[Haskell-cafe] Partially-applied type synonyms?

Carl Witty cwitty at newtonlabs.com
Tue Aug 31 14:40:09 EDT 2004

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>"):


Carl Witty

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