DrRacket BNF grammar

Question

I write a BNF grammar like this:

#lang pl

#| BNF for the LE language:
   <LE> ::= <num>
          | <null>
|#

(define-type LE
  [Num Number]
)

but I am not sure how to check if this code are good... how to check in racket the the only thing that we can use its null and numbers?

I think something like that:

(test 5)

but

(test '())

working too, and I am not setting List in my BNF

(if this code are not good - I will be happy for some BNF example and checking...)

Since the language pl is not part of the main distribution, you will need to give some more information. Is the pl language from Bremner's course? (cs.unb.ca/~bremner/teaching/cs3613/racket-setup ). If so which version of Racket is it for? — soegaard

soegaard soegaard · Accepted Answer · 2016-04-10T14:48:35

Without testing I suggest trying the following program:

#lang pl

#| BNF for the LE language:
   <LE> ::= <num>
          | <null>
|#

(define-type LE
  [Num Number]
  [Nul Null]
  [Kons LE LE])

(: test : LE -> LE)
(define (test x)
   x)

(test (Num 5))        ; this is accepted since 5 is a Number
(test (Nul '())
(test (Kons (Num 1) (Num 2)))
; (test (Num "foo"))  ; this provokes an error (as it should)

Note that (: test : LE -> LE) declares the types of the test function. Since in (test '()) the empty list does not match the LE type, you ought to get an error.

EDIT: The examples has been update to use (Num 5) and not just 5.

EDIT 2: Added Kons

DrRacket BNF grammar

1 Answers