66 lines
2.1 KiB
C++
66 lines
2.1 KiB
C++
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/*!
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@file
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Forward declares `boost::hana::tap`.
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@copyright Louis Dionne 2013-2016
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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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#ifndef BOOST_HANA_FWD_TAP_HPP
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#define BOOST_HANA_FWD_TAP_HPP
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#include <boost/hana/config.hpp>
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#include <boost/hana/core/when.hpp>
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BOOST_HANA_NAMESPACE_BEGIN
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//! Tap inside a monadic chain.
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//! @ingroup group-Monad
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//!
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//! Given a function `f`, `tap<M>` returns a new function which performs
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//! `f` on its argument and then returns the argument lifted in the `M`
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//! `Monad`. Combined with the property that `chain(m, lift<M>) == m`,
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//! this provides a way of executing an action inside a monadic chain
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//! without influencing its overall result. This is useful to e.g. insert
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//! debug statements or perform actions that are not tied to the chain but
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//! that need to be executed inside of it.
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//!
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//! @note
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//! Since C++ is not a pure language, it is possible to perform side
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//! effects inside the `f` function. Actually, side effects are the
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//! only reason why one might want to use `tap`. However, one should
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//! not rely on the side effects being done in any specific order.
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//!
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//!
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//! @tparam M
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//! The tag (a `Monad`) of the monads in the tapped monadic chain.
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//!
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//! @param f
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//! A function to be executed inside a monadic chain. It will be called
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//! as `f(x)`, where `x` is a value inside the previous monad in the
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//! chain. The result of `f` is always discarded.
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//!
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//!
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//! Example
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//! -------
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//! @include example/tap.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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template <typename M>
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constexpr auto tap = [](auto&& f) {
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return tag-dispatched;
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};
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#else
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template <typename M, typename = void>
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struct tap_impl : tap_impl<M, when<true>> { };
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template <typename M>
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struct tap_t;
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template <typename M>
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constexpr tap_t<M> tap{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_TAP_HPP
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