tcfail099.hs:9:20: error:
    • Couldn't match expected type ‘a’ with actual type ‘p’
      ‘a’ is a rigid type variable bound by
        a pattern with constructor: C :: forall a. (a -> Int) -> DS,
        in an equation for ‘call’
        at tcfail099.hs:9:7-9
      ‘p’ is a rigid type variable bound by
        the inferred type of call :: DS -> p -> Int
        at tcfail099.hs:9:1-22
    • In the first argument of ‘f’, namely ‘arg’
      In the expression: f arg
      In an equation for ‘call’: call (C f) arg = f arg
    • Relevant bindings include
        arg :: p (bound at tcfail099.hs:9:12)
        f :: a -> Int (bound at tcfail099.hs:9:9)
        call :: DS -> p -> Int (bound at tcfail099.hs:9:1)