[plt-scheme] Unit/sig and hygiene

[Sorry for the delay...]

At Thu, 28 Sep 2006 10:54:36 +0100, Dave Gurnell wrote:
> We have a problem with the interaction between signed units and  
> hygiene.

I don't think your problem has anything to do with units. It has to do
with `syntax-local-introduce' and modules:

>    (define-syntax (define-snooze-interface stx)
>      (syntax-case stx ()
>        [(define-snooze-interface db@)
>          (syntax-local-introduce
>           #'(begin
>           [...]

Here's a simpler program with the same problem:

  (module ma mzscheme
    (define-syntax (def-a stx)
      (syntax-case stx ()
        [(_)
         (syntax-local-introduce
          #'(begin
              (define a 5)

              (define-syntax get-a
                (syntax-rules ()
                  [(_) a]))

              (provide get-a)))]))
    (provide def-a))

  (module mb mzscheme
    (require ma)
    (def-a)
    (printf "~a\n" (get-a)))

I think the problem is best characterized as a bug in MzScheme's
handling of module-top-level bindings. MzScheme's expander doesn't
introduce a suitable of renaming for `a', because the defined `a'
appears not to come from a macro (since the macro uses
`syntax-local-introduce' to hide the introduction).

I'll look into fixing it, but I'll have to think more about whether
syntax-local-introduce' can cause other, similar problems. Meanwhile, I
recommend trying to use `datum->syntax-object' instead of
`syntax-local-introduce'.

Matthew