How to implement fold right for Tree in purescript

Question

I am new to Purescript so I was trying to implement fold left and fold right (aka reduce) for my custom Tree data structure, but I am having problem implementing logic for fold right:

data Tree a = Leaf a | Node (Tree a) (Tree a)

instance foldableTree :: Foldable Tree where
    foldl f d (Node left right) = foldl f (foldl f d left) right
    foldl f d (Leaf a) = f (d) (a)
    foldr f d (Node left right) = ???
    foldr f d (Leaf a) = ???

I only got so far with implementation:

instance foldableTree :: Foldable Tree where
    foldl f d (Node left right) = foldl f (foldl f d left) right
    foldl f d (Leaf a) = f (d) (a)
    foldr f d (Node left right) = f (foldr f d left) (foldr f d right)
    foldr f d (Leaf a) = a

But this implementation is flawed because deafult value is not passed to the fold

Answer 1

I solved it, I only needed to use same logic as on fold left but with swapped arguments for Leaf and for the recursive function "foldr" by first providing right side then left side:

instance foldableTree :: Foldable Tree where
    foldl f d (Node left right) = foldl f (foldl f d left) right
    foldl f d (Leaf a) = f d a
    foldr f d (Node left right) = foldr f (foldr f d right) left
    foldr f d (Leaf a) = f a d