Haskell-book/15/optional/src/Optional.hs


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module Optional where

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

instance Monoid a => Monoid (Optional a) where
    mempty = Nada
    mappend x Nada = x
    mappend Nada x = x
    mappend (Only x) (Only y) = Only (mappend x y)