@example
@group
(setq a 123)
     @result{} 123
@end group
@group
(eval 'a)
     @result{} 123
@end group
@group
a
     @result{} 123
@end group
@end example

  The symbols @code{nil} and @code{t} are treated specially, so that the
value of @code{nil} is always @code{nil}, and the value of @code{t} is
always @code{t}; you cannot set or bind them to any other values.  Thus,
these two symbols act like self-evaluating forms, even though
@code{eval} treats them like any other symbol.

@node Classifying Lists
@subsection Classification of List Forms
@cindex list form evaluation

  A form that is a nonempty list is either a function call, a macro
call, or a special form, according to its first element.  These three
kinds of forms are evaluated in different ways, described below.  The
remaining list elements constitute the @dfn{arguments} for the function,
macro, or special form.

  The first step in evaluating a nonempty list is to examine its first
element.  This element alone determines what kind of form the list is
and how the rest of the list is to be processed.  The first element is
@emph{not} evaluated, as it would be in some Lisp dialects such as
Scheme.

@node Function Indirection
@subsection Symbol Function Indirection
@cindex symbol function indirection
@cindex indirection
@cindex void function

  If the first element of the list is a symbol then evaluation examines
the symbol's function cell, and uses its contents instead of the
original symbol.  If the contents are another symbol, this process,
called @dfn{symbol function indirection}, is repeated until it obtains a
non-symbol.  @xref{Function Names}, for more information about using a
symbol as a name for a function stored in the function cell of the
symbol.

  One possible consequence of this process is an infinite loop, in the
event that a symbol's function cell refers to the same symbol.  Or a
symbol may have a void function cell, in which case the subroutine
@code{symbol-function} signals a @code{void-function} error.  But if
neither of these things happens, we eventually obtain a non-symbol,
which ought to be a function or other suitable object.

@kindex invalid-function
@cindex invalid function
  More precisely, we should now have a Lisp function (a lambda
expression), a byte-code function, a primitive function, a Lisp macro, a
special form, or an autoload object.  Each of these types is a case
described in one of the following sections.  If the object is not one of
these types, the error @code{invalid-function} is signaled.

  The following example illustrates the symbol indirection process.  We
use @code{fset} to set the function cell of a symbol and
@code{symbol-function} to get the function cell contents
(@pxref{Function Cells}).  Specifically, we store the symbol @code{car}
into the function cell of @code{first}, and the symbol @code{first} into
the function cell of @code{erste}.

@smallexample
@group
;; @r{Build this function cell linkage:}
;;   -------------       -----        -------        -------
;;  | #<subr car> | <-- | car |  <-- | first |  <-- | erste |
;;   -------------       -----        -------        -------
@end group
@end smallexample

@smallexample
@group
(symbol-function 'car)
     @result{} #<subr car>
@end group
@group
(fset 'first 'car)
     @result{} car
@end group
@group
(fset 'erste 'first)
     @result{} first
@end group
@group
(erste '(1 2 3))   ; @r{Call the function referenced by @code{erste}.}
     @result{} 1
@end group
@end smallexample

  By contrast, the following example calls a function without any symbol
function indirection, because the first element is an anonymous Lisp
function, not a symbol.

@smallexample
@group
((lambda (arg) (erste arg))
 '(1 2 3)) 
     @result{} 1
@end group
@end smallexample

@noindent
Executing the function itself evaluates its body; this does involve
symbol function indirection when calling @code{erste}.

  The built-in function @code{indirect-function} provides an easy way to
perform symbol function indirection explicitly.

@c Emacs 19 feature
@defun indirect-function function
This function returns the meaning of @var{function} as a function.  If
@var{function} is a symbol, then it finds @var{function}'s function
definition and starts over with that value.  If @var{function} is not a
symbol, then it returns @var{function} itself.

Here is how you could define @code{indirect-function} in Lisp:

@smallexample
(defun indirect-function (function)
  (if (symbolp function)
      (indirect-function (symbol-function function))
    function))
@end smallexample
@end defun

@node Function Forms
@subsection Evaluation of Function Forms
@cindex function form evaluation
@cindex function call

  If the first element of a list being evaluated is a Lisp function
object, byte-code object or primitive function object, then that list is
a @dfn{function call}.  For example, here is a call to the function
@code{+}:

@example
(+ 1 x)
@end example

  The first step in evaluating a function call is to evaluate the
remaining elements of the list from left to right.  The results are the
actual argument values, one value for each list element.  The next step
is to call the function with this list of arguments, effectively using
the function @code{apply} (@pxref{Calling Functions}).  If the function
is written in Lisp, the arguments are used to bind the argument
variables of the function (@pxref{Lambda Expressions}); then the forms
in the function body are evaluated in order, and the value of the last
body form becomes the value of the function call.

@node Macro Forms
@subsection Lisp Macro Evaluation
@cindex macro call evaluation

  If the first element of a list being evaluated is a macro object, then
the list is a @dfn{macro call}.  When a macro call is evaluated, the
elements of the rest of the list are @emph{not} initially evaluated.
Instead, these elements themselves are used as the arguments of the
macro.  The macro definition computes a replacement form, called the
@dfn{expansion} of the macro, to be evaluated in place of the original
form.  The expansion may be any sort of form: a self-evaluating
constant, a symbol, or a list.  If the expansion is itself a macro call,
this process of expansion repeats until some other sort of form results.

