#4981: inconsistent class requirements with TypeFamilies and FlexibleContexts
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Reporter: ganesh | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 7.0.1
Keywords: | Testcase:
Blockedby: | Difficulty:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: GHC rejects valid
program
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Comment(by simonpj):
* Removing this silent superclass parameters is reasonable, and would
suppress this particular error, but the underlying problem would remain.
* Giving priority to equalities is also probably reasonable for
efficiency reasons, and would also suppress this error, but again it's not
a solution; for example, the equality in question might be somehow hidden
inside a superclass equality of a class constraint that arises from a use
of an instance...
* I think the real solution is to ''refrain from using an instance
declaration if there is an overlap with a Given''. This is what we do
already for overlapping instances (unless you use
`-XAllowIncoherentInstances`); we just need to extend it for givens.
I'll discuss this with D.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4981#comment:3>
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