pwm

  data Attr a = Leaf a | Node (AttrSet a)
    deriving stock (Show, Functor)

  newtype AttrSet a = AttrSet (HashMap Text (Attr a))
    deriving stock (Show, Functor)
    deriving newtype (Semigroup, Monoid)

  type Overlay a = AttrSet a -> AttrSet a -> AttrSet a

  apply :: forall a. [Overlay a] -> AttrSet a -> AttrSet a
  apply overlays attrSet = fix go
    where
      go :: AttrSet a -> AttrSet a
      go final =
        let fs = map (\overlay -> overlay final) overlays
        in foldr (\f as -> f as <> as) attrSet fs

  find :: AttrSet a -> Text -> Maybe (Attr a)
  find (AttrSet m) k = HMap.lookup k m

  set :: AttrSet a -> Text -> Attr a -> AttrSet a
  set (AttrSet m) k v = AttrSet $ HMap.insert k v m

  overlayed =
    apply
      [ \final prev -> set prev "a" $ maybe (Leaf 0) (fmap (* 2)) (find final "b"),
        \_final prev -> set prev "b" $ Leaf 2
      ]
      (AttrSet $ HMap.fromList [("a", Leaf 1), ("b", Leaf 1)])

  λ overlayed
  AttrSet (fromList [("a",Leaf 4),("b",Leaf 2)])

  data Thing
    = ThingA Int
    | ThingB String Bool
    | ThingC

  f :: Either Error Stuff
  f = do
    foo <- getFoo ...
    bar <- getBar ...
    ...

  > :t fmap
  fmap :: Functor f => (a -> b) -> f a -> f b

  > :t bimap
  bimap :: Bifunctor p => (a -> b) -> (c -> d) -> p a c -> p b d

  > :t bimap @(,)
  bimap @(,) :: (a -> b) -> (c -> d) -> (a, c) -> (b, d)