(define-datatype expression expression?
  (lit-exp
   (value scheme-value?))
  (begin-exp
   (exps (list-of expression?)))
  (var-exp
   (id symbol?))
;;;  (assignment-exp
;;;   (id symbol?)
;;;   (val expression?))
  (if-exp
   (test-exp expression?)
   (true-exp expression?)
   (false-exp expression?))
  (cond-exp
   (exps (list-of (list-of expression?))))
  (lambda-exp
   (ids (list-of symbol?))
   (bodies (list-of expression?)))
  (let-exp
   (ids (list-of symbol?))
   (vals (list-of expression?))
   (bodies (list-of expression?)))
  (app-exp
   (rator expression?)
   (rands (list-of expression?))))

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

(define parse-expression
  (lambda (datum)
    (cond [(symbol? datum) (var-exp datum)]
	  [(number? datum) (lit-exp datum)]
	  [(string? datum) (lit-exp datum)]
	  [(boolean? datum) (lit-exp datum)]
	  [(null? datum) (lit-exp datum)]
	  [(pair? datum)
	   (cond [(eqv? (car datum) 'lambda)
		  (lambda-exp (cadr datum)
			      (map parse-expression (cddr datum)))]
		 [(eq? (cadr datum) '+) (app-exp (parse-expression (cadr datum))
						 (map parse-expression (cons (car datum) (cddr datum))))]
		 [(eq? (cadr datum) '-) 'plus]
		 [(eq? (cadr datum) '*) 'plus]
		 [(eq? (cadr datum) '/) 'plus]

		 [(eqv? (car datum) 'cond)
		  (cond-exp (map (lambda (x) (list (parse-expression (car x))
						   (parse-expression (cadr x))))
				 (cdr datum)))]

;;;		 [(eq? (cadr datum) '=)
;;;		  (assignment-exp (car datum)
;;;				  (parse-expression (caddr datum)))]
		 [(eqv? (car datum) 'let)
		  (let-exp (map car (cadr datum))
			   (map parse-expression (map cadr (cadr datum)))
			   (map parse-expression (cddr datum)))]
		 [(eqv? (car datum) 'begin)
		  (begin-exp (map parse-expression (cdr datum)))]
		 [(eqv? (car datum) 'if)
		  (if-exp (parse-expression (cadr datum))
			  (parse-expression (caddr datum))
			  (parse-expression (cadddr datum)))]
		 [(eqv? (car datum) 'quote)
		  (lit-exp (cadr datum))]
		 [else (app-exp
			(parse-expression (car datum))
			(map parse-expression (cdr datum)))])]
	  [else (eopl:error 'parse-expression
			    "Invalid concrete syntax ~s" datum)])))