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10.5 Eval

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 eval works, 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 funcall.

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 funcall or 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.

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.

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
;; Here eval receives argument foo
(eval 'foo)
     ⇒ bar
;; Here eval receives argument 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).

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 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.

By default, 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 load-read-function (see How Programs Do Loading) to specify this function, but it is more robust to use the read-function argument.

eval-region does not move point. It always returns nil.

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 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 load-history (see Unloading), and defaults to buffer-file-name (see Buffer File Name). If unibyte is non-nil, 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 eval, apply, and 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 eval, apply, and 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.

variable values​

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, C-x C-e, and similar evaluation commands 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 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