[plt-dev] Hunting down an elusive certification bug

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.