(module data-structures (lib "eopl.ss" "eopl")

  (require "lang.scm")                  ; for expression?

  (provide (all-defined))               ; too many things to list

;;;;;;;;;;;;;;;; expressed values ;;;;;;;;;;;;;;;;

;;; an expressed value is either a number, a boolean or a procval.

  (define-datatype expval expval?
    (num-val
      (value number?))
    (bool-val
      (boolean boolean?))
    (proc-val 
      (proc proc?))
    (ref-val
      (ref vector?)))

;;; extractors:

  (define expval->num
    (lambda (v)
      (cases expval v
	(num-val (num) num)
	(else (expval-extractor-error 'num v)))))

  (define expval->bool
    (lambda (v)
      (cases expval v
	(bool-val (bool) bool)
	(else (expval-extractor-error 'bool v)))))

  (define expval->proc
    (lambda (v)
      (cases expval v
	(proc-val (proc) proc)
	(else (expval-extractor-error 'proc v)))))

  (define expval->ref
    (lambda (v)
      (cases expval v
	     (ref-val (ref) ref)
	     (else (expval-extractor-error 'reference v)))))

  (define expval-extractor-error
    (lambda (variant value)
      (eopl:error 'expval-extractors "Looking for a ~s, found ~s"
	variant value)))

;;;;;;;;;;;;;;;; procedures ;;;;;;;;;;;;;;;;

  (define-datatype proc proc?
    (procedure
      (bvars (list-of symbol?))
      (body expression?)))
  
)