; Scanner specification defines terminals (tokens).
(define scanner-spec-lec20
  '((white-sp
     (whitespace)                   skip)
    (comment
     ("%" (arbno (not #\newline)))  skip)
    (identifier
     (letter (arbno (or letter digit "?")))
                                    symbol)
    (number
     (digit (arbno digit))          number)))

; Grammar specification describes allowable syntax.
(define grammar-lec20
  '((expr (number) lit-expr)
    (expr (identifier) var-expr)
    (expr ("(" subexpr ")") sub-expr)
    (subexpr ("lambda" "(" (arbno identifier) ")"
                  expr) lambda-expr)
    (subexpr ("if" expr expr expr) if-expr)
    (subexpr ((arbno expr)) app-expr)))

; Automagically define a scanner and parser.
(define scan&parse (sllgen:make-string-parser
                   scanner-spec-lec20
                   grammar-lec20))

; Automagically generate the define-datatype records for an
; Abstract Syntax Tree representation of the grammar.
(sllgen:make-define-datatypes scanner-spec-lec20
                              grammar-lec20)