ch10_10.1-i.hs


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{-# LANGUAGE FunctionalDependencies #-}

-- Exercise 10.1-i
-- Defunctionalize `listToMaybe :: [a] -> Maybe a`.
newtype ListToMaybe a = ListToMaybe [a]

class Eval l t | l -> t where
  eval :: l -> t

instance Eval (ListToMaybe a) (Maybe a) where
  eval (ListToMaybe []) = Nothing
  eval (ListToMaybe (x : _)) = Just x