#179: Instance match failure on openTypeKind
--------------------------------------+-------------------------------------
Reporter: simonpj | Owner: simonpj
Type: bug | Status: assigned
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 6.4.1
Severity: normal | Resolution: None
Keywords: | Os: Unknown
Difficulty: Unknown | Architecture: Unknown
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Changes (by simonmar):
* architecture: => Unknown
* difficulty: => Unknown
* version: None => 6.4.1
* os: => Unknown
Old description:
> {{{
> 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.
> }}}
New description:
{{{
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.
}}}
--
Ticket URL: <http://cvs.haskell.org/trac/ghc/ticket/179>
GHC <http://www.haskell.org/ghc/>
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