ExPatFail.hs:12:15: error:
    • Couldn't match expected type ‘p’ with actual type ‘a’
      ‘a’ is a rigid type variable bound by
        a pattern with constructor:
          MkT :: forall a. Integral a => a -> Int -> T,
        in a pattern binding
        at ExPatFail.hs:12:11-17
      ‘p’ is a rigid type variable bound by
        the inferred type of f :: T -> p
        at ExPatFail.hs:(12,1)-(13,10)
    • In the pattern: MkT y _
      In a pattern binding: MkT y _ = x
      In the expression: let MkT y _ = x in y
    • Relevant bindings include
        f :: T -> p (bound at ExPatFail.hs:12:1)