gistfile1.scm

(define (my-eval exp env)
  (match exp
    ((int i) => i)
    ((add a b) => (+ (my-eval a env) (my-eval b env)))
    ((abs p b) => (list 'cls exp env))
    ((var a) => (cond ((assoc a env) => cdr)
                      (else (error "unbound variable" a))))
    ((app f a) =>  (match (my-eval f env)
                     ((cls (abs p b) cenv) => (my-eval b (cons (cons p (my-eval a env)) cenv)))
                     (else (error "not a valid function: " f))))
    (else (error "invalid expression: " exp))))

(my-eval '(app (app (abs a (abs b (add (var a) (var b)))) (int 1)) (int 2)) '())
;; => 3