Simple Question Again

[Jerry]

Hi, I'm sorry to bother everyone again with this simple append' stuff

-- below is my revised append' function 

1) append' :: [[a]] -> a -> [[a]]
2) append' [] y = [[y]]
3) append' (x:xs) y =
4)     case xs of [] -> foldr (:) [y] x
5)                (z:zs) -> (init (x:xs)) ++ [(last xs)++[y]]

-- to achieve

append' [] 1 = [[1]]
append' [[1]] 2 = [[1, 2]]
append' [[1], [2]] 3 = [[1], [2, 3]]

-- and ghc gives the following compile error:

p3a.hs:4:
    Cannot unify the type-signature variable `a' with the type `[a]'
        Expected type: [a]
        Inferred type: a
    In the list element: y
    In the second argument of `foldr', namely `([y])'
make: *** [p3a] Error 1

-- now this is something I _really_ don't understand:
-- x is of type [a], [y] is of type [a], and isn't foldr (:) [a] [a]
-- perfectly valid?!

Much thanx in advance

--
Regards,
Jerry