ceph/src/boost/libs/hana/example/optional/applicative.complex.cpp

   1 // Copyright Louis Dionne 2013-2016
   2 // Distributed under the Boost Software License, Version 1.0.
   3 // (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
   4
   5 #include <boost/hana/ap.hpp>
   6 #include <boost/hana/assert.hpp>
   7 #include <boost/hana/bool.hpp>
   8 #include <boost/hana/config.hpp>
   9 #include <boost/hana/equal.hpp>
  10 #include <boost/hana/if.hpp>
  11 #include <boost/hana/lift.hpp>
  12 #include <boost/hana/optional.hpp>
  13 namespace hana = boost::hana;
  14
  15
  16 template <char op>
  17 constexpr auto function = hana::nothing;
  18
  19 template <>
  20 BOOST_HANA_CONSTEXPR_LAMBDA auto function<'+'> = hana::just([](auto x, auto y) {
  21     return x + y;
  22 });
  23
  24 template <>
  25 BOOST_HANA_CONSTEXPR_LAMBDA auto function<'-'> = hana::just([](auto x, auto y) {
  26     return x - y;
  27 });
  28
  29 // and so on...
  30
  31 template <char n>
  32 constexpr auto digit = hana::if_(hana::bool_c<(n >= '0' && n <= '9')>,
  33     hana::just(static_cast<int>(n - 48)),
  34     hana::nothing
  35 );
  36
  37 template <char x, char op, char y>
  38 BOOST_HANA_CONSTEXPR_LAMBDA auto evaluate = hana::ap(function<op>, digit<x>, digit<y>);
  39
  40 int main() {
  41     BOOST_HANA_CONSTEXPR_CHECK(evaluate<'1', '+', '2'> == hana::just(1 + 2));
  42
  43     BOOST_HANA_CONSTANT_CHECK(evaluate<'?', '+', '2'> == hana::nothing);
  44     BOOST_HANA_CONSTANT_CHECK(evaluate<'1', '?', '2'> == hana::nothing);
  45     BOOST_HANA_CONSTANT_CHECK(evaluate<'1', '+', '?'> == hana::nothing);
  46     BOOST_HANA_CONSTANT_CHECK(evaluate<'?', '?', '?'> == hana::nothing);
  47
  48     static_assert(hana::lift<hana::optional_tag>(123) == hana::just(123), "");
  49 }