On Sat, 28 Feb 2009, Matthew Flatt wrote:
At Sat, 28 Feb 2009 15:45:11 -0500 (EST), Dimitris Vyzovitis wrote:
Where foo is syntax-local-value looked up as a macro, and then called
on the syntax (directly, as a function without local expand).
The result is deconstructed as #'(pat ...)
That sounds like the problem. When you call a transformer directly,
then you're taking on the role of the macro expander, and you're
obligated to do all the same sort of things that the expander does.
If `local-expand' applies, but if you need to pull a syntax-object
vector apart, then my guess is that the deconstruction is still going
to cause certificate problems. You might be able to avoid the
certificate problem by expecting a `begin' form back instead of a
vector --- just because `local-expand' by default pushes certificates
down one level when it see a `begin' form.
The more general solution is to do what the macro expander does, which
is to record a certifier along with the transformer and use it on the
result, possibly use `syntax-recertify' on pieces that you pull apart,
and so on. You can see examples of this in `match' (look for
`match-expander-transform' in the implementation) and `for'.
Unfortunately, this is all a bit complex, but that's the trade-off in
implementing your own sub-expander.
This seems to work with minimum pain:
Expansion for splice with:
...
(let ((intro (make-syntax-introducer)))
(syntax-local-introduce
(intro (macro (intro (syntax-local-introduce stx))))))
Where the splicing macro is defined capturing the certifier:
...
(let (cert (syntax-local-certifier))
(lambda (x)
(syntax-case x ()
((_ arg ...)
(with-syntax (((body (... ...)) (list (cert (syntax rule)) ...)))
(syntax/loc x (body (... ...))))))))))
-- vyzo
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