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

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

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

(define extend-env
  (lambda (syms vals env)
    (extended-env-record syms (list->vector 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)
            (vector-ref vals pos)
            (apply-env env sym)))])))

(trace apply-env)

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

(define e1 (extend-env '(a b) '(3 4) (empty-env)))
(define e2 (extend-env '(c d) '(5 6) e1))