[plt-scheme] match library for r6rs

At Fri, 09 May 2008 15:32:22 -0700, Derick Eddington wrote:
> > But using this library with DrScheme leads to following error message:
> > 
> > match.ss:377:6: compile: bad syntax; function application is not
> > allowed, because no #%app syntax transformer is bound
> 
> I think the reason for this error could be a bug in MzScheme's R6RS
> support.  I think the macro experts here can tell us.  I think the issue
> has to do with export levels of bindings.  IIRC (rnrs) is supposed to be
> imported for phase `expand' by default.  The problem can be reduced to
> this example, which only works in MzScheme if the (rnrs) library is
> explicitly imported for phase `expand':
> 
> #!r6rs
> (import (for (rnrs) run #;expand))
> 
> (define-syntax A
>   (let ()
>     (define-syntax B
>       (syntax-rules () [(_ x) x]))
>     (define (h)
>       (B 1))
>     (lambda (ignored)
>       (h))))
> 
> (write (A)) (newline)
> 
> ERROR=>
> 
> compile: bad syntax; function application is not allowed, because no #%
> app syntax transformer is bound in: (syntax-rules () ((_ x) x))
> 
> Uncomment the `expand' in the import form, and it works.

The use of `syntax-rules' is in phase level 2 (i.e., expand time
relative to expand time). The `(rnrs)' language exports `syntax-rules'
at phase level 1 (= `expand'), but not 2.

A `(run .... expand)' import of `(rnrs)' fixes the problem, because it
shifts the phase level 1 export of `syntax-rules' to phase level 2.

Matthew