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50 lines
1.3 KiB
C++
50 lines
1.3 KiB
C++
/*
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@copyright Louis Dionne 2014
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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*/
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#include <boost/hana/assert.hpp>
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#include <boost/hana/bool.hpp>
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#include <boost/hana/detail/constexpr.hpp>
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#include <boost/hana/maybe.hpp>
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using namespace boost::hana;
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//! [applicative]
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template <char op>
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constexpr auto function = nothing;
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template <>
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BOOST_HANA_CONSTEXPR_LAMBDA auto function<'+'> = just([](auto x, auto y) {
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return x + y;
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});
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template <>
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BOOST_HANA_CONSTEXPR_LAMBDA auto function<'-'> = just([](auto x, auto y) {
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return x - y;
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});
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// and so on...
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template <char n>
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BOOST_HANA_CONSTEXPR_LAMBDA auto digit = if_(bool_<(n >= '0' && n <= '9')>,
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just(static_cast<int>(n - 48)),
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nothing
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);
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template <char x, char op, char y>
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BOOST_HANA_CONSTEXPR_LAMBDA auto evaluate = ap(function<op>, digit<x>, digit<y>);
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int main() {
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BOOST_HANA_CONSTEXPR_CHECK(evaluate<'1', '+', '2'> == just(1 + 2));
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BOOST_HANA_CONSTANT_CHECK(evaluate<'?', '+', '2'> == nothing);
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BOOST_HANA_CONSTANT_CHECK(evaluate<'1', '?', '2'> == nothing);
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BOOST_HANA_CONSTANT_CHECK(evaluate<'1', '+', '?'> == nothing);
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BOOST_HANA_CONSTANT_CHECK(evaluate<'?', '?', '?'> == nothing);
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BOOST_HANA_CONSTEXPR_CHECK(lift<Maybe>(123) == just(123));
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}
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//! [applicative]
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