Lastoid.hs

-- I think this is a a monoid. Basically it is used to either mappend to a value or replace the value
data Lastoid a = Replace a | Mappend a | Nope

instance Semigroup a => Semigroup (Lastoid a) where
  x <> y = case (x, y) of
    (r@Replace {}, _        ) -> r
    (Mappend a   , Replace b) -> Replace $ a <> b
    (Mappend a   , Mappend b) -> Mappend $ a <> b
    (Nope        , x        ) -> x

instance Monoid a => Monoid (Lastoid a) where
  mempty = Nope
  mappend = (<>)