[racket] Managing communication between two macro invocations at the top-level

Stick the identifier into a define-syntax and retrieve it elsewhere. 

My idiom is 

> #lang racket
> 
> (define-for-syntax it #f)
> 
> (begin-for-syntax 
>   (set! it #'a))
> 
> (define-syntax (m stx)
>   (syntax-case stx ()
>     [(_ . x) it]))
> 
> (define a 10)
> 
> (m)


I have used this on one occasion. Does it work for you? 




On Jul 20, 2014, at 6:27 PM, Asumu Takikawa <asumu at ccs.neu.edu> wrote:

> Hi all,
> 
> I'm stuck on a macro issue with the top-level and was wondering if
> anyone had any thoughts.
> 
> Certain macros (I'm thinking of parts of Typed Racket) manage
> communication between different parts of a program (different macros or
> different instantiations of the same macro) by storing a syntax-time
> table that maps, say, identifiers that should be defined in one place
> and referenced in another.
> 
> The trouble is that this approach doesn't work so well at the top-level
> because of the issues described in Reference §1.2.3.9. Namely because at
> the top-level if one invocation of a macro expands to:
> 
>  (my-macro) =>
> 
>  (begin
>    (define-values (generated-id) something-to-use-later)
>    ... other stuff ...)
> 
> and then another invocation expands to:
> 
>  (my-macro) =>
> 
>  (begin
>    (generated-id)
>    ... other stuff ...)
> 
> you get an unbound identifier error because `generated-id` is only bound
> within the first `begin` block.
> 
> Is there any way I can get around this issue? I thought maybe syntax
> lifting would help out, but I need to use a particular generated
> identifier and not a fresh one obtained from lifting. That and TR
> catches all lifts at the top-level anyway (and sticks them in a `begin`)
> so I don't think it'll help.
> 
> I also noticed that you can get around the 1.2.3.9 limitation using
> `eval` if you have a non-gensymed identifier, but it doesn't appear to
> help with a gensymed one.
> 
> Cheers,
> Asumu
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