Lens traversals

traversals.hs

import Control.Lens
import Control.Applicative

data T a =
  T (Maybe a)
  deriving (Eq, Show)

traverseT ::
  Traversal' (T a) a
traverseT _ (T Nothing) =
  pure (T Nothing)
traverseT f (T (Just a)) =
  T . Just <$> f a
-- \f (T a) -> T <$> traverse f a

data U a =
  U (Maybe a) [a] Int
  deriving (Eq, Show)

traverseU ::
  Traversal' (U a) a
traverseU f (U a as i) =
  U <$> traverse f a <*> traverse f as <*> pure i

data V a =
  V Int a
  deriving (Eq, Show)

traverseV ::
  Traversal' (V a) Int
traverseV f (V n a) =
  V <$> f n <*> pure a