Intro to Functional Prog. by Bird, Section 2.5

[Shawn P. Garbett]

I've been working on understanding Haskell using hugs98 and playing with some 
of the examples in Bird's book.

In section 2.5, page 46 of the second edition, he gives an example of 
dependent types:

But I get this error I've been unable to solve:

----------------------------------------------------------------------------------------------------------------
--Type checking
--ERROR sec2-5.hs:8 - Type error in application
--*** Expression     : case' (Left' f,Right' g)
--*** Term           : (Left' f,Right' g)
--*** Type           : (Either' (d -> e) f,Either' g (h -> i))
--*** Does not match : (a -> b,c -> b)

data Either' a b =  Left' a | Right' b

case' :: (a->g, b->g) -> Either' a b -> g
case' (f, g) (Left' x) = f x
case' (f, g) (Right' y) = g y

plus' :: (a->b , g->d) -> Either' a g -> Either' b d
plus' (f,g) = case' (Left' f, Right' g)
-----------------------------------------------------------------------------

Please note that the version I've entered is correct by the errata published 
for this section.