[rurban]

I certainly won't use that. It's not type safe, and doesn't even allow names for its pattern matching sugar. Why he calls this simple struct matching sugar via tagged unions "Algebraic Data Types" is beyond my understanding. He cannot even do nested structs nor unions.

  datatype(
    BinaryTree,
    (Leaf, int),
    (Node, BinaryTree *, int, BinaryTree *)
  );

No names for the struct fields, so you need to rely on the position.

And then used:

    int sum(const BinaryTree *tree) {
    match(*tree) {
        of(Leaf, x) return *x;
        of(Node, lhs, x, rhs) return sum(*lhs) + *x + sum(*rhs);
    }

    // Invalid input (no such variant).
    return -1;
    } 

Where lhs, x, rhs magically match the types defined above. What a nonsense design!