#include <vector>

#include <iostream>

#include <string>

#include <charconv>

#include <optional>

#include <ranges>

using namespace std;

using namespace literals;

template <typename T>

using vector_ex = vector<optional<T>>;

auto to_int(const string_view s) -> optional<int>

{

if (int r; from_chars(s.data(), s.data()+s.size(), r).ec == errc{})

return r;

return nullopt;

}

int main(int argc, char* argv[])

{

vector_ex<string> v = { "1234", "15 foo", "bar", "42", "5000", " 5" };

// NOTICE

// transform == fmap (map)

// and_then == bind (flatmap)

auto filter = [](auto&& o) { // optional<string>,

return o.and_then(to_int) // bind (or flatmap) from str to int

.transform([](auto n) { return n + 1; })

.transform([](auto n) { return to_string(n); })

.or_else ([] { return optional("null"s); });

};

for (auto&& x : v | views::transform(filter))

cout << *x << '\n';

return 0;

}