[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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