T9017.hs:8:7: error: [GHC-25897]
    • Couldn't match kind ‘k2’ with ‘*’
      When matching types
        a0 :: * -> * -> *
        a :: k1 -> k2 -> *
      Expected: a b (m b)
        Actual: a0 b0 (m0 b0)
      ‘k2’ is a rigid type variable bound by
        the type signature for:
          foo :: forall {k1} {k2} (a :: k1 -> k2 -> *) (b :: k1)
                        (m :: k1 -> k2).
                 a b (m b)
        at T9017.hs:7:1-16
    • In the expression: arr return
      In an equation for ‘foo’: foo = arr return
    • Relevant bindings include
        foo :: a b (m b) (bound at T9017.hs:8:1)