<html><head><meta http-equiv="Content-Type" content="text/html charset=windows-1252"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div>I found this on line 2327 of racket/src/racket/src/env.c:</div><div><pre style="box-sizing: border-box; overflow: auto; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; margin-top: 0px; margin-bottom: 0px; color: rgb(51, 51, 51); line-height: 18px;"><div class="line" id="LC2327" style="box-sizing: border-box; padding-left: 10px; height: 18px; background-color: rgb(255, 255, 204);">static Scheme_Object *</div><div class="line" id="LC2328" style="box-sizing: border-box; padding-left: 10px; height: 18px;">local_introduce(int argc, Scheme_Object *argv[])</div><div class="line" id="LC2329" style="box-sizing: border-box; padding-left: 10px; height: 18px;">{</div><div class="line" id="LC2330" style="box-sizing: border-box; padding-left: 10px; height: 18px;"> Scheme_Comp_Env *env;</div><div class="line" id="LC2331" style="box-sizing: border-box; padding-left: 10px; height: 18px;"> Scheme_Object *s;</div><div class="line" id="LC2332" style="box-sizing: border-box; padding-left: 10px; height: 18px;"> </div><div class="line" id="LC2333" style="box-sizing: border-box; padding-left: 10px; height: 18px;"> env = scheme_current_thread->current_local_env;</div><div class="line" id="LC2334" style="box-sizing: border-box; padding-left: 10px; height: 18px;"> if (!env)</div><div class="line" id="LC2335" style="box-sizing: border-box; padding-left: 10px; height: 18px;"> not_currently_transforming("syntax-local-introduce");</div><div class="line" id="LC2336" style="box-sizing: border-box; padding-left: 10px; height: 18px;"> </div><div class="line" id="LC2337" style="box-sizing: border-box; padding-left: 10px; height: 18px;"> s = argv[0];</div><div class="line" id="LC2338" style="box-sizing: border-box; padding-left: 10px; height: 18px;"> if (!SCHEME_STXP(s))</div><div class="line" id="LC2339" style="box-sizing: border-box; padding-left: 10px; height: 18px;"> scheme_wrong_contract("syntax-local-introduce", "syntax?", 0, argc, argv);</div><div class="line" id="LC2340" style="box-sizing: border-box; padding-left: 10px; height: 18px;"> </div><div class="line" id="LC2341" style="box-sizing: border-box; padding-left: 10px; height: 18px;"> if (scheme_current_thread->current_local_mark)</div><div class="line" id="LC2342" style="box-sizing: border-box; padding-left: 10px; height: 18px;"> s = scheme_add_remove_mark(s, scheme_current_thread->current_local_mark);</div><div class="line" id="LC2343" style="box-sizing: border-box; padding-left: 10px; height: 18px;"> </div><div class="line" id="LC2344" style="box-sizing: border-box; padding-left: 10px; height: 18px;"> return s;</div><div class="line" id="LC2345" style="box-sizing: border-box; padding-left: 10px; height: 18px;">}</div></pre><div>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)</div></div><div> <div><div>On Aug 1, 2014, at 6:45 PM, J. Ian Johnson <<a href="mailto:ianj@ccs.neu.edu">ianj@ccs.neu.edu</a>> wrote:</div> <blockquote type="cite">Well one problem is expander application his its own mark to deal with, so you can't cancel the mark on x. <a href="https://github.com/plt/racket/blob/master/racket/collects/racket/match/parse-helper.rkt#L157">https://github.com/plt/racket/blob/master/racket/collects/racket/match/parse-helper.rkt#L157</a> 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@knauth.org> To: "J. Ian Johnson" <ianj@ccs.neu.edu> Cc: "racket users list" <users@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@ccs.neu.edu> wrote: <blockquote type="cite">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@knauth.org> To: "racket users list" <users@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@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@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 ____________________ Racket Users list: http://lists.racket-lang.org/users </blockquote> </blockquote></div> </div></body></html>