hana/example/lazy.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/assert.hpp>
#include <boost/hana/bool.hpp>
#include <boost/hana/detail/constexpr.hpp>
#include <boost/hana/lazy.hpp>
using namespace boost::hana;


int main() {

{

//! [lazy]
BOOST_HANA_CONSTEXPR_LAMBDA auto f = lazy([](auto x) {
    return 1 / x;
});
BOOST_HANA_CONSTEXPR_LAMBDA auto g = lazy([](auto x) {
    return x + 1;
});

BOOST_HANA_CONSTEXPR_CHECK(eval(if_(false_, f(0), g(0))) == 0 + 1);
//! [lazy]

}{

//! [functor]
BOOST_HANA_CONSTEXPR_LAMBDA auto double_ = [](auto x) {
    return x * 2;
};

BOOST_HANA_CONSTEXPR_LAMBDA auto one_over = [](auto x) {
    return 1 / x;
};

BOOST_HANA_CONSTEXPR_CHECK(eval(fmap(lazy(one_over)(4), double_)) == 1 / 2);

fmap(lazy(one_over)(0), double_); // never evaluated
//! [functor]

}

}