[plt-scheme] macro question

From: Matthias Felleisen (matthias at ccs.neu.edu)
Date: Sat Jun 7 13:06:46 EDT 2008

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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