Well, I just hacked racket/match to allow match-expanders to take a third
argument that is the introducer for the application. No permutation of mark
applications works, I think since syntax-local-introduce does not include all
the marks from the top use of match-define all the way through internal macros
to the transformer call. I'm currently looking into getting a delta-introducer
sent to the transformer that has the delta between the original match syntax
and the match-expander syntax, but I'm fighting the implementation in actually
getting the orig-stx parameter set to the right thing.
-Ian
----- Original Message -----
From: "Alexander D. Knauth" <[email protected]>
To: "J. Ian Johnson" <[email protected]>
Cc: "racket users list" <[email protected]>
Sent: Friday, August 1, 2014 7:17:53 PM GMT -05:00 US/Canada Eastern
Subject: Re: [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 < [email protected] > 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" <[email protected]>
To: "J. Ian Johnson" <[email protected]>
Cc: "racket users list" <[email protected]>
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 <[email protected]> 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" <[email protected]>
To: "racket users list" <[email protected]>
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 < [email protected] > 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 < [email protected] > 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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