(module interp (lib "eopl.ss" "eopl")
  
  (require "drscheme-init.scm")

  (require "lang.scm")
  (require "data-structures.scm")
  (require "environments.scm")

  (provide value-of-program value-of)

;;;;;;;;;;;;;;;; the interpreter ;;;;;;;;;;;;;;;;

  ;; value-of-program : Program -> ExpVal

  (define value-of-program 
    (lambda (pgm)
      (cases program pgm
        (a-program (exp1)
          (value-of exp1 (init-env))))))

  ;; value-of : Exp * Env -> ExpVal

  (define value-of
    (lambda (exp env)
      (cases expression exp

        (const-exp (num) (num-val num))

        (var-exp (var) (apply-env env var))

        (diff-exp (exp1 exp2)
          (let ((val1 (value-of exp1 env))
                (val2 (value-of exp2 env)))
            (let ((num1 (expval->num val1))
                  (num2 (expval->num val2)))
              (num-val
                (- num1 num2)))))

        (zero?-exp (exp1)
          (let ((val1 (value-of exp1 env)))
            (let ((num1 (expval->num val1)))
              (if (zero? num1)
                (bool-val #t)
                (bool-val #f)))))
              
        (if-exp (exp1 exp2 exp3)
          (let ((val1 (value-of exp1 env)))
            (if (expval->bool val1)
              (value-of exp2 env)
              (value-of exp3 env))))

        (let-exp (var exp1 body)       
          (let ((val1 (value-of exp1 env)))
            (value-of body
              (extend-env var val1 env))))

        )))


  )