On Tue, 2004-08-31 at 10:00, Chung-chieh Shan wrote:
> The rationale for disallowing matching partially-applied type synonyms
> is that higher-order unification is undecidable.
Higher-order unification is worse than just undecidable (after all,
GHC's extended Haskell already includes constructs which are
undecidable, which means that sometimes the compiler will loop forever);
it's ambiguous. There can be multiple unifiers, none of which is the
most general. See my earlier Haskell-cafe message for the trouble this
can cause (search for "<technical note>"):
http://www.haskell.org/pipermail/haskell-cafe/2004-March/005965.html
Carl Witty
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