Fw: [plt-scheme] macro question

From: Jos Koot (jos.koot at telefonica.net)
Date: Sat Jun 7 16:47:00 EDT 2008


 No, it is not a bug. I think you are programming at top level, where every
 variable that has no other binding is bound at top level.  Your problem
 would have been exposed when programming in modules.
 Jos
>
> ----- Original Message ----- 
> From: "Michael Vanier" <mvanier at cs.caltech.edu>
> To: "Matthias Felleisen" <matthias at ccs.neu.edu>
> Cc: "mzscheme Mailing List" <plt-scheme at list.cs.brown.edu>
> Sent: Saturday, June 07, 2008 9:58 PM
> Subject: Re: [plt-scheme] macro question
>
>
>> I'm using 3.72[3m]; I didn't know that 4.0 was ready yet.  I may not know 
>> what I'm doing, but when I hit the "macro stepper" button it expands the 
>> entire Scheme file form-by-form.  When it gets to the get-stack-values 
>> macro calls it just expands them naively with no error messages.  I don't 
>> know if this is a bug in 3.72 or if I'm not doing it right.
>>
>> Completely off-topic, I have to say that I am really keen on the idea of 
>> typed Scheme (which I haven't used yet, though I've read the paper).  I 
>> found that about 90% of the bugs in my code were stupid type errors. 
>> Contracts would probably have helped a lot too.
>>
>> Mike
>>
>> Matthias Felleisen wrote:
>>>
>>> Michael, when I run the macro stepper (in 4.0), I get
>>>
>>>> expand: unbound variable in module
>>>> es
>>>
>>> with high-lighting, exactly where I expected it. Same for RUN.
>>>
>>> Jos explained the why and the how to fix it.
>>>
>>> I'm surprised to hear that the macro stepper wouldn't show you this 
>>> error. What version are you using? What language?
>>>
>>> Thanks -- Matthias
>>>
>>>
>>>
>>>
>>> On Jun 7, 2008, at 5:17 AM, Michael Vanier wrote:
>>>
>>>> I've run into a problem with a macro I don't know how to solve.  Here's 
>>>> the macro:
>>>>
>>>> (define-syntax get-stack-vals
>>>>   (syntax-rules ()
>>>>     ((_ n (env first rest) expr ...)
>>>>      (let*-values (((env stk) (es-values es))
>>>>                    ((first rest) (take n stk)))
>>>>        expr ...))))
>>>>
>>>> Here's a use:
>>>>
>>>> (define (do-def es)
>>>>   (get-stack-vals 2 (env first rest)
>>>>      (let ((sym (cadr first))
>>>>            (val (car first)))
>>>>        (if (symbol? sym)
>>>>            (let ((env2 (env-add env sym val)))
>>>>              (make-es env2 rest))
>>>>            (error "cannot define non-symbol: " sym)))))
>>>>
>>>> which should expand to:
>>>>
>>>> (define (do-def es)
>>>>   (let*-values (((env stk) (es-values es))
>>>>                 ((first rest) (take 2 stk)))
>>>>     (let ((sym (cadr first))
>>>>           (val (car first)))
>>>>       (if (symbol? sym)
>>>>           (let ((env2 (env-add env sym val)))
>>>>             (make-es env2 rest))
>>>>           (error "cannot define non-symbol: " sym)))))
>>>>
>>>> and apparently does, at least according to the macro stepper.  However, 
>>>> when I call do-def I get an error message: "reference to undefined 
>>>> identifier: es".  This seems odd, because es is in scope at the macro 
>>>> expansion location.  Is the es in the get-stack-vals macro bound in the 
>>>> lexical scope of that macro?  All the other external identifiers in the 
>>>> macro are bound at the top-level, so it works either way for them.  If 
>>>> this is the case, how do I get es to be bound the way I want it to be? 
>>>> I realize that I'm trying to do something a bit naughty since I'm 
>>>> assuming a binding for es will exist at all macro expansion locations; 
>>>> alternative suggestions are welcome (other than "don't give up your day 
>>>> job").
>>>>
>>>> I'm a macro newbie, so please be gentle ;-)  PLT Scheme's macro system 
>>>> is an intricate and beautiful thing that I'm just beginning to wrap my 
>>>> head around.
>>>>
>>>> Mike
>>>>
>>>>
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>>>
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> 



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