#lang racket
(require "chez-init.rkt")

(define-datatype environment environment? 
  [empty-env-record]
  [extended-env-record
    (syms (listof symbol?))
    (vals (listof racket-value?))
    (env environment?)])

(define racket-value? (lambda (v) #t))

(define empty-env
  (lambda ()
    (empty-env-record)))

(define extend-env
  (lambda (syms vals env)
    (extended-env-record syms vals env)))

(define apply-env
  (lambda (env sym)
    (cases environment env
      [empty-env-record ()
        (error 'apply-env "No binding for ~s" sym)]
      [extended-env-record (syms vals env)
        (let ((pos (find-position sym syms 0)))
          (if (number? pos)
            (list-ref vals pos)
            (apply-env env sym)))])))

(define find-position
  (lambda (sym ls pos)
    (cond [(null? ls) #f]
	  [(eq? sym (car ls)) pos]
	  [else (find-position sym (cdr ls) (+ pos 1))])))