Requesting Evaluation

Evaluation of an expression can be explicitly requested using the function eval. (eval x ) gives the result of evaluating the value of the expression x: eval performs an extra level of evaluation. The argument of eval is evaluated to produce some Lisp code; then eval causes that Lisp code to be evaluated.

(define formula '(* 3.14159 (expt radius 2)))

(eval (subst 10.0 'radius formula))
  =  314.159

eval allows Lisp code to create new Lisp functions:

(define (define-alias new old)
  (eval (list 'define new old)) )

> (define-alias 'rest 'cdr)
> (rest '(a b c))
(b c)

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