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
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.
evalis a function, the argument expression that appears in a call to
evalis evaluated twice: once as preparation before
evalis called, and again by the
evalfunction 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
evalis limited to
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
evalon them until the end of the region is reached, or until an error is signaled and not handled.
eval-regiondoes 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
readto 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 read-function argument.
eval-regiondoes not move point. It always returns
This is similar to
eval-region, but the arguments provide different optional features.
eval-bufferoperates 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
niland 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
load-history(see Unloading), and defaults to
buffer-file-name(see Buffer File Name). If unibyte is non-
readconverts strings to unibyte whenever possible.
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-depthtoo 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-sizeprovides 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 first.(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
valuesitself, 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