[racket-dev] cross-phase syntax constants

From: Matthew Flatt (mflatt at cs.utah.edu)
Date: Fri Jul 6 11:00:50 EDT 2012

I find the example too abstract to understand why the computation
happens at the "wrong phase" when you adjust the code with
`begin-for-syntax'. Can you explain a little more, maybe with a more
concrete example?

Just in case, here's the code that I think you have in mind for "wrong

 #lang racket/load

 (module m1 racket
   (define l (list #'l))
   (provide l))

 (module m2 racket
   (require (for-syntax 'm1))
   (define-syntax (mac stx)
        (module* sub #f
          (length (list #,(car l))))))
   (provide mac))

 (module m3 racket
   (require 'm2)

 (require (submod 'm3 sub))

At Thu, 5 Jul 2012 17:17:08 -0400, Sam Tobin-Hochstadt wrote:
> I'd like to write a program basically like this:
> #lang racket/load
> (module m1 racket
>   (define l (list #'l))
>   (provide l))
> (module m2 racket
>   (require (for-syntax 'm1))
>   (define-syntax (mac stx)
>     #`(module* sub #f
>         (length (list #,(car l)))))
>   (provide mac))
> (module m3 racket
>   (require 'm2)
>   (mac))
> But I can't come up with any way to `require` m1 appropriately so that
> `l` is bound in the generated submodule.  If I wrap the body of the
> submodule in `begin-for-syntax`, then it works, but then loading the
> submodule does the computation at the wrong phase.  So another
> solution would be `dynamic-require-for-template`, if that's feasible.
> -- 
> sam th
> samth at ccs.neu.edu

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