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List:       haskell-cafe
Subject:    Re: [Haskell-cafe] Partially-applied type synonyms?
From:       Carl Witty <cwitty () newtonlabs ! com>
Date:       2004-08-31 18:40:09
Message-ID: 1093977608.6796.7.camel () flare
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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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