listTree :: Tree a -> [a]
        listTree = foldTree f z
            where
            f x y z  = x + y + z
            z = []

This is what I have so far, but my f is wrong because the expected type is Tree a -> [a] but the actual is Tree [a] -> [a]

        data Tree a
           = Tip
           | Bin (Tree a) a (Tree a)
           deriving (Show, Eq)


        foldTree :: (b -> a -> b -> b) -> b -> Tree a -> b
        foldTree f z Tip         = z
        foldTree f z (Bin l x r) = f (foldTree f z l) x (foldTree f z r)

This is fold and data type for tree

Need help with defining f.

>Solution :

First of all, combining lists uses ++ in haskell. + is only used for adding numbers. Second, the variable y is of type a, not [a], so you cannot use ++ on it directly.

listTree :: Tree a -> [a]
listTree = foldTree f z
    where
    f x y z = x ++ [y] ++ z
    z = []