eval

eval Function

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.”

Expanded Reference: eval

Basic Usage: Evaluating a Form

eval evaluates a form in the current dynamic environment and the null lexical environment.

(eval '(+ 1 2 3))
=> 6

(eval '(list 'a 'b 'c))
=> (A B C)

Evaluating a Symbol: Dynamic Lookup

eval looks up variables in the dynamic (global) environment, not in any lexical scope.

(setq my-var 42)
=> 42

(eval 'my-var)
=> 42

;; Lexical bindings are NOT visible to eval:
(let ((my-var 100))
  (eval 'my-var))
=> 42

Two Levels of Evaluation

When calling eval, the argument is first evaluated normally, then the result is evaluated by eval.

(setq form '(+ 10 20))
=> (+ 10 20)

(eval form)
=> 30

(eval 'form)
=> (+ 10 20)

Constructing and Evaluating Forms Dynamically

(eval (list '* 6 7))
=> 42

(eval (list 'if t ''yes ''no))
=> YES

eval vs. symbol-value

For simply obtaining the dynamic value of a symbol, symbol-value is preferred over eval.

(defvar *counter* 10)
=> *COUNTER*

(symbol-value '*counter*)
=> 10

(eval '*counter*)
=> 10