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GNU Emacs Lisp Reference Manual

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9.4 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 (see section 15. 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 (see section 9.2 Kinds of 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. See section 19.4 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 section 15.1 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. See section 11.3 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)
     => ((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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