Question

I’m needing some help in figuring out how to create a leftSpine function in Haskell.
Basically, it is supposed to take all the left most leafs and put them into a list, but whenever I run my code I get an empty list. Any help would be appreciated.

Here is my code.

data Tree x = Leaf | Node (Tree x) x (Tree x)
    deriving Show

leftSpine :: Tree x -> [x]
leftSpine Leaf  = []
leftSpine (Node lt x rt) = (leftSpine lt)

And here is my code to test it.

leftSpine (Node (Node (Node Leaf 1 Leaf) 2 (Node Leaf 3 Leaf))
                4
                (Node (Node Leaf 5 Leaf) 6 (Node Leaf 7 Leaf)))

It should equal [4,2,1] but it just comes up as [].

Answer 1

leftSpine :: Tree x -> [x]
leftSpine   Leaf  = []
leftSpine   (Node lt x rt) =  x:leftSpine lt

You weren’t actually putting anything into the list. The difference is the x:leftSpine lt instead of leftSpine lt.