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.
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 (Note:Loading).
*Note:* 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
This is the basic function evaluating an expression. It evaluates
FORM in the current environment and returns the result. How the
evaluation proceeds depends on the type of the object (Note:Forms).
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.
If STREAM is non-`nil', the values that result from evaluating the
expressions in the region are printed using STREAM. Note: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' (Note:How Programs Do Loading) to specify this function, but it is
more robust to use the READ-FUNCTION argument.
`eval-region' always returns `nil'.
- Command: eval-current-buffer &optional stream
This is like `eval-region' except that it operates on the whole
buffer.
- Variable: 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 that Lisp
avoids infinite recursion on an ill-defined function.
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 300. 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. Note: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. 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