{-# LANGUAGE ImpredicativeTypes #-}

-- Sept 16: now failing, because I've further reduced the scop
--          of impredicative types

module Base1 where
-- basic examples of impredicative instantiation of variables

data MEither a b = MLeft a
                 | MRight b
                 | MEmpty

type Sid = forall a. a -> a

-- no need for impredicativity
test0 = MRight id

-- requires impredicativity
test1 :: Sid -> MEither Sid b
test1 fid = MLeft fid

test2 :: MEither b Sid -> Maybe (Sid,Sid)
test2 m = case (test1 id) of
             MLeft x -> case m of
                          MRight y -> Just (x,y)
                          _ -> Nothing
             _ -> Nothing

test3 :: MEither a b -> b
test3 (MRight x) = x

test4 = test3 (test1 id)

