ForceElems.hs

-- Solution to challenge: https://github.com/effectfully/haskell-challenges/tree/master/force-elems
-- (Spoiler)

data EC = Elem | Con
data T a = T {ec :: !EC, unT :: a}

appEC :: EC -> (a -> b) -> a -> b
appEC Elem f a = f $! a
appEC Con  f a = f a

instance Functor T where
  fmap f ta = T Con $ appEC (ec ta) f (unT ta)

instance Applicative T where
  pure    = T Con
  f <*> a = T Con $ appEC (ec a) (unT f) (unT a)

forceElems :: Traversable t => t a -> t a
forceElems = unT . traverse (T Elem)