#2307: Poor warning for conflicting functional dependencies
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Reporter: NeilMitchell | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 6.10 branch
Component: Compiler | Version: 6.8.2
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Testcase: | Architecture: Unknown
Os: Unknown |
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Changes (by igloo):
* difficulty: => Unknown
* milestone: => 6.10 branch
Comment:
Thanks for the report! Here is a minimal testcase:
{{{
{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies,
OverlappingInstances, UndecidableInstances,
IncoherentInstances,
FlexibleInstances #-}
module Foo where
class C a b c | a -> b, a -> c
instance C Int (Maybe String) Float
instance C Int (Maybe Bool) Double
}}}
With the HEAD:
{{{
Proof_default.hs:8:0:
Functional dependencies conflict between instance declarations:
instance [incoherent] C Int (Maybe String) Float
-- Defined at Proof_default.hs:8:0-34
instance [incoherent] C Int (Maybe Bool) Double
-- Defined at Proof_default.hs:9:0-35
instance [incoherent] C Int (Maybe Bool) Double
-- Defined at Proof_default.hs:9:0-35
}}}
Before just nubbing the instances, we should make sure that it's not a bug
that it is finding the same one multiple times in the first place.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2307#comment:1>
GHC <http://www.haskell.org/ghc/>
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