It's true that `internal-definition-context-binding-identifier` is unusual
in not applying `syntax-local-introduce` to the identifiers it returns, but
that doesn't capture the whole problem. Let's imagine we made things
consistent by mapping `syntax-local-introduce` over the returned names.
Let's also, though, bind an identifier that has an extra scope that should
make it distinct from the `var` we look up:
#lang racket
(define-for-syntax (sanity-check var)
(define ctx (syntax-local-make-definition-context))
(syntax-local-bind-syntaxes (list ((make-syntax-introducer) var)) #f ctx)
(cond
; should be non-#f; we just bound it!
[(member (internal-definition-context-introduce ctx var)
(map syntax-local-introduce
(internal-definition-context-binding-identifiers ctx))
free-identifier=?)
'ok]
[else
(error "failed check")]))
(define-syntax (m1 stx)
#`'#,(sanity-check #'test))
(m1)
We find that they're `free-identifier=?`, even though we added the extra
scope to the binder!
The problem here is that neither `(internal-definition-context-introduce
ctx var)` or the identifiers in `(map syntax-local-introduce
(internal-definition-context-binding-identifiers ctx))` now have the
introduction scope, because we've removed it to match the "negative space"
within the macro where syntax without an introduction scope should be
considered introduced, rather than the "postive space" outside of the macro
where the introduction scope means it the syntax was introduced. So when
`free-identifier=?` resolves each of the two references, it finds them both
unbound. And unbound references with the same symbol are
`free-identifier=?`.
To get the right answer, you need to ask `free-identifier=?` of identifiers
in "positive space". In this case you should write:
(member (syntax-local-introduce (internal-definition-context-introduce ctx
var))
(internal-definition-context-binding-identifiers ctx)
free-identifier=?)
On Tuesday, July 24, 2018 at 8:32:56 AM UTC-4, Matthew Flatt wrote:
>
> At Tue, 24 Jul 2018 00:47:19 -0700 (PDT), Milo Turner wrote:
> > Hello Racketeers.
> >
> > I'm having trouble understanding why this free-identifier=? check is
> > failing: [...]
> >
> > This suggests that internal definition contexts are aware of the macro
> > introduction scope in some way. The documentation doesn't mention this
> sort
> > of thing, so I'm wondering if there is some explanation I'm missing?
>
> I think it's a documentation issue, or perhaps a design issue with
> `internal-definition-context-binding-identifiers`.
>
> Various operations implicitly use `syntax-local-introduce`, and I have
> never been clear on when/where that needs to be specified. For example,
> `local-expand` implicitly uses `syntax-local-introduce` on its
> argument, because that's going back out of the current macro in a
> sense, and it uses `syntax-local-introduce` again on the result. The
> implicit uses of `syntax-local-introduce` make things "just work", and
> so the uses are normally not mentioned in the documentation.
>
> Occasionally, however, there's a mismatch. In this case,
> `syntax-local-bind-syntaxes` implicitly uses `syntax-local-introduce`
> on its argument, but `internal-definition-context-binding-identifiers`
> doesn't use `syntax-local-introduce` on its result. I think that's the
> mismatch you're seeing.
>
> Overall, the documentation should be improved/corrected to mention the
> implicit `syntax-local-introduce`s, and then exceptions like
> `internal-definition-context-binding-identifiers` can be highlighted.
>
>
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