tcfail198.hs:6:36: error:
    • Couldn't match expected type ‘a1’ with actual type ‘a’
      ‘a1’ is a rigid type variable bound by
        an expression type signature:
          forall a1. a1
        at tcfail198.hs:6:41
      ‘a’ is a rigid type variable bound by
        the inferred type of f3 :: [a] -> [a]
        at tcfail198.hs:6:1-44
    • In the expression: x :: a
      In the second argument of ‘(++)’, namely ‘[x :: a]’
      In the expression: xs ++ [x :: a]
    • Relevant bindings include
        xs :: [a] (bound at tcfail198.hs:6:21)
        x :: a (bound at tcfail198.hs:6:19)
        f3 :: [a] -> [a] (bound at tcfail198.hs:6:6)