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 that 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`.