{-# LANGUAGE TypeFamilies, UndecidableSuperClasses, UndecidableInstances #-}

module BidirectionalInstances where

import Data.Constraint

class Evidence a => Foo a where
  type Evidence a :: Constraint
  type Evidence a = ()


instance Foo Bool
instance Foo Int


instance Evidence (a, b) => Foo (a, b) where
  type Evidence (a, b) = (Foo a, Foo b)
  
  
forwards
  :: Dict (Foo a)
  -> Dict (Foo b)
  -> Dict (Foo (a, b))
forwards Dict Dict = Dict


backwards
  :: Dict (Foo (a, b))
  -> (Dict (Foo a), Dict (Foo b))
backwards Dict = (Dict, Dict)