How to define listTree so that it returns a list containing all the elements in order?-CodePudding

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.

CodePudding user response：

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 = []