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.
Note: it is generally cleaner and more flexible to call functions that are stored in data structures, rather than to evaluate expressions stored in data structures. Using functions provides the ability to pass information to them as arguments.
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 section Loading).
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 ;;eval
receives argumentbar
, which is the value offoo
(eval foo) => baz (eval 'foo) => bar
The number of currently active calls to eval
is limited to
max-lisp-eval-depth
(see below).
eval
on them until the end of the region is
reached, or until an error is signaled and not handled.
If stream is supplied, standard-output
is bound to it
during the evaluation.
You can use the variable load-read-function
to specify a function
for eval-region
to use instead of read
for reading
expressions. See section How Programs Do Loading.
eval-region
always returns nil
.
eval-region
except that it operates on the whole
buffer.
eval
,
apply
, and funcall
before an error is signaled (with error
message "Lisp nesting exceeds max-lisp-eval-depth"
). This counts
internal uses of those functions, such as for calling the functions
mentioned in Lisp expressions, and recursive evaluation of function call
arguments and function body forms.
This limit, with the associated error when it is exceeded, is one way that Lisp avoids infinite recursion on an ill-defined function.
The default value of this variable is 200. If you set it to a value less than 100, Lisp will reset it to 100 if the given value is reached.
max-specpdl-size
provides another limit on nesting.
See section Local Variables.
(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
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