#include <variant>
#include <memory>
#include <iostream>

struct Node;
struct Number { double value; };
struct BinaryOp {
    char op;
    std::unique_ptr<Node> left;
    std::unique_ptr<Node> right;
};
struct Node {
    std::variant<Number, BinaryOp> expr;
    double eval() const {
        struct Visitor {
            double operator()(const Number& n) const { return n.value; }
            double operator()(const BinaryOp& b) const {
                double l = b.left->eval();
                double r = b.right->eval();
                return b.op == '+' ? l + r : 0.0;
            }
        };
        return std::visit(Visitor{}, expr);
    }
};

int main() {
    // Build (1 + 2) + 3 = 6
    auto leaf1 = std::make_unique<Node>(Node{Number{1}});
    auto leaf2 = std::make_unique<Node>(Node{Number{2}});
    auto inner = std::make_unique<Node>(Node{BinaryOp{'+', std::move(leaf1), std::move(leaf2)}});
    auto root  = std::make_unique<Node>(Node{BinaryOp{'+', std::move(inner), std::make_unique<Node>(Node{Number{3}})}});
    std::println("result: {}", root->eval());
}