The evaluation of expressions in Emacs Lisp is performed by the
Lisp interpreter---a program that receives a Lisp object as input
and computes its 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 eval.
A Lisp object that is intended for evaluation is called an expression or a 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 read whether this
object is a form to be evaluated, or serves some entirely different
purpose. See section 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 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.
See section Command Loop.
Evaluation is a recursive process. That is, evaluation of a form may
call 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 (car x): the subform x must first be evaluated
recursively, so that its value can be passed as an argument to the
function car.
The evaluation of forms takes place in a context called the environment, which consists of the current values and bindings of all Lisp variables.(2) 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. See section Variables.
Evaluation of a form may create new environments for recursive
evaluation by binding variables (see section 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 side effects. An example of a form that produces side
effects is (setq foo 1).
Finally, evaluation of one particular function call, byte-code,
invokes the byte-code interpreter on its arguments. Although the
byte-code interpreter is not the same as the Lisp interpreter, it uses
the same environment as the Lisp interpreter, and may on occasion invoke
the Lisp interpreter. (See section Byte Compilation.)
The details of what evaluation means for each kind of form are described below (see section Kinds of Forms).
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 eval function.
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 (see section Loading).
Since eval is a function, the argument expression that appears
in a call to eval is evaluated twice: once as preparation before
eval is called, and again by the eval function itself.
Here is an example:
(setq foo 'bar)
=> bar
(setq bar 'baz)
=> baz
;; eval receives argument bar, which is the value of foo
(eval foo)
=> baz
(eval 'foo)
=> bar
The number of currently active calls to eval is limited to
max-lisp-eval-depth (see below).
eval on them until the end of the
buffer is reached, or until an error is signaled and not handled.
If stream is supplied, the variable standard-output is
bound to stream during the evaluation (see section Output Functions).
eval-current-buffer always returns nil.
eval on them until the end of the region is
reached, or until an error is signaled and not handled.
If stream is supplied, standard-output is bound to it
during the evaluation.
eval-region always returns nil.
eval,
apply, and funcall before an error is signaled (with error
message "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.
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.
max-specpdl-size provides another limit on nesting.
See section Local Variables.
(setq x 1)
=> 1
(list 'A (1+ 2) auto-save-default)
=> (A 3 t)
values
=> ((A 3 t) 1 ...)
This variable is useful for referring back to values of forms recently
evaluated. It is generally a bad idea to print the value of
values itself, since this may be very long. Instead, examine
particular elements, like this:
;; Refer to the most recent evaluation result.
(nth 0 values)
=> (A 3 t)
;; That put a new element on,
;; so all elements move back one.
(nth 1 values)
=> (A 3 t)
;; This gets the element that was next-to-most-recent
;; before this example.
(nth 3 values)
=> 1
A Lisp object that is intended to be evaluated is called a 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.
A 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 "foo" evaluates to the string "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.
'123 ; An object, shown without evaluation.
=> 123
123 ; Evaluated as usual--result is the same.
=> 123
(eval '123) ; Evaluated "by hand"---result is the same.
=> 123
(eval (eval '123)) ; Evaluating twice changes nothing.
=> 123
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; however, it is possible to construct Lisp expressions containing these types by means of a Lisp program. Here is an example:
;; Build an expression containing a buffer object.
(setq buffer (list 'print (current-buffer)))
=> (print #<buffer eval.texi>)
;; Evaluate it.
(eval buffer)
-| #<buffer eval.texi>
=> #<buffer eval.texi>
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 section Variables.
In the following example, we set the value of a symbol with
setq. Then we evaluate the symbol, and get back the value that
setq stored.
(setq a 123)
=> 123
(eval 'a)
=> 123
a
=> 123
The symbols nil and t are treated specially, so that the
value of nil is always nil, and the value of t is
always t; you cannot set or bind them to any other values. Thus,
these two symbols act like self-evaluating forms, even though
eval treats them like any other symbol.
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 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 not evaluated, as it would be in some Lisp dialects such as Scheme.
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 symbol function indirection, is repeated until it obtains a non-symbol. See section Naming a Function, 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
symbol-function signals a 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.
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 invalid-function is signaled.
The following example illustrates the symbol indirection process. We
use fset to set the function cell of a symbol and
symbol-function to get the function cell contents
(see section Accessing Function Cell Contents). Specifically, we store the symbol car
into the function cell of first, and the symbol first into
the function cell of erste.
;; Build this function cell linkage: ;; ------------- ----- ------- ------- ;; | #<subr car> | <-- | car | <-- | first | <-- | erste | ;; ------------- ----- ------- -------
(symbol-function 'car)
=> #<subr car>
(fset 'first 'car)
=> car
(fset 'erste 'first)
=> first
(erste '(1 2 3)) ; Call the function referenced by erste.
=> 1
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.
((lambda (arg) (erste arg))
'(1 2 3))
=> 1
Executing the function itself evaluates its body; this does involve
symbol function indirection when calling erste.
The built-in function indirect-function provides an easy way to
perform symbol function indirection explicitly.
Here is how you could define indirect-function in Lisp:
(defun indirect-function (function)
(if (symbolp function)
(indirect-function (symbol-function function))
function))
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 function call. For example, here is a call to the function
+:
(+ 1 x)
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 apply (see section Calling Functions). If the function
is written in Lisp, the arguments are used to bind the argument
variables of the function (see section 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.
If the first element of a list being evaluated is a macro object, then the list is a macro call. When a macro call is evaluated, the elements of the rest of the list are not initially evaluated. Instead, these elements themselves are used as the arguments of the macro. The macro definition computes a replacement form, called the 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:
(defmacro cadr (x) (list 'car (list 'cdr x)))
an expression such as (cadr (assq 'handler list)) is a macro
call, and its expansion is:
(car (cdr (assq 'handler list)))
Note that the argument (assq 'handler list) appears in the
expansion.
See section Macros, for a complete description of Emacs Lisp macros.
A 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.
and
catch
catch and throw
cond
condition-case
defconst
defmacro
defun
defvar
function
if
interactive
let
let*
or
prog1
prog2
progn
quote
save-excursion
save-restriction
save-window-excursion
setq
setq-default
track-mouse
unwind-protect
while
with-output-to-temp-buffer
Common Lisp note: Here are some comparisons of special forms in GNU Emacs Lisp and Common Lisp.
setq,if, andcatchare special forms in both Emacs Lisp and Common Lisp.defunis a special form in Emacs Lisp, but a macro in Common Lisp.save-excursionis a special form in Emacs Lisp, but doesn't exist in Common Lisp.throwis 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).
The 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. See section Autoload.
The special form quote returns its single argument
"unchanged".
Because quote is used so often in programs, Lisp provides a
convenient read syntax for it. An apostrophe character (`'')
followed by a Lisp object (in read syntax) expands to a list whose first
element is quote, and whose second element is the object. Thus,
the read syntax 'x is an abbreviation for (quote x).
Here are some examples of expressions that use quote:
(quote (+ 1 2))
=> (+ 1 2)
(quote foo)
=> foo
'foo
=> foo
"foo
=> (quote foo)
'(quote foo)
=> (quote foo)
['foo]
=> [(quote foo)]
Other quoting constructs include function (see section Anonymous Functions), which causes an anonymous lambda expression written in Lisp
to be compiled, and ` (see section Backquote), which is used to quote
only part of a list, while computing and substituting other parts.