At Mon, 27 Feb 2017 18:43:28 -0800, Matthew Butterick wrote:
> > On Feb 27, 2017, at 5:34 PM, gfb <g...@cs.toronto.edu> wrote:
> > If you want that to be equivalent to having `(require
> > math/number-theory)` at the top-level then I'm not sure the precise
> > semantics: should it affect the meaning of already-transformed code
> > from before that `lifted-require` was encountered?
> Currently, I'm doing the job by hand: that is, in the expansion of the
> `#%module-begin` of my #lang, I search the inbound parse tree and move the
> `require` forms to the top level. This works fine.
> So I suppose my question boils down to "is there something in the
> syntax-function zoo that does this more neatly?"
> As it stands, I think the answer is "no" because my manual technique is
> essentially pre-empting the macro expander.
The `syntax-local-lift-require` function works the way it does to avoid
the potential ambiguity Gary suggests, where adding a `require` from a
nested expression could create a binding that would affect expansion of
surrounding forms (but it's too late for that).
By adding the `require` in a fresh scope and then indirectly giving you
the scope, the effect of the `require` is consistently limited to
expressions that are afterwards given the new scope.
At Mon, 27 Feb 2017 17:34:53 -0800 (PST), gfb wrote:
> Recall that `requires` are noted before expansion of other forms,
> so, e.g., this works:
> #lang racket
> divides? ; Even though the require for this appears after
> (require math/number-theory)
Well, ok... there is some ambiguity here. It's not that `require`s are
noted before other expansion, but that module-level expansions are
partial until all the `defines` and `requires` are found. So `divides?`
is determined not to be a definition by partial expansion, and further
expansion is delayed under the assumption that `divides?` is an
expression form. If `divides?` turns out to be a definition form, it
will trigger an error. In contrast, if `divides?` is put after
`(require math/number-theory)`, then it's ok for `divides?` to expand
as a definition.
This partial-expansion idea has worked ok, and so we've put up with
this much order-sensitive behavior, but we try to minimize it.
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