(module grammar3
  (lib "eopl.ss" "eopl")
  
  (require "drscheme-init.scm")
  
  (define lex0
  '((whitespace (whitespace)                       skip)
    (comment    ("%" (arbno (not #\newline)))      skip)
    (identifier (letter (arbno (or letter digit))) symbol)
    (number     (digit (arbno digit))              number)))

  (define grammar3
  '((expression (number) lit-exp)
    (expression (identifier) var-exp)
    (expression
      ("let" (arbno identifier "=" expression) "in" expression)
      let-exp)
    (expression
      ("(" "-" (arbno expression) ")")
      sum-exp)
    ))
  

  ;; build a scanner with empty grammar 
  (define scan0 (sllgen:make-string-scanner lex0 '()))
  
  ;; build data-types
  (sllgen:make-define-datatypes lex0 grammar3)
  
  ;; build scanner with grammar 
  (define just-scan
    (sllgen:make-string-scanner lex0 grammar3))
  
  ;; build parser 
  (define scan&parse 
    (sllgen:make-string-parser lex0 grammar3))
  
  ;; show the datatypes 
  (define show-the-datatypes 
    (lambda () (sllgen:show-define-datatypes lex0 grammar3)))
  
  (define stmt1 "let x = (- y z) u = (- g h) v = (- i j) 
                 in (- (- x u) (- v k))")
  
  )