alternative.cpp

#include <iostream>
#include <optional>
#include <vector>
using namespace std;

template<template<typename A> typename F>
struct Alternative {

  template<typename A>
  static F<A> empty();

  template<typename A>
  static F<A> alt(F<A> &&, F<A> &&);

};

template<>
struct Alternative<std::optional> {

  template<typename A>
  static std::optional<A> empty() {
    return std::nullopt;
  }

  template<typename A>
  static std::optional<A> alt(std::optional<A> &&x, std::optional<A> &&y) {
    return x.has_value() ? x : y;
  }

};

template<template<typename A> typename F, typename A>
F<A> asum(std::vector<F<A>> fs) {
  F<A> result;
  for (auto &&f : fs) {
    result = Alternative<F>::alt(std::move(result), std::move(f));
  }
  return result;
}

int main() {
  std::vector<std::optional<int>> options {
    std::nullopt,
    std::make_optional(1),
    std::make_optional(2),
  };
  std::cout << asum<std::optional, int>(options).value() << '\n';
}