[racket] macros in local namespaces?...

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" <matthias at ccs.neu.edu>
To: "Rüdiger Asche" <rac at ruediger-asche.de>
Cc: <users at 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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