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2c86ba2390
- Rename fold{l,r,l1,r1} to fold.{right,left} with overloads.
- Rename foldlM/foldrM to monadic_fold.{left,right}
- Add no-state variants of the monadic folds
- Improve the fold's documentation
- Deprecate the previous folds; they'll be removed soon
Fixes #18.
63 lines
1.6 KiB
C++
63 lines
1.6 KiB
C++
/*
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@copyright Louis Dionne 2015
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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/ext/std/utility.hpp>
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#include <boost/hana/maybe.hpp>
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#include <boost/hana/tuple.hpp>
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#include <boost/hana/type.hpp>
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#include <type_traits>
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using namespace boost::hana;
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//! [monadic_fold.left]
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auto builtin_common_t = sfinae([](auto t, auto u) -> decltype(type<
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std::decay_t<decltype(true ? traits::declval(t) : traits::declval(u))>
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>) { return {}; });
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template <typename ...T>
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struct common_type { };
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template <typename T, typename U>
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struct common_type<T, U>
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: std::conditional_t<std::is_same<std::decay_t<T>, T>{} &&
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std::is_same<std::decay_t<U>, U>{},
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decltype(builtin_common_t(type<T>, type<U>)),
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common_type<std::decay_t<T>, std::decay_t<U>>
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>
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{ };
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template <typename T1, typename ...Tn>
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struct common_type<T1, Tn...>
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: decltype(monadic_fold<Maybe>.left(
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tuple_t<Tn...>,
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type<std::decay_t<T1>>,
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sfinae(metafunction<common_type>)
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))
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{ };
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template <typename ...Ts>
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using common_type_t = typename common_type<Ts...>::type;
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static_assert(std::is_same<
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common_type_t<char, short, char, short>, int>{}
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, "");
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static_assert(std::is_same<
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common_type_t<char, double, short, char, short, double>, double>{}
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, "");
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static_assert(std::is_same<
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common_type_t<char, short, float, short>, float
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>{}, "");
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static_assert(
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sfinae(metafunction<common_type>)(type<int>, type<int>, type<int*>) == nothing
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, "");
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//! [monadic_fold.left]
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int main() { }
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