; environment

(load "utils.scm")

(define list-of
  (lambda (base-type)
    (lambda (val)
      (and 
         (list? val)
         (fold-left val (lambda (e x) (and e (base-type x))) #t)
         )
      )
    )
  )

(define list-find-position
  (lambda (sym los)
    (list-index (lambda (sym1) (eqv? sym1 sym)) los)
    )
  )

(define list-index
  (lambda (pred ls)
    (cond
      ((null? ls) #f)
      ((pred (car ls)) 0)
      (else (let ((list-index-r (list-index pred (cdr ls))))
              (if (number? list-index-r)
                (+ list-index-r 1)
                #f)))
      )
    )
  )

(define-datatype environment environment?
  (empty-env-rec)
  (extend-env-rec
     (syms (list-of symbol?))
     (vals (list-of (lambda (x) #t)))
     (env environment?))
  )

; environment operations

(define empty-env
  (lambda ()
    (empty-env-rec)
    )
  )

(define extend-env
  (lambda (syms vals env)
    (extend-env-rec syms vals env)
    )
  )

(define apply-env
  (lambda (env sym)
    (cases environment env
      (empty-env-rec () (eopl:error 'apply-env "No binding for ~s" sym))
      (extend-env-rec (syms vals old-env)
         (let ((pos (list-find-position sym syms)))
              (if (number? pos)
                  (list-ref vals pos)
                  (apply-env old-env sym))))
      )
    )
  )