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 is not needed and something else should be used instead.
For example, to get the value of a variable, while
symbol-value is preferable; or rather than store expressions
in a property list that then need to go through
eval, it is better to
store functions instead that are then passed to
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 Loading).
It is generally cleaner and more flexible to store a function in a
data structure, and call it with
to store an expression in the data structure and evaluate it. Using
functions provides the ability to pass information to them as
This is the basic function for evaluating an expression. It evaluates form in the current environment, and returns the result. The type of the form object determines how it is evaluated. See Forms.
The argument lexical specifies the scoping rule for local
variables (see Variable Scoping). If it is omitted or
that means to evaluate form using the default dynamic scoping
rule. If it is
t, that means to use the lexical scoping rule.
The value of lexical can also be a non-empty alist specifying a
particular lexical environment for lexical bindings; however,
this feature is only useful for specialized purposes, such as in Emacs
Lisp debuggers. See Lexical Binding.
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 ;; Here
foo(eval 'foo) ⇒ bar ;; Here
bar, which is the value of
foo(eval foo) ⇒ baz
The number of currently active calls to
eval is limited to
max-lisp-eval-depth (see below).
This function evaluates the forms in the current buffer in the region
defined by the positions start and end. It reads forms from
the region and calls
eval on them until the end of the region is
reached, or until an error is signaled and not handled.
eval-region does not produce any output. However,
if stream is non-
nil, any output produced by output
functions (see Output Functions), as well as the values that
result from evaluating the expressions in the region are printed using
stream. See Output Streams.
If read-function is non-
nil, it should be a function,
which is used instead of
read to read expressions one by one.
This function is called with one argument, the stream for reading
input. You can also use the variable
(see How Programs Do Loading)
to specify this function, but it is more robust to use the
eval-region does not move point. It always returns
This is similar to
eval-region, but the arguments provide
different optional features.
eval-buffer operates on the
entire accessible portion of buffer buffer-or-name.
buffer-or-name can be a buffer, a buffer name (a string), or
nil (or omitted), which means to use the current buffer.
stream is used as in
eval-region, unless stream is
nil and print non-
nil. In that case, values that
result from evaluating the expressions are still discarded, but the
output of the output functions is printed in the echo area.
filename is the file name to use for
(see Unloading), and defaults to
(see Buffer File Name). If unibyte is non-
read converts strings to unibyte whenever possible.
eval-current-buffer is an alias for this command.
This variable defines the maximum depth allowed in calls to
funcall before an error is signaled (with error
"Lisp nesting exceeds max-lisp-eval-depth").
This limit, with the associated error when it is exceeded, is one way
Emacs Lisp avoids infinite recursion on an ill-defined function. If
you increase the value of
max-lisp-eval-depth too much, such
code can cause stack overflow instead.
The depth limit counts internal uses of
funcall, such as for calling the functions mentioned in Lisp
expressions, and recursive evaluation of function call arguments and
function body forms, as well as explicit calls in Lisp code.
The default value of this variable is 400. If you set it to a value less than 100, Lisp will reset it to 100 if the given value is reached. Entry to the Lisp debugger increases the value, if there is little room left, to make sure the debugger itself has room to execute.
max-specpdl-size provides another limit on nesting.
See Local Variables.
The value of this variable is a list of the values returned by all the
expressions that were read, evaluated, and printed from buffers
(including the minibuffer) by the standard Emacs commands which do
this. (Note that this does not include evaluation in
*ielm* buffers, nor evaluation using C-j in
lisp-interaction-mode.) The elements are ordered most recent
(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