error.txt

[1 of 1] Compiling ZZ               ( hang.hs, hang.o )

hang.hs:19:31:
    Could not deduce (Foo' (Base a)) arising from a use of ‘setBase’
    from the context (Foo' a)
      bound by the type signature for setBase :: Foo' a => a
      at hang.hs:16:12-32
    In the expression: setBase
    In an equation for ‘d’: d = setBase
    In the expression:
      let
        d = setBase
        v = setFooBase def d
      in v

zz.h

{-# 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 :: *
    getFooBase :: Proxy a -> Maybe (Proxy (Base a))
    setFooBase :: a -> Base a -> a

class (Foo a, Foo (Base a)) => Foo' 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