[plt-scheme] Macro changing behaviour

On Tue, Apr 21, 2009 at 10:44 PM, Ryan Culpepper <ryanc at ccs.neu.edu> wrote:
> On Apr 21, 2009, at 6:30 PM, Paulo J. Matos wrote:
>
>> Hi all,
>>
>> I have a simple issue:
>> (define-syntax-rule (test proc . args)
>>  (apply proc args))
>> (test (lambda () (+ 1 1)))
>>
>> doesn't work. Generates:
>> #%app: missing procedure expression; probably originally (), which is
>> an illegal empty application in: (#%app)
>>
>> But this works:
>> (apply (lambda () (+ 1 1)) '())
>>
>> So, somewhere this goes wrong. Can someone explain me what's the issue?
>
> Your example expands into this expression (see using the macro stepper):
>
> (apply (lambda () (+ 1 1)) ())
>
> The second argument, (), is illegal in PLT Scheme v4. The error says '#%app'
> because it is interpreted as an empty application, which is not allowed.
>
> --
>
> Let's back up and look at the skeleton of your macro again:
>
> (define-syntax-rule (test proc . args)
>  (apply proc ???))
>
> What is 'args'?
> Well, it's probably supposed to be a (syntax) list of expressions.
>
> What do you need for the final argument of 'apply'?
> It must be an expression resulting in a list. (Not the same as a list of
> expressions!)
>
> How do you form an expression resulting in a list from a list of
> expressions?
> Use the list procedure.
>
> Here's the macro, rewritten:
>
> (define-syntax-rule (test proc . args)
>  (apply proc (list . args))
>
> or, more idiomatically:
>
> (define-syntax-rule (test proc arg ...)
>  (apply proc (list arg ...))
>

Thanks, got the mistake! :)

> Ryan
>
>



-- 
Paulo Jorge Matos - pocmatos at gmail.com
Webpage: http://www.personal.soton.ac.uk/pocm