base.hs

{-# LANGUAGE ScopedTypeVariables, TypeFamilies, GADTs #-}
module ZZ where

import Data.Proxy

class Def a where
    def :: a

data TWrapper a b where
    TW :: (Foo a, Foo b, Base a ~ b) => Proxy a -> Maybe (Proxy b) -> TWrapper a b

class Def a => Foo a where
    type Base a :: *
    getFooWrapper :: Proxy a -> TWrapper a (Base a)
    setFooBase :: a -> Base a -> a

readBase :: forall a b. TWrapper a b -> a
readBase (TW pa pb) = case pb of
            Nothing -> def
            Just p -> let w = getFooWrapper p
                          d = (readBase w) :: b
                          v = setFooBase def d
                       in v