#7395: DefaultSignatures conflict with default implementations
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Reporter: cgaebel | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.6.1
Keywords: DefaultSignatures | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: GHC rejects valid program
Difficulty: Unknown | Testcase: http://hpaste.org/77290
Blockedby: | Blocking:
Related: |
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Comment(by tibbe):
A test case is linked under the "Test Case" attribute above. Here's the
same test case, inlined:
{{{
{-# LANGUAGE DefaultSignatures, TypeOperators, FlexibleContexts,
DeriveGeneric #-}
module Main ( main ) where
import GHC.Generics
class Klass a where
func :: a -> Int
func _ = 0
-- This implementation is incompatible with the one above. What should
-- happen instead is that this instance is selected whenever
availible,
-- and the one above should be used as a fallback.
default func :: (Generic a, GKlass (Rep a)) => a -> Int
func = gfunc . from
class GKlass f where
gfunc :: f a -> Int
instance GKlass U1 where
gfunc _ = 0
instance Klass a => GKlass (K1 i a) where
gfunc = (+1) . func . unK1
instance GKlass a => GKlass (M1 i c a) where
gfunc = gfunc . unM1
instance (GKlass a, GKlass b) => GKlass (a :*: b) where
gfunc (x :*: y) = 1 + gfunc x + gfunc y
instance (GKlass a, GKlass b) => GKlass (a :+: b) where
gfunc (L1 x) = 1 + gfunc x
gfunc (R1 x) = 1 + gfunc x
instance Klass Int where
func = id
data A = A { unA :: Int }
deriving Generic
-- A has a Generic instance. Therefore, should use gfunc.
instance Klass A
data B = B { unB :: Int }
-- B does not have a Generic instance. Therefore, should use func.
instance Klass B
main :: IO ()
main = print (func $ A 3, func $ B 3)
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7395#comment:2>
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