Hi Tim,
Racket's macros expand one step at a time. Your module begin will expand in one
step before the inner expr ... all get expanded, thus leaving the counter at 0.
What you might want to do is local expand the body of your module and count the
define-values forms in the module-begin itself. You could also just do a
local-expand for the effect on counter to get your result, but I don't
recommend this.
-Ian
----- Original Message -----
From: "Tim Brown" <tim.br...@cityc.co.uk>
To: "Racket Users" <users@racket-lang.org>
Sent: Wednesday, January 16, 2013 9:24:44 AM GMT -05:00 US/Canada Eastern
Subject: [racket] variables within macros
Folks,
I would like to write a module that uses a macro to transform "define"s
and accumulate a value based on those transformations.
In a fairly simple way, I want to do something like the below. However, it
seems the module-begin is being transformed before the my-defines. How do
I get counter to be incremented before I need it?
Tim
===================================================================================
"counted-defines.rkt" (similar to what's on the 17.1.2 Using #lang s-exp
Guide Page)
===================================================================================
#lang racket
(provide (except-out (all-from-out racket) #%module-begin define)
(rename-out (module-begin #%module-begin) (my-define define)))
(require (for-syntax syntax/parse))
(define-for-syntax counter 0)
(define-syntax (my-define stx)
(syntax-parse
stx
[(_ i v) (set! counter (add1 counter)) #`(define i v)]
[(_ (i args ...) v ...+) (set! counter (add1 counter))
#`(define (i args ...) v ...)]))
(define-syntax (module-begin stx)
(syntax-parse
stx
[(_ expr ...)
#`(#%module-begin expr ...
(define defines-count #,counter)
(provide defines-count))]))
==========================
"test-counted-defines.rkt"
==========================
#lang s-exp "counted-defines.rkt"
(define woo 2) ; should have pushed counter up by one?
==========================
I then run: racket -e '(require "test-counted-defines.rkt") defines-count'
Which returns me "0"
--
Tim Brown <tim.br...@cityc.co.uk> | City Computing Limited |
T: +44 20 8770 2110 | City House, Sutton Park Road |
F: +44 20 8770 2130 | Sutton, Surrey, SM1 2AE, GB |
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