Challenge 3: A Recursive Tree With a Summing Function — Possible Solution
====================================================================

Main.hs:

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

treeSum :: Tree Int -> Int
treeSum Leaf = 0                                    -- base case
treeSum (Node left value right) = treeSum left + value + treeSum right  -- recursive case

sampleTree :: Tree Int
sampleTree = Node
               (Node Leaf 1 Leaf)
               2
               (Node (Node Leaf 3 Leaf) 4 Leaf)

main :: IO ()
main = print (treeSum sampleTree)

Output:

    10

Explanation:
Tree a is genuinely recursive -- a Node holds a value AND two more
Trees (its left and right children), which can themselves be Nodes or
Leafs. treeSum mirrors this shape exactly: the base case handles Leaf
(an empty subtree contributes 0), and the recursive case handles Node
by summing the left subtree, adding the node's own value, and adding
the right subtree -- calling itself on both children. sampleTree
contains four real values (1, 2, 3, 4), and treeSum correctly adds
them all to 10, regardless of how deeply nested any particular value
is.

WHY THIS WORKS AS AN ANSWER
------------------------------
This defines the exact recursive Tree a shape the chapter introduces
and writes a properly recursive treeSum function matching its own
recursive structure, tested on a tree with genuine nesting (not just
a flat list of children) to confirm the recursion actually traverses
correctly.