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{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE UndecidableInstances #-}
-- Provided by Christian Maeder; broke
-- a pre-release GHC 7.0
module HasKey where
class Ord key => HasKey x key | x -> key where
toKey :: x -> key
newtype Keyed x = Keyed { unKey :: x }
lift :: (HasKey x1 key1,HasKey x2 key2)
=> (key1 -> key2 -> a) -> (Keyed x1 -> Keyed x2 -> a)
lift f x1 x2 = f (toKey . unKey $ x1) (toKey . unKey $ x2)
instance HasKey x key => Eq (Keyed x) where
(==) = lift (==)
instance HasKey x key => Ord (Keyed x)
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