[plt-scheme] Struggling with macro...

On 5/23/07, Robert Nikander <nikander at nc.rr.com> wrote:
> Paulo J. Matos wrote:
> >  (define-syntax (machine stx)
> >    (syntax-case stx ()
> >      [(_ mname sexps ...)
> >       (identifier? #'mname) ; fender
> >       #'(let ([mc (make-mc () () () () ())])
> >           #,(with-syntax ([variables (syntax-case #'(sexps ...) (variables)
> >                                        ((variables (name value) ...)
> >                                         (andmap identifier? #'(name ...))
> >                                         ((add-variable-to-mc #'name
> > #'value) ...)))])
> >               #'variables
> >               #'mc))]))
> >
> > This gives me: syntax: no pattern variables before ellipses in template
> > in: ...
>
> The first problem is the #' before the let.  Since you are using #,
> (unquote-syntax) then you need #` (or quasisyntax), not #' (or syntax).
>   In other words, it must be:
>
> #`(let ([mc .....
>

Yes, thank you...

> Another problem is that in the inner syntax case #'(sexps ...) actually
> starts as:
>
> ((variables (x 1)
>
> not
>
> (variables (x 1)
>
> So, it won't match the one clause in the syntax-case.  One way to help
> see this is to add lines like this to your macro, to run at transform
> time, not runtime.
>
> (printf "(sexps ...) = ~a~n" (syntax-object->datum (syntax (sexps ...))))
>
> Here is something that works, but it may not fit your real situation:
>
> (define-syntax (machine stx)
>    (syntax-case stx (variables)
>      [(_ mname (variables (name value) ...))
>       (identifier? #'mname) ; fender
>       #'(let ([mc (make-mc () () () () ())])
>                   (add-variable-to-mc name value) ...
>            mc)]))
>
>

Indeed, although this works I don't want to do this in one go because
I have several more things besides variable declarations, so I'll
eventually need to call the macro again, kind like a cond macro, but
one which has to be wrapped around a (let ([mc... so that every code
in the expansion is able to access mc.

> Rob
>
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>


-- 
Paulo Jorge Matos - pocm at soton.ac.uk
http://www.personal.soton.ac.uk/pocm
PhD Student @ ECS
University of Southampton, UK