module MonoidSemigroup where

data Optional a =
    Nada
  | Only a
  deriving (Eq, Show)

instance Semigroup a => Semigroup (Optional a) where
  Only a <> Nada = Only a
  Nada <> Only a = Only a
  Only a <> Only b = Only (a <> b)

instance Semigroup a => Monoid (Optional a) where
  mempty = Nada