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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
.
9.1 Introduction to Evaluation | Evaluation in the scheme of things. | |
9.2 Kinds of Forms | How various sorts of objects are evaluated. | |
9.3 Quoting | Avoiding evaluation (to put constants in the program). | |
9.4 Eval | How to invoke the Lisp interpreter explicitly. |
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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.
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
.
Evaluation of a function call ultimately calls the function specified in it. See section 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 (see section Byte Compilation).
The evaluation of forms takes place in a context called the environment, which consists of the current values and bindings of all Lisp variables.(3) Whenever a form refers to a variable without creating a new binding for it, the value of the variable’s 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)
.
The details of what evaluation means for each kind of form are described below (see section Kinds of Forms).
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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, one by one, starting with the “all other types” which are self-evaluating forms.
9.2.1 Self-Evaluating Forms | Forms that evaluate to themselves. | |
9.2.2 Symbol Forms | Symbols evaluate as variables. | |
9.2.3 Classification of List Forms | How to distinguish various sorts of list forms. | |
9.2.4 Symbol Function Indirection | When a symbol appears as the car of a list, we find the real function via the symbol. | |
9.2.5 Evaluation of Function Forms | Forms that call functions. | |
9.2.6 Lisp Macro Evaluation | Forms that call macros. | |
9.2.7 Special Forms | “Special forms” are idiosyncratic primitives, most of them extremely important. | |
9.2.8 Autoloading | Functions set up to load files containing their real definitions. |
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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 ; A number, 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. 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 print-exp (list 'print (current-buffer)))
⇒ (print #<buffer eval.texi>)
;; Evaluate it.
(eval print-exp)
-| #<buffer eval.texi>
⇒ #<buffer eval.texi>
|
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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 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 symbol whose name
starts with ‘:’ also self-evaluates in the same way; likewise,
its value ordinarily cannot be changed. See section Variables that Never Change.
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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.
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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 |
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.
This function returns the meaning of function as a function. If function is a symbol, then it finds function’s function definition and starts over with that value. If function is not a symbol, then it returns function itself.
Here is how you could define indirect-function
in Lisp:
(defun indirect-function (function) (if (symbolp function) (indirect-function (symbol-function function)) function)) |
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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.
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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 evaluated.
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.
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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
see section Constructs for Combining Conditions
catch
see section Explicit Nonlocal Exits: catch
and throw
cond
see section Conditionals
condition-case
see section Writing Code to Handle Errors
defconst
see section Defining Global Variables
defmacro
see section Defining Macros
defun
see section Defining Functions
defvar
see section Defining Global Variables
function
see section Anonymous Functions
if
see section Conditionals
interactive
see section Interactive Call
let
let*
see section Local Variables
or
see section Constructs for Combining Conditions
prog1
prog2
progn
see section Sequencing
quote
see section Quoting
save-current-buffer
see section The Current Buffer
save-excursion
see section Excursions
save-restriction
see section Narrowing
save-window-excursion
see section Window Configurations
setq
see section How to Alter a Variable Value
setq-default
see section Creating and Deleting Buffer-Local Bindings
track-mouse
see section Mouse Tracking
unwind-protect
see section Nonlocal Exits
while
see section Iteration
with-output-to-temp-buffer
see section Temporary Displays
Common Lisp note: Here are some comparisons of special forms in GNU Emacs Lisp and Common Lisp.
setq
,if
, andcatch
are special forms in both Emacs Lisp and Common Lisp.defun
is a special form in Emacs Lisp, but a macro in Common Lisp.save-excursion
is a special form in Emacs Lisp, but doesn’t exist in Common Lisp.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).
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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.
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The special form 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.)
This special form returns object, without evaluating it.
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.
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