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- // Copyright Louis Dionne 2013-2017
- // 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/ap.hpp>
- #include <boost/hana/assert.hpp>
- #include <boost/hana/bool.hpp>
- #include <boost/hana/config.hpp>
- #include <boost/hana/equal.hpp>
- #include <boost/hana/if.hpp>
- #include <boost/hana/lift.hpp>
- #include <boost/hana/optional.hpp>
- namespace hana = boost::hana;
- template <char op>
- constexpr auto function = hana::nothing;
- template <>
- BOOST_HANA_CONSTEXPR_LAMBDA auto function<'+'> = hana::just([](auto x, auto y) {
- return x + y;
- });
- template <>
- BOOST_HANA_CONSTEXPR_LAMBDA auto function<'-'> = hana::just([](auto x, auto y) {
- return x - y;
- });
- // and so on...
- template <char n>
- constexpr auto digit = hana::if_(hana::bool_c<(n >= '0' && n <= '9')>,
- hana::just(static_cast<int>(n - 48)),
- hana::nothing
- );
- template <char x, char op, char y>
- BOOST_HANA_CONSTEXPR_LAMBDA auto evaluate = hana::ap(function<op>, digit<x>, digit<y>);
- int main() {
- BOOST_HANA_CONSTEXPR_CHECK(evaluate<'1', '+', '2'> == hana::just(1 + 2));
- BOOST_HANA_CONSTEXPR_CHECK(evaluate<'4', '-', '2'> == hana::just(4 - 2));
- BOOST_HANA_CONSTANT_CHECK(evaluate<'?', '+', '2'> == hana::nothing);
- BOOST_HANA_CONSTANT_CHECK(evaluate<'1', '?', '2'> == hana::nothing);
- BOOST_HANA_CONSTANT_CHECK(evaluate<'1', '+', '?'> == hana::nothing);
- BOOST_HANA_CONSTANT_CHECK(evaluate<'?', '?', '?'> == hana::nothing);
- static_assert(hana::lift<hana::optional_tag>(123) == hana::just(123), "");
- }
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