I think you need to use a trampoline for the module body's expansion,
where you expand only one module-body form at a time and let
`#%module-begin` finish up forms like `require` or `define-syntaxes`
before you macro continues with later forms.
The general pattern is that you have a macro `do-module-begin` where
(do-module-begin
expr-0
expr ...)
turns into
(begin
expanded-expr-0
(do-module-begin
expr ...))
where `expr-0` is `local-expand`ed and transformed as needed. The base
module-body expander will process `expanded-expr-0`, importing and
binding macros as needed, before continuing with `(do-module-begin expr
...)` for the rest of the module.
For an example, see the implementation of `syntax/wrap-modbeg`.
This pattern shows up frequently, but I've never hit on quite the right
abstraction to turn the pattern into a library. It's probably just a
matter of trying harder.
At Fri, 29 Jan 2016 21:29:16 -0800 (PST), reilithion wrote:
> I'm trying to make a little EDSL that has one form that is special and needs
> to be collected and later processed. I want to allow a user of the EDSL to
> write a macro that expands to the special form. I have a module-begin that
> (so
> far) looks like this:
>
> (define-syntax (module-begin stx)
> (syntax-parse stx
> ((_ module-contents ...)
> (let-values
> ([(tasks other)
> (partition
> (syntax-parser #:literals (task-internal) ((task-internal _ ...) #t)
> (_ #f))
> (rest
> ;; Problems start here
> (syntax->list
>(local-expand
> #'(#%plain-module-begin module-contents ...)
> 'module-begin
> (list #'task-internal #'module* #'#%app)])
> #`(#%module-begin
> #,@other
> (define all-tasks (list #,@tasks))
> (process all-tasks))
>
> If I don't bother with the local-expand and just use (rest (syntax->list
> stx)), it works basically fine but won't treat macros that expand to
> task-internal specially. I tried mapping local-expand over (syntax->list
> stx),
> but that led to strange errors when I tried to define macros within the EDSL.
>
> This current implementation fails because it expands too much too soon. A
> macro will happily expand into (task-internal args ...), then (task-internal1
> args ...), then (#%app task-internal1 args ...), and then the call to
> partition won't see what it's expecting (in fact, I think it CAN'T see what
> it
> needs to, because I suspect the local-expand of the #%plain-module-begin
> effectively hides the task-internal identifier).
>
> I added a bunch of stuff to the stop-ids list, hoping it would bail before
> transforming task-internal beyond recognition, but nothing I added seemed to
> work.
>
> I also tried calling local-expand on #'(begin module-contexts ...) with the
> 'top-level context-v, but of course begin is automatically added to the
> stop-ids list and local-expand stops right away.
>
> Can I somehow get local-expand to only mostly expand a module? How do I do
> what I'm trying to do here?
>
> Thanks!
> Lucas Paul
>
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