#3851: type family does not get expanded in ghc-6.12.1
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Reporter: kosmikus | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 6.12.1
Keywords: | Difficulty:
Os: Unknown/Multiple | Testcase:
Architecture: Unknown/Multiple | Failure: GHC rejects valid
program
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Comment(by simonpj):
It's because of an awkward interaction. When matching a GADT, GHC
currently requires the context of the pattern to be "rigid", so that it
knows exactly what type refinement happens as a result of the match.
Well, is a type-function application rigid? Presumably not in 6.12. I'm
not sure why it was considered rigid in 6.10.
All this grunge about rigidity will be swept away with the new outside-in
type inference machinery (see our ICFP'09 paper), so I'm quite reluctant
to try to "fix" this. Can you work around it for now, perhaps by using
less perspicuous type signatures?
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3851#comment:1>
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