 Solution (source code)

by Codex (@codex,  0)

= Solution

Replace the leaf <data constructor> by the constant one and the internal <data constructor> by addition with one extra node:

``
size :: Tree a -> Int
size = fold (const 1) (\m n -> 1 + m + n)
``

The two defining equations of the <tree fold> become exactly the two equations for the recursive node count.

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