?
{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-}
class GEq a b where
geq :: a -> b -> Bool
instance {-# OVERLAPPABLE #-} (Eq a) => GEq a a where
geq = (==)
instance {-# OVERLAPPING #-} GEq a b where
geq _ _ = False
GHCi, version 8.0.2: http://www.haskell.org/ghc/ :? for help
Prelude> :load GEq.hs
[1 of 1] Compiling Main ( GEq.hs, interpreted )
Ok, modules loaded: Main.
*Main> geq () ()
<interactive>:2:1: error:
• Overlapping instances for GEq () () arising from a use of ‘geq’
Matching instances:
instance [overlapping] [safe] GEq a b -- Defined at GEq.hs:9:30
instance [overlappable] [safe] Eq a => GEq a a
-- Defined at GEq.hs:6:31
• In the expression: geq () ()
In an equation for ‘it’: it = geq () ()
*Main>
Trac metadata
Trac field | Value |
---|---|
Version | 8.0.2 |
Type | FeatureRequest |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |