CSE 341 - Programming Languages - Winter 2010
Scheme

Eval and Apply

eval

The eval function takes a Scheme object evaluates it. Examples: 
(define fn '*)
(define x 3)
(define y (list '+ x 5))
(define z (list fn 10 y))
x  =>  3
y  => (+ 3 5)
z  => (* 10 (+ 3 5))
(eval '(+ 6 6))  => 12
(eval y)  => 8
(eval z)  => 80
An example of variables whose values are atoms: 
(define a 'b)
(define b 'c)
(define c 50)
a => b
(eval a)  => c
(eval (eval a))  => 50
Numbers just evaluate to themselves, so: 
(eval (eval (eval a)))  => 50
(eval (eval (eval (eval a))))  => 50
The top level of the Scheme interpreter is a read-eval-print loop: read in an expression, evaluate it, and print the result.

Quote suppresses evaluation; eval causes evaluation. They can cancel each other out. 

(define x 3)
x  =>  3
'x => x
(eval 'x)  => 3

apply

The apply function applies a function to a list of its arguments. Examples: 
(apply factorial '(3))  => 6
(apply + '(1 2 3 4))  => 10
A useful programming trick is to use apply to define a function that takes a list of arguments, if you have available a function that arbitrary number of arguments. Example: 
(define (sum s) (apply + s))
(sum '(1 2 3))  => 6