Pseudo closed type class

{-# LANGUAGE DataKinds, FlexibleContexts, FlexibleInstances, MultiParamTypeClasses
  , ScopedTypeVariables, TypeApplications, TypeFamilies, UndecidableInstances
#-}

module Overlap2 where

import Data.Monoid ((<>))

data ViewP (a :: ViewR) = ViewP

data ViewR = Default | Overload

type family ViewF a :: ViewR where
    ViewF (a, b) = 'Overload
    ViewF a = 'Default

class View a (r :: ViewR) where
    view' :: a -> ViewP r -> String

instance Show a => View a 'Default where
    view' = const . show

instance forall a b. (View a (ViewF a), View b (ViewF b)) => View (a, b) 'Overload where
    view' (x, y) _ = "(" <> view x <> ", " <> view y <> ")"

view :: forall a. (View a (ViewF a)) => a -> String
view x = view' x (ViewP @(ViewF a))