#1897: Ambiguous types and rejected type signatures
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Reporter: guest | Owner: chak
Type: bug | Status: reopened
Priority: normal | Milestone: 6.12 branch
Component: Compiler (Type checker) | Version: 6.9
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Testcase: | Os: Linux
Architecture: x86 |
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Comment (by simonpj):
Yet another (killer) type-family example:
{{{
class Key k where
type Map k :: * -> *
empty :: Map k v
instance Key a => Key (Maybe a) where
type Map (Maybe a) = MM a
empty = MM Nothing empty
data MM a v = MM (Maybe v) (Map a v)
}}}
where we get
{{{
Map.hs:27:21:
Couldn't match expected type `Map a' against inferred type `Map k'
In the second argument of `MM', namely `empty'
In the expression: MM Nothing empty
In the definition of `empty': empty = MM Nothing empty
}}}
The terrible thing is that no amount of type signatures will fix this!
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1897#comment:16>
GHC <http://www.haskell.org/ghc/>
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