型クラスのインスタンスに型制約を指定したい時のGHC拡張

YesNo.hs

{-# LANGUAGE FlexibleInstances    #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE IncoherentInstances  #-}

class YesNo a where
  yesno :: a -> Bool

instance (Num a, Eq a) => YesNo a where
  yesno 0 = False
  yesno _ = True

instance YesNo [a] where
  yesno [] = False
  yesno _  = True

instance YesNo Bool where
  yesno = id

instance YesNo (Maybe a) where
  yesno (Just _) = True
  yesno Nothing  = False