#6118: Kind variable falls out of scope in instance declaration
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Reporter: goldfire | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.5
Keywords: PolyKinds | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: None/Unknown
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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Changes (by simonpj):
* difficulty: => Unknown
Comment:
I'm on this. When fixed I get this message for the `Maybe` instance:
{{{
T6118.hs:20:3:
Variable occurs more often than in instance head
in the type family application: Demote k (Any k)
(Use -XUndecidableInstances to permit this)
}}}
This is because of the
[http://www.haskell.org/ghc/docs/latest/html/users_guide/type-
families.html#type-family-decidability conservative termination-checking
rules for type functions]. These rules could perhaps be loosened a bit to
account for kind polymorphism, but I'm not quite sure ''how'' they should
be loosened.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6118#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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