[racket-dev] Trouble with state, submodules, and module-begin

From: Matthew Flatt (mflatt at cs.utah.edu)
Date: Mon Jul 2 09:07:36 EDT 2012

Thoughts so far:

I think you need a new communication channel to get information from
the expansion of an enclosing module to the expansion of its submodule.

Expansion-time state is the right kind of channel, but I think it's
important to start every submodule's expansion in a fresh store, at
least usually. Otherwise, many syntactic extensions won't work
correctly in a submodule.

To give the programmer more control, we could add a
`local-expand-submodule' function that is like `local-expand', but (1)
it works only for `module' and `module*' forms in a 'module context,
and (2) it accepts module paths to attach from the current expansion
context to the submodule's expansion context.

Using this addition, when expanding a `(module* name #f ....)'
submodule, Typed Racket could attach a compile-time module that houses
the "in a typed context" flag --- the same one that Typed Racket's
`#%module-begin' sets. Does it sound like that would work?

I worry that `local-expand-submodule' might be used to break a
syntactic form by attaching a module that isn't intended to be attached
to multiple expansion stores. I think a macro that abuses
`local-expand-submodule' could only harm itself or some other module
that trusts the macro, but it's difficult to be sure.

At Mon, 25 Jun 2012 16:47:36 -0600, Matthew Flatt wrote:
> At Mon, 25 Jun 2012 17:50:27 -0400, Sam Tobin-Hochstadt wrote:
> > The problem (I
> > think) is that the implicit `require` of `(submod "..")` happens
> > *before* the expansion of `#%module-begin` inside the submodule.
> That's the same for a top-level module M whose initial language is some
> other module L, right? The require of `L' happens before the
> `#%module-begin' expansion in `M'... and it can't be any other way,
> because `#%module-begin' comes from `L'.
> > The
> > key bit of code is the residual snippet left in the outer module:
> > 
> >          (begin-for-syntax
> >              (when (unbox is-typed?)
> >                (set-box! type-env 1)))
> > 
> > Currently, in TR, the code in the begin-for-syntax is unconditional,
> > and therefore it gets re-run in the store used for expanding the inner
> > submodule.  However, if I add the `when`, then the `set-box!` doesn't
> > happen, and the expansion of `m` fails.  I'd like to be able to add
> > this conditional, so I'd like to change the order of effects slightly
> > here.
> I see what you mean, but I don't think it makes sense to change the
> order of things in the way that you're suggesting. I don't have any
> immediate ideas, but I'll think about it more.
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