problem.hs

data Free f a = Free (f (Free f a)) | Pure a
  deriving Functor

liftF :: Functor f => f a -> Free f a
liftF action = Free (fmap Pure action)

data Ex x = Ex x (Maybe x)
  deriving Functor

ex x y = liftF (Ex x y)

{-  Error:
    Occurs check: cannot construct the infinite type: a ~ Free Ex a
    Expected type: Maybe a
      Actual type: Maybe (Free Ex a)
    Relevant bindings include
      xs :: [a] (bound at src/Free.hs:17:12)
      x :: a (bound at src/Free.hs:17:10)
      problem :: [a] -> Free Ex a (bound at src/Free.hs:16:1)
    In the second argument of ‘($)’, namely ‘Just $ problem xs’
    In the expression: ex x $ Just $ problem xs
    In an equation for ‘problem’:
        problem (x : xs) = ex x $ Just $ problem xs
-}

problem (x:[]) = ex x Nothing
problem (x:xs) = ex x $ Just $ problem xs