From 8368cc56411d5526e1478ad9de2abe408f460335 Mon Sep 17 00:00:00 2001 From: Jack Firth Date: Tue, 9 Dec 2014 22:54:58 -0800 Subject: [PATCH] Update README.md Provide an explanation of the package --- README.md | 27 +++++++++++++++++++++++++++ 1 file changed, 27 insertions(+) diff --git a/README.md b/README.md index cc7a0c1..3a4ea1f 100644 --- a/README.md +++ b/README.md @@ -2,3 +2,30 @@ 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 a way for their syntax to be extended. The Racket match form is an example, new patterns can be created for match using define-match-expander. This racket 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 racket package provides a form, define-syntax-with-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`.