  Ordinary evaluation of a macro call finishes by evaluating the
expansion.  However, the macro expansion is not necessarily evaluated
right away, or at all, because other programs also expand macro calls,
and they may or may not evaluate the expansions.

  Normally, the argument expressions are not evaluated as part of
computing the macro expansion, but instead appear as part of the
expansion, so they are computed when the expansion is computed.

  For example, given a macro defined as follows:

@example
@group
(defmacro cadr (x)
  (list 'car (list 'cdr x)))
@end group
@end example

@noindent
an expression such as @code{(cadr (assq 'handler list))} is a macro
call, and its expansion is:

@example
(car (cdr (assq 'handler list)))
@end example

@noindent
Note that the argument @code{(assq 'handler list)} appears in the
expansion.

@xref{Macros}, for a complete description of Emacs Lisp macros.

@node Special Forms
@subsection Special Forms
@cindex special form evaluation

  A @dfn{special form} is a primitive function specially marked so that
its arguments are not all evaluated.  Most special forms define control
structures or perform variable bindings---things which functions cannot
do.

  Each special form has its own rules for which arguments are evaluated
and which are used without evaluation.  Whether a particular argument is
evaluated may depend on the results of evaluating other arguments.

  Here is a list, in alphabetical order, of all of the special forms in
Emacs Lisp with a reference to where each is described.

@table @code
@item and
@pxref{Combining Conditions}

@item catch
@pxref{Catch and Throw}

@item cond
@pxref{Conditionals}

@item condition-case
@pxref{Handling Errors}

@item defconst
@pxref{Defining Variables}

@item defmacro
@pxref{Defining Macros}

@item defun
@pxref{Defining Functions}

@item defvar
@pxref{Defining Variables}

@item function
@pxref{Anonymous Functions}

@item if
@pxref{Conditionals}

@item interactive
@pxref{Interactive Call}

@item let
@itemx let*
@pxref{Local Variables}

@item or
@pxref{Combining Conditions}

@item prog1
@itemx prog2
@itemx progn
@pxref{Sequencing}

@item quote
@pxref{Quoting}

@item save-excursion
@pxref{Excursions}

@item save-restriction
@pxref{Narrowing}

@item save-window-excursion
@pxref{Window Configurations}

@item setq
@pxref{Setting Variables}

@item setq-default
@pxref{Creating Buffer-Local}

@item track-mouse
@pxref{Mouse Tracking}

@item unwind-protect
@pxref{Nonlocal Exits}

@item while
@pxref{Iteration}

@item with-output-to-temp-buffer
@pxref{Temporary Displays}
@end table

@cindex CL note---special forms compared
@quotation
@b{Common Lisp note:} Here are some comparisons of special forms in
GNU Emacs Lisp and Common Lisp.  @code{setq}, @code{if}, and
@code{catch} are special forms in both Emacs Lisp and Common Lisp.
@code{defun} is a special form in Emacs Lisp, but a macro in Common
Lisp.  @code{save-excursion} is a special form in Emacs Lisp, but
doesn't exist in Common Lisp.  @code{throw} is a special form in
Common Lisp (because it must be able to throw multiple values), but it
is a function in Emacs Lisp (which doesn't have multiple
values).@refill
@end quotation

@node Autoloading
@subsection Autoloading

  The @dfn{autoload} feature allows you to call a function or macro
whose function definition has not yet been loaded into Emacs.  It
specifies which file contains the definition.  When an autoload object
appears as a symbol's function definition, calling that symbol as a
function automatically loads the specified file; then it calls the real
definition loaded from that file.  @xref{Autoload}.

@node Quoting
@section Quoting
@cindex quoting

  The special form @code{quote} returns its single argument, as written,
without evaluating it.  This provides a way to include constant symbols
and lists, which are not self-evaluating objects, in a program.  (It is
not necessary to quote self-evaluating objects such as numbers, strings,
and vectors.)

@defspec quote object
This special form returns @var{object}, without evaluating it.
@end defspec

@cindex @samp{'} for quoting
@cindex quoting using apostrophe
@cindex apostrophe for quoting
Because @code{quote} is used so often in programs, Lisp provides a
convenient read syntax for it.  An apostrophe character (@samp{'})
followed by a Lisp object (in read syntax) expands to a list whose first
element is @code{quote}, and whose second element is the object.  Thus,
the read syntax @code{'x} is an abbreviation for @code{(quote x)}.

Here are some examples of expressions that use @code{quote}:

@example
@group
(quote (+ 1 2))
     @result{} (+ 1 2)
@end group
@group
(quote foo)
     @result{} foo
@end group
@group
'foo
     @result{} foo
@end group
@group
''foo
     @result{} (quote foo)
@end group
@group
'(quote foo)
     @result{} (quote foo)
@end group
@group
['foo]
     @result{} [(quote foo)]
@end group
@end example

  Other quoting constructs include @code{function} (@pxref{Anonymous
Functions}), which causes an anonymous lambda expression written in Lisp
to be compiled, and @samp{`} (@pxref{Backquote}), which is used to quote
only part of a list, while computing and substituting other parts.