[racket] getting one macro to tell another macro to define something

I found this on line 2327 of racket/src/racket/src/env.c:
static Scheme_Object *
local_introduce(int argc, Scheme_Object *argv[])
{
  Scheme_Comp_Env *env;
  Scheme_Object *s;

  env = scheme_current_thread->current_local_env;
  if (!env)
    not_currently_transforming("syntax-local-introduce");

  s = argv[0];
  if (!SCHEME_STXP(s))
    scheme_wrong_contract("syntax-local-introduce", "syntax?", 0, argc, argv);

  if (scheme_current_thread->current_local_mark)
    s = scheme_add_remove_mark(s, scheme_current_thread->current_local_mark);

  return s;
}
What would happen if match-expander-transform somehow set scheme_current_thread->current_local_mark to the mark produced by the new syntax-introducer?  And would that even be possible?  (without fundamental changes to racket)

On Aug 1, 2014, at 6:45 PM, J. Ian Johnson <ianj at ccs.neu.edu> wrote:

> Well one problem is expander application his its own mark to deal with, so you can't cancel the mark on x.
> 
> https://github.com/plt/racket/blob/master/racket/collects/racket/match/parse-helper.rkt#L157
> 
> Another problem is expecting the implementation of match-define to not have any inner macros that would change syntax-local-introduce to a less helpful extent.
> What would be ideal is if racket/match could change some "parameter" so that syntax-local-introduce used the introducer defined in the above link, since the generated temporary does not have x's mark that x has annihilated. Instead x's mark will be added to tmp after the transformer returns, and there's nothing you can do about it :(
> 
> -Ian
> ----- Original Message -----
> From: "Alexander D. Knauth" <alexander at knauth.org>
> To: "J. Ian Johnson" <ianj at ccs.neu.edu>
> Cc: "racket users list" <users at racket-lang.org>
> Sent: Friday, August 1, 2014 6:31:59 PM GMT -05:00 US/Canada Eastern
> Subject: Re: [racket] getting one macro to tell another macro to define something
> 
> Well, if the match-expander is invoked in the “dynamic extent” of the match-define form, then would syntax-local-introduce apply that syntax-mark?  
> 
> On Aug 1, 2014, at 6:20 PM, J. Ian Johnson <ianj at ccs.neu.edu> wrote:
> 
>> Well that's a pickle. I can tell you that (mac . args) gets expanded as (X[mac^m] . X[args^m])^m where m is a fresh mark and X expands a form. If m is applied to something with m already, they annihilate each other (see Syntactic Abstraction in Scheme for how this totally works).
>> The syntax-local-introduce form allows you to apply the macro application's mark to an arbitrary piece of syntax, so later on the application's mark will annihilate it and voila`, it's like it was textually given to the macro application itself.
>> 
>> Here, however, a match expander is not treated as a macro invocation. There is no mark for that match-expander use to introduce. There is, however, the mark from match-define that you'll want to introduce to this temporary you've generated. I think. I haven't quite worked out how to make this work.
>> -Ian
>> ----- Original Message -----
>> From: "Alexander D. Knauth" <alexander at knauth.org>
>> To: "racket users list" <users at racket-lang.org>
>> Sent: Friday, August 1, 2014 5:55:57 PM GMT -05:00 US/Canada Eastern
>> Subject: Re: [racket] getting one macro to tell another macro to define something
>> 
>> 
>> 
>> 
>> 
>> On Aug 1, 2014, at 5:37 PM, J. Ian Johnson < ianj at ccs.neu.edu > wrote: 
>> 
>> 
>> It's best to expand into a begin-for-syntax that does the desired mutation, rather than mutate within the transformer. You currently _cannot_ do this outside top level forms. 
>> 
>> 
>> 
>> The reason I can’t do that is because in the real program, sender is actually a match-expander. 
>> 
>> 
>> You are also right about the marks. The call to receiver adds additional marks to the definitions that it pulls out, so you'll need to apply syntax-local-introduce. ... 
>> 
>> 
>> 
>> 
>> On Aug 1, 2014, at 5:39 PM, Ryan Culpepper < ryanc at ccs.neu.edu > wrote: 
>> 
>> 
>> Use syntax-local-introduce when putting syntax into a side-channel or getting it back out across macro calls. This only matters when the syntax represents a definition or more generally contains binders whose references are not all in the same syntax. 
>> ... 
>> 
>> 
>> Thanks, the syntax-local-introduce got it working for that example, but for some reason it’s not working when sender is a match-expander. 
>> 
>> I’m still not very clear on when to use syntax-local-introduce and when not to, or even what it does (other than get that example working), so could someone point me in the right direction? 
>> 
>> 
>> 
>> #lang racket 
>> (require racket/stxparam 
>> (for-syntax syntax/parse 
>> racket/syntax 
>> racket/set)) 
>> ;; current-defs : (syntax-parameter-of (or/c set-mutable? #f)) 
>> (define-syntax-parameter current-defs #f) 
>> (define-match-expander sender 
>> (lambda (stx) 
>> (syntax-parse stx 
>> [(sender x) 
>> #:with tmp (generate-temporary #'x) 
>> (define defs (syntax-parameter-value #'current-defs)) 
>> (set-add! defs (syntax-local-introduce #'(define x tmp))) 
>> #'tmp]))) 
>> (define-syntax reciever 
>> (lambda (stx) 
>> (syntax-parse stx 
>> [(reciever) 
>> (define defs (syntax-parameter-value #'current-defs)) 
>> (with-syntax ([(def ...) (map syntax-local-introduce (set->list defs))]) 
>> #'(begin def ...))]))) 
>> 
>> 
>> (syntax-parameterize ([current-defs (mutable-set)]) 
>> (match-define (sender x) 1) 
>> (reciever) 
>> x) 
>> 
>> 
>> ;x3: unbound identifier; 
>> ; also, no #%top syntax transformer is bound in: x3 
>> 
>> 
>> 
>> 
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> 

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