#lang r5rs

(define (error x)
  (display x)
  (newline))

(define (make-frame variables values)
  (if (not (= (length variables)
              (length values)))
      (error "Number of variables and values should be the same -- MAKE-FRAME")
      (map cons variables values)))

(define (frame-variables frame)
  (map car frame))

(define (frame-values frame)
  (map cdr frame))

(define (add-binding-to-frame! var val frame)
  (define (add-to-end-of-frame! b f)
    (if (null? (cdr f))
        (set-cdr! f b)
        (add-to-end-of-frame! b (cdr f))))
  (add-to-end-of-frame! (cons var val) frame))

(define (lookup-variable-value var env)
  (define (env-loop env)
    (define (scan frame)
      (if (null? frame)
          (env-loop (enclosing-environment env))
          (let ((p (car frame)))
            (if (eq? var (car p))
                (cdr p)
                (scan (cdr frame))))))
    (if (eq? env the-empty-environment)
        (error "variable not defined")
        (let ((f (first-frame env)))
          (scan f))))
  (env-loop env))

(define (set-variable-value! var val env)
  (define (env-loop env)
    (define (scan frame)
      (if (null? frame)
          (env-loop (enclosing-environment env))
          (let ((p (car frame)))
            (if (eq? var (car p))
                (set-cdr! p val)
                (scan (cdr frame))))))
    (if (eq? env the-empty-environment)
        (error "variable not defined")
        (let ((f (first-frame env)))
          (scan f))))
  (env-loop env))

(define (define-variable! var val env)
  (let ((frame (first-frame env)))
    (define (scan frame)
      (if (null? frame)
          (add-binding-to-frame var val frame)
          (let ((p (car frame)))
            (if (eq? (car p) var)
                (set-cdr! p val)
                (scan (cdr frame))))))
    (scan frame)))