Thanks for the fast and on-the-spot response!
Is there a way to run the preprocessor only so that I can see what code that
construction expands to before being passed to the read-eval loop?
----- Original Message -----
From: "Matthias Felleisen" <matth...@ccs.neu.edu>
To: "Rüdiger Asche" <r...@ruediger-asche.de>
Cc: <users@racket-lang.org>
Sent: Wednesday, February 01, 2012 6:03 PM
Subject: Re: [racket] macros in local namespaces?...
Something like this might be what you want:
(letrec-syntaxes+values ([(absfn)
(syntax-rules ()
[(_ varname) (lambda (x) (+ (+ x localc)
varname))])])
([(a) 2]
[(b) 3]
[(localc) 4]
[(afn) (absfn a)]
[(bfn) (absfn b)])
(afn (bfn 3)))
BUT, I strongly recommend abstraction the macro over both variables.
On Feb 1, 2012, at 11:45 AM, Rüdiger Asche wrote:
Hi there,
I'm trying to get a grip on macros. Here is a very simple Racket
expression (1):
(letrec [(a 2)
(b 3)
(afn (lambda (x) (+ x a)))
(bfn (lambda (x) (+ x b)))]
(afn (bfn 2)))
Now I need a syntactic abstraction for afn and bfn. The following will do
in first approximation (2):
(define-syntax-rule (absfn varname) (lambda (x) (+ x varname)))
(letrec [(a 2)
(b 3)
(afn (absfn a))
(bfn (absfn b))]
(afn (bfn 2)))
However, it will fail due to scoping rules in the following example (3):
(define-syntax-rule (absfn varname) (lambda (x) (+ (+ x localc) varname)))
(letrec [(a 2)
(b 3)
(localc 4)
(afn (absfn a))
(bfn (absfn b))]
(afn (bfn 2)))
In other words, my syntactic extension absfn needs to be embedded in the
namespace of the sorrounding expression (or as a "dumb" macro which simply
does lexical replacement without considering scoping, but needless to say
such a macro would be unhygienic).
I suspect that letrec-syntax was meant for that purpose, but I can't
figure out how the parameters to define-syntax-rule would translate to
those of letrec-syntax.
Does anyone have sample code for how to get (3) above to work?
Thanks!
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