yesno.hs

-- Javascript っぽい真偽値判定型を定義する
class YesNo a where
  yesno :: a -> Bool

instance YesNo Int where
  yesno 0 = False
  yesno _ = True

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

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

main = print $ yesno $ length []