generic-syntax-expanders

A Racket package for creating macros with built-in support for defining sub-macros within them, similar to match expanders

Sometimes complex Racket macros need ways to extend their syntax. For example, in the match form new patterns can be created for match using define-match-expander. This package provides a form for defining a macro with the ability to define expanders for the macro. Consider this contrived example:

(define-syntax call-each
  (syntax-parser
    [(_ f (expr ...))
    #'(begin (f expr) ...)]))
    
(call-each displayln (1 2 3))

If a user of this macro wished for a way to specify a range of numbers as the arguments, the user must define their own version of call-each with support for ranges that expands to using the original call-each. However, this package provides a form, define-syntax-with-expanders:

(require generic-syntax-expanders)

(define-syntax-with-expanders call-each
  (syntax-parser
    [(_ f (expr ...))
    #'(begin (f expr) ... )]))

(define-call-each-expander foo
  (syntax-parser
    [(_ low:number high:number)
    #`(#,@(range (syntax-e #'low) (syntax-e #'high)))]))
    
(call-each displayln (foo 1 4))

The define-syntax-with-expanders form creates a define-call-each-expander form, which defines syntax transformers that are only used inside the body of a call-each form and expand before call-each does.

You can install this package with raco package install generic-syntax-expanders.