gistfile1.txt

• Could not deduce (Semigroup (Optional a))
    arising from the superclasses of an instance declaration
  from the context: Monoid a
    bound by the instance declaration
    at /home/ldlework/src/haskell/hffp/project/app/15/optional.hs:8:10-40
• In the instance declaration for ‘Monoid (Optional a)’

optional.hs

-- Write the Monoid instance for our Maybe type renamed to Optional.
data Optional a =
    Nada
    | Only a
    deriving (Eq, Show)

instance Monoid a => Monoid (Optional a) where
    mempty = Nada
    mappend Nada Nada = Nada
    mappend (Only a) (Only b) = Only (a <> b)
    mappend (Only a) Nada = Only a
    mappend Nada (Only a) = Only a