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ANSI Common Lisp  3 Evaluation and Compilation 3.8 Dictionary Evaluation and Compilation
- Syntax:
-
eval
form
 {result}*
- Arguments and Values:
-
form - a form.
results - the values yielded by the evaluation of form.
- Description:
-
Evaluates form in the current dynamic environment
and the null lexical environment.
eval is a user interface to the evaluator.
The evaluator expands macro calls as if through the use of macroexpand-1.
Constants appearing in code
processed by eval are
not copied nor coalesced. The code resulting from the execution of
eval
references objects
that are eql to the corresponding objects in
the source code.
- Examples:
-
(setq form '(1+ a) a 999) 999
(eval form) 1000
(eval 'form) (1+ A)
(let ((a '(this would break if eval used local value))) (eval form))
1000
- See Also:
-
macroexpand-1,
Section 3.1.2 The Evaluation Model
- Notes:
-
To obtain the current dynamic value of a symbol,
use of symbol-value is equivalent (and usually preferable)
to use of eval.
Note that an eval form involves two levels of evaluation
for its argument. First, form is evaluated by the
normal argument evaluation mechanism as would occur with any call.
The object that results from this normal argument evaluation
becomes the value of the form parameter, and is then
evaluated as part of the eval form.
For example:
(eval (list 'cdr (car '((quote (a . b)) c)))) b
The argument form (list 'cdr (car '((quote (a . b)) c))) is evaluated
in the usual way to produce the argument (cdr (quote (a . b)));
eval then evaluates its argument, (cdr (quote (a . b))), to produce b.
Since a single evaluation already occurs for any argument form
in any function form,
eval is sometimes said to perform "an extra level of evaluation."
- Allegro CL Implementation Details:
-
None.
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