customGenerator.cpp

// Simulate a network fetch generator, e.g. something that gets
// a paginated list of posts/values/etc, and yields one item at a time.
 
#include <array>
#include <iostream>
#include <z/all.hpp>
 
using std::optional;
using z::core::generator;
using z::system::sleep;
 
int main() {
    std::array<std::array<int, 5>, 3> data;
    data[0] = {0, 1, 2, 3, 4};
    data[1] = {5, 6, 7, 8, 9};
    data[2] = {10, 11, 12, 13, 14};
 
    // Note how initially we have to use std::array<int> as the data type,
    // because that's what the generator *actually* produces.
    // Afterwards, we can unchunk it.
    auto getNextPage = [&data](int &page) -> optional<std::array<int, 5>> {
        std::cout << "Fetching data... " << std::flush;
        // Simulate a network fetch.
        sleep(500);
 
        if (page >= data.size()) {
            std::cout << "No more data!" << std::endl;
            return {};
        }
 
        std::cout << " Done." << std::endl;
        return data[page++];
    };
 
    // Build the generator and unchunk it.
    // This generator yields one post/value at a time,
    // only fetching the next page when it needs to.
    auto myGenerator = generator<std::array<int, 5>, int>(0, getNextPage).flatten();
 
    for (auto value : myGenerator) {
        std::cout << value << std::endl;
    }
}