mirror of
https://github.com/boostorg/hana.git
synced 2026-02-09 11:12:30 +00:00
48 lines
1.3 KiB
C++
48 lines
1.3 KiB
C++
/*
|
|
@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/bool.hpp>
|
|
#include <boost/hana/detail/assert.hpp>
|
|
#include <boost/hana/detail/constexpr.hpp>
|
|
#include <boost/hana/foreign.hpp>
|
|
#include <boost/hana/maybe.hpp>
|
|
using namespace boost::hana;
|
|
|
|
|
|
template <char op>
|
|
BOOST_HANA_CONSTEXPR_LAMBDA auto function = nothing;
|
|
|
|
template <>
|
|
BOOST_HANA_CONSTEXPR_LAMBDA auto function<'+'> = just([](auto x, auto y) {
|
|
return x + y;
|
|
});
|
|
|
|
template <>
|
|
BOOST_HANA_CONSTEXPR_LAMBDA auto function<'-'> = just([](auto x, auto y) {
|
|
return x - y;
|
|
});
|
|
|
|
// and so on...
|
|
|
|
template <char n>
|
|
BOOST_HANA_CONSTEXPR_LAMBDA auto digit = if_(bool_<(n >= '0' && n <= '9')>,
|
|
just(static_cast<int>(n - 48)), nothing
|
|
);
|
|
|
|
template <char x, char op, char y>
|
|
BOOST_HANA_CONSTEXPR_LAMBDA auto evaluate = ap(function<op>, digit<x>, digit<y>);
|
|
|
|
int main() {
|
|
BOOST_HANA_CONSTEXPR_ASSERT(evaluate<'1', '+', '2'> == just(1 + 2));
|
|
|
|
BOOST_HANA_CONSTANT_ASSERT(evaluate<'?', '+', '2'> == nothing);
|
|
BOOST_HANA_CONSTANT_ASSERT(evaluate<'1', '?', '2'> == nothing);
|
|
BOOST_HANA_CONSTANT_ASSERT(evaluate<'1', '+', '?'> == nothing);
|
|
BOOST_HANA_CONSTANT_ASSERT(evaluate<'?', '?', '?'> == nothing);
|
|
|
|
BOOST_HANA_CONSTEXPR_ASSERT(lift<Maybe>(123) == just(123));
|
|
}
|