[racket] datum<->syntax with same context, yet "unbound identifier"

From: Robby Findler (robby at eecs.northwestern.edu)
Date: Wed Nov 21 11:55:28 EST 2012

Oh, good point! Yes, that sounds like it should work well, sorry I didn't
think of it.


On Wednesday, November 21, 2012, Tobias Hammer wrote:

> On Wed, 21 Nov 2012 15:33:06 +0100, Greg Hendershott <
> greghendershott at gmail.com> wrote:
>  The lexical context you're picking up to put on #'args in the lam
>>> macro is the context that's sitting on the parentheses in the first
>>> argument to 'lam' in the body of 'expand-to-lam'. that context has no
>>> "x" bound.
>> OK, that explains why x is not bound. Thanks!
>>  This is the trouble with these kinds of macros: you have to be careful
>>> to propagate the correct lexical information thru. In this case, you
>>> can either change expand-to-lam to carry its argument's lexical
>>> context onto that open paren there or you can change lam to use a
>>> different spot to get its lexical information (one that's already
>>> being carried thru, eg the context on the actual variables for
>>> example).
>> I want lam to be usable by other macros without the latter needing to
>> cooperate explicitly; I want them to "just work". So I prefer the
>> second way.
> As Robby explained you are using the context of the list of args for all
> arguments,
> which is in this case a wrong one. One context that should be valid in all
> cases
> in your example is the one of the arguments itself. If you unpack the
> arguments, do whatever you want to them and then reapply the context you
> stripped before,
> it should (as far as i understand) work:
> (define-syntax (lam stx)
>   (syntax-case stx ()
>     [(_ args body ...)
>      (with-syntax ([conv-args (for/list ([a (syntax->list #'args)])
>                                 (define a-datum (syntax->datum a))
>                                 ;; do whatever you want with a-datum
>                                 (datum->syntax a a-datum))])
>      #`(lambda conv-args
>          body ...))]))
> Other macros only have to cooperate in the 'normal' way, i.e. they have to
> make sure
> that the arguments they pass in have the right context.
> Tobias
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