zz.hs

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

import Data.Proxy

class Def a where
    def :: a

class Def a => Foo a where
    type Base a :: *

class (Foo a, Foo (Base a)) => Foo' a  where
    getFooBase :: Proxy a -> Maybe (Proxy (Base a))
    setFooBase :: a -> Base a -> a

setBase :: forall a. Foo' a => a
setBase = case getFooBase (Proxy :: Proxy a) of
            Nothing -> def
            Just p -> let d = setBase
                          v = setFooBase def d
                       in v