CSE 341 -- Eval and Apply

`eval`

The eval function takes a Scheme object and an environment, and evaluates the Scheme object. 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) user-initial-environment)  => 12
(eval y user-initial-environment)  => 8
(eval z user-initial-environment)  => 80

An example of variables whose values are atoms:

(define a 'b)
(define b 'c)
(define c 50)
a => b
(eval a user-initial-environment)  => c
(eval (eval a user-initial-environment) user-initial-environment)  => 50

The top level of the Scheme interpreter is a read-eval-print loop: read in an expression, evaluate it, and print the result.

user-initial-environment is bound to an environment and is pre-defined. There are also functions to get the environment for any procedure, etc. See the MIT Scheme User's Manual section on read-eval-print loop.

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

(define x 3)
x  =>  3
'x => x
(eval 'x user-initial-environment)  => 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