Transform a function from `R f(P ... ps)` to `std::optional<R> f(std::optional<P> ... ps)` and use it monadic style. needs C++17.

optional_monad.cpp

#include <iostream>
#include <optional>

/// library part

template <typename R, typename ... P>
auto lift_optional(R (*f)(P ... ps))
{
    return [f](std::optional<P> ... xs) -> std::optional<R> {
        if ((xs && ...)) {
            return {f(*(xs)...)};
        } else {
            return {};
        }
    };
}

template <typename T>
std::ostream& operator<<(std::ostream &os, const std::optional<T> &mt)
{
    if (mt) {
        return os << "Just " << *mt;
    } else {
        return os << "Nothing";
    }
}

/// user code

int f(int a, int b)
{
    return a + b;
}

int g(int a)
{
    return 2 * a;
}

auto mf = lift_optional(f);
auto mg = lift_optional(g);

int main()
{
    std::cout << mg(mf({1}, {})) << '\n';
    std::cout << mg(mf({1}, {2})) << '\n';
}