meow.rkt

#lang racket

(define lookup (compose cdr assoc))
(define (extend-env x e env) (cons `(,x . ,e) env))

(define init-env
  `((zero? . ,zero?)
    (sub1 . ,sub1)
    (add1 . ,add1)
    (* . ,*)
    (+ . ,+)
    (- . ,-)
    (/ . ,/)))

(define (eval foo)
  (let eval ([foo foo]
             [env init-env])
    (match foo
      [(? symbol? x) (lookup x env)]
      [(? number? x) x]
      [`(if ,e ,t ,f) (if (eval e env) (eval t env) (eval f env))]
      [`(lambda (,x) ,e) (lambda (y) (eval e (extend-env x y env)))]
      [`(,e1 ,e2) ((eval e1 env) (eval e2 env))])))


(module+ test
  (require rackunit)
  (check-equal? 0 (eval `(sub1 1)))
  (check-equal? 2 (eval `(add1 1))))