A tiny evaluator of scheme expressions

expression

• define-expr

define "{" 
    var value-expr | 
    proc"(" [arg,] ")" 
    "{" 
         expr [, expr]
    "}" 
"}"

• assign-expr

set var value-expr

• if-expr

if predict-expr
    "{" 
    expr [, expr]
    "}"

• lambda-expr

lambda"("[arg,] ")" "{" expr [,expr] "}" ["(" [,para] ")"]

• application-expr

proc-name"("[,para]")" 

• perform-expr

perform application-expr

Remark

Core of evaluator is interleaving of eval and apply loop. Everytime eval enconters a application it call apply to make actually calling. A compound procedure is a sequence of expression to be evaluated within a given environment.

Example

compute fibonacci with evaluator:

Eval-go INPUT:
define fib(n) { 
    if ==(n, 0) {0} 
    if ==(n, 1) {1} 
    +(fib(-(n,1)), fib(-(n,2))) 
    };
Eval-go INPUT:
fib(10);
Eval-go OUTPUT:
 55