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. Often
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
apply, than to store an expression in the data structure and evaluate it. Using functions provides the ability to pass information to them as arguments.
function eval form \&optional lexical
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.
lexical specifies the scoping rule for local variables (see Variable Scoping). If it is omitted or
nil, 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)
(setq bar 'baz)
;; Here eval receives argument foo
;; Here eval receives argument bar, which is the value of foo
The number of currently active calls to
eval is limited to
max-lisp-eval-depth (see below).
command eval-region start end \&optional stream read-function
This function evaluates the forms in the current buffer in the region defined by the positions
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.
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
load-read-function (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
command eval-buffer \&optional buffer-or-name stream filename unibyte print
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 (see Narrowing in The GNU Emacs Manual).
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
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
load-history (see Unloading), and defaults to
buffer-file-name (see Buffer File Name). If
unibyte is non-
read converts strings to unibyte whenever possible.
eval-current-buffer is an alias for this command.
user option max-lisp-eval-depth
This variable defines the maximum depth allowed in calls to
funcall before an error is signaled (with error message
"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. On some systems, this overflow can be handled. In that case, normal Lisp evaluation is interrupted and control is transferred back to the top level command loop (
top-level). Note that there is no way to enter Emacs Lisp debugger in this situation. See Error Debugging.
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 800. 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-x C-e, and similar evaluation commands in
lisp-interaction-mode.) The elements are ordered most recent first.
(setq x 1)
(list 'A (1+ 2) auto-save-default)
⇒ (A 3 t)
⇒ ((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)