11.4 Options and user definitions

Options act as parameters that are transmitted to the parser on the call to read/rp. Local defines are user functions inserted in the produced parser, at the same level as the pre-defined ignore function.

Here is an example of grammar using both

(define gram
   (regular-grammar (x y)
      
      (define (foo s)
	 (cons* 'foo x s (ignore)))
      (define (bar s)
	 (cons* 'bar y s (ignore)))

      ((+ #\a) (foo (the-string)))
      ((+ #\b) (bar (the-string)))
      (else '())))

This grammar uses two options x and y. Hence when invokes it takes two additional values such as:

(with-input-from-string "aabb"
   (lambda ()
      (read/rp gram (current-input-port) 'option-x 'option-y)))
   ⇒ (foo option-x aa bar option-y bb)

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