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|---|---|---|
| 7c673cae FG |
1 | /*! |
| 2 | @file | |
| 3 | Forward declares `boost::hana::tap`. | |
| 4 | ||
| 5 | @copyright Louis Dionne 2013-2016 | |
| 6 | Distributed under the Boost Software License, Version 1.0. | |
| 7 | (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) | |
| 8 | */ | |
| 9 | ||
| 10 | #ifndef BOOST_HANA_FWD_TAP_HPP | |
| 11 | #define BOOST_HANA_FWD_TAP_HPP | |
| 12 | ||
| 13 | #include <boost/hana/config.hpp> | |
| 14 | #include <boost/hana/core/when.hpp> | |
| 15 | ||
| 16 | ||
| 17 | BOOST_HANA_NAMESPACE_BEGIN | |
| 18 | //! Tap inside a monadic chain. | |
| 19 | //! @ingroup group-Monad | |
| 20 | //! | |
| 21 | //! Given a function `f`, `tap<M>` returns a new function which performs | |
| 22 | //! `f` on its argument and then returns the argument lifted in the `M` | |
| 23 | //! `Monad`. Combined with the property that `chain(m, lift<M>) == m`, | |
| 24 | //! this provides a way of executing an action inside a monadic chain | |
| 25 | //! without influencing its overall result. This is useful to e.g. insert | |
| 26 | //! debug statements or perform actions that are not tied to the chain but | |
| 27 | //! that need to be executed inside of it. | |
| 28 | //! | |
| 29 | //! @note | |
| 30 | //! Since C++ is not a pure language, it is possible to perform side | |
| 31 | //! effects inside the `f` function. Actually, side effects are the | |
| 32 | //! only reason why one might want to use `tap`. However, one should | |
| 33 | //! not rely on the side effects being done in any specific order. | |
| 34 | //! | |
| 