[plt-dev] parser macros

From: Dave Herman (dherman at ccs.neu.edu)
Date: Wed Sep 16 09:05:03 EDT 2009

Floating an idea:

I'd love to see an extension to the `parser' form that allows  
programmers to define grammar macros. Imagine if you could do  
something like this:

  (start Expr)
  (end EOF)
  (tokens Tokens)
    (define-macro (List x)
      [(LPAREN (ListTail x))
    (define-macro (ListTail x)
      [(RPAREN) '()]
      [(x COMMA (ListTail x))
       (cons $1 $3)]))
      [(Expr (List Expr))
       (make-app-expr $1 $2)]
      [(LAMBDA ID (List ID))
       (make-lambda-expr $2 $3)]
      ;; etc...

and imagine the macros would be expanded away as part of the expansion  
of the `parser' form. This would give you the ability to define  
abstractions in the grammar language in a manner that results in code  
duplication rather than sharing. Obviously, it's a bit of a blunt  
instrument, but in an LALR framework this kind of duplication is  
sometimes necessary to avoid conflicts.

Perhaps better yet, it'd be cool if these macros could be defined  
independently, like match-expanders:

     (define-grammar-expander (ListTail x)
       [(RPAREN) '()]
       [(x COMMA (ListTail x))
        (cons $1 $3)])


BTW, who maintains the parser-tools these days?


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