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48 lines
1.3 KiB
C++
48 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/bool.hpp>
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#include <boost/hana/detail/assert.hpp>
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#include <boost/hana/either.hpp>
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#include <boost/hana/foreign.hpp>
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#include <string>
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using namespace boost::hana;
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template <char op>
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auto function = left(op + std::string{" is not a valid operation"});
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template <>
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auto function<'+'> = right([](auto x, auto y) {
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return x + y;
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});
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template <>
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auto function<'-'> = right([](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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auto digit = if_(bool_<(n >= '0' && n <= '9')>,
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right(static_cast<int>(n - 48)),
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left(n + std::string{" is not a valid digit"})
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);
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template <char x, char op, char y>
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auto evaluate = ap(function<op>, digit<x>, digit<y>);
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int main() {
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BOOST_HANA_RUNTIME_ASSERT(evaluate<'1', '+', '2'> == right(1 + 2));
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BOOST_HANA_RUNTIME_ASSERT(evaluate<'?', '+', '2'> == left("? is not a valid digit"));
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BOOST_HANA_RUNTIME_ASSERT(evaluate<'1', '?', '2'> == left("? is not a valid operation"));
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BOOST_HANA_RUNTIME_ASSERT(evaluate<'1', '+', '?'> == left("? is not a valid digit"));
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BOOST_HANA_RUNTIME_ASSERT(evaluate<'?', '?', '?'> == left("? is not a valid operation"));
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}
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