hana/example/range/foldable/lazy_foldr.cpp

/*
@copyright Louis Dionne 2014
Distributed under the Boost Software License, Version 1.0.
(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
 */

#include <boost/hana/functional.hpp>
#include <boost/hana/integral.hpp>
#include <boost/hana/logical.hpp>
#include <boost/hana/range.hpp>

#include <cassert>
#include <sstream>
using namespace boost::hana;
using namespace literals;


int main() {
    //! [main]
    auto to_string = [](auto x) { return (std::ostringstream{} << x).str(); };

    auto up_to = [=](auto n) {
        return [=](auto x, auto y) {
            auto y_ = if_(x() == n, always("..."), y)();
            return "(" + to_string(x()) + " + " + to_string(y_) + ")";
        };
    };

    constexpr struct { } state{}; // this will never be evaluated
    assert(
        lazy_foldr(up_to(4_c), state, range(1_c, 999999_c)) == "(1 + (2 + (3 + (4 + ...))))"
    );
    //! [main]
}