Bugs item #807249, was opened at 2003-09-16 16:37
Message generated for change (Comment added) made by simonmar
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Category: Compiler (Type checker)
Group: None
Status: Open
Resolution: None
Priority: 6
Submitted By: Simon Peyton Jones (simonpj)
Assigned to: Simon Peyton Jones (simonpj)
Summary: Instance match failure on openTypeKind
Initial Comment:
Consider
instance Show (a->b) where ...
foo x = show (\ _ -> True)
This fails with:
No instance for (Show (t -> Bool))
arising from use of `show' at Foo.hs:5
Reason: the type of (\_ -> True) is (t -> Bool) where
t has an "openTypeKind". It's possible that the function
will be applied to say an Int#, and the openTypeKind
records that this is OK.
BUT, the instance decl Show (a->b) has
a::liftedTypeKind, and that doesn't match an
openTypeKind type variable.
This bug relates to GHC's unsatisfactory treatment of
the variants of kind "type", for which there are at least 2
other SourceForge bugs registered (753780 and
753777). It's very obscure, so I'm not going to fix it
today.
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>Comment By: Simon Marlow (simonmar)
Date: 2005-07-11 10:36
Message:
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ghci015 now tests for this bug.
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Comment By: Simon Peyton Jones (simonpj)
Date: 2005-05-23 12:57
Message:
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user_id=50165
I'm bumping up the priority of this bug, because it also
happens if, in GHCi, you say
Prelude> :m +Text.Show.Functions
Text.Show.Functions> print (\x -> x)
(this elicits a no-such-instance error)
It's even more perplexing that this does not happen if you say
> print id
becuase 'id' has kind-defaulted type variables in its type.
Sigh.
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