[plt-dev] syntax/module-reader and #%module-begin

On Dec  1, Sam TH wrote:
> On Tue, Dec 1, 2009 at 11:29 AM, Eli Barzilay <eli at barzilay.org> wrote:
> > On Dec  1, Sam TH wrote:
> >> On Tue, Dec 1, 2009 at 10:59 AM, Eli Barzilay <eli at barzilay.org> wrote:
> >> > On Nov 30, Stevie Strickland wrote:
> >> >>
> >> >> I've tried making the following change (surrounded by -----):
> >> >>
> >> >> -----
> >> >> Index: collects/syntax/module-reader.ss
> >> >> ===================================================================
> >> >> --- collects/syntax/module-reader.ss  (revision 17113)
> >> >> +++ collects/syntax/module-reader.ss  (working copy)
> >> >> @@ -170,7 +170,11 @@
> >> >>                                          (- (or (syntax-position modpath) (add1 pos))
> >> >>                                             pos)))
> >> >>                            v))]
> >> >> -           [r `(,(tag-src 'module) ,(tag-src name) ,lang . ,body)])
> >> >> +           [wrapped-body (let ([wrapped `(#%module-begin . ,body)])
> >> >> +                           (if stx?
> >> >> +                               (datum->syntax #f wrapped all-loc)
> >> >> +                               wrapped))]
> >> >> +           [r `(,(tag-src 'module) ,(tag-src name) ,lang ,wrapped-body)])
> >> >>        (if stx? (datum->syntax #f r all-loc) r)))
> >> >>
> >> >>    (define (wrap lang port read modpath src line col pos)
> >> >> -----
> >> >
> >> > Isn't it better to do the wrapping only when `#%module-begin' is not
> >> > there?
> >>
> >> What would this mean?  Looking for the symbolic name
> >> '#%module-begin?
> >
> > Yes.
> >
> >
> >> That seems like it adds another special case, where the benefit of
> >> the change is to remove such special cases, especially since it
> >> wouldn't help if you wrote #%plain-module-begin instead.
> >
> > It is already a special case -- it's a symbolic name that gets added.
> 
> Right, but currently the rule for #%module-begin insertion is
> complicated.  With this change, it would become simpler - if you use
> #lang, #%module-begin is always inserted.  With your suggestion, the
> rule grows a special case.

It's not *that* special -- if you use `#lang', the result will get a
`#%module-begin' unless it has one.


> > (IIRC, `#%plain-module-begin' is not a special case, and intended
> > only to be provided from a language as `#%module-begin'.)
> 
> No, you can certainly write
> 
> (module m scheme (#%plain-module-begin 'x 'y 'z))
> 
> Thus, if you wanted to support that with #lang as well, you could
> add the symbol '#%plain-module-begin to this special case as well.

Oh, right -- but I think that that happens only after bindings are
resolved.  At least it looks like that, since the C code doesn't have
any mention of `#%plain-module-begin'.  Also, this works:

  (module fooish scheme
    (provide (rename-out [#%module-begin foo]) (all-from-out scheme)))
  (module bar 'fooish (foo 1 2 3))

If this is the case, then module readers should definitely not try to
deal with anything other than `#%module-begin' since that involves
more than context-less identifiers.


> > The reason I've asked this is that a reader that adds
> > `#%module-begin' will be broken if it's added by
> > `syntax/module-reader' unconditionally.
> 
> That is a potential problem.  Are there any languages that do that
> right now?

I don't know -- that's (part of) my question.

-- 
          ((lambda (x) (x x)) (lambda (x) (x x)))          Eli Barzilay:
                    http://barzilay.org/                   Maze is Life!