[racket] macros in local namespaces?...

The macro stepper is your friend. 



On Feb 2, 2012, at 2:50 AM, Rüdiger Asche wrote:

> 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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