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supki/eblo.hs

Created September 28, 2013 08:49
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eblo.hs
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE OverlappingInstances #-}
module Just where
class Just a b where
collapse :: Maybe a -> Maybe b
instance Just a a where
collapse = id
instance Just a b => Just (Maybe a) b where
collapse Nothing = Nothing
collapse (Just a) = collapse a
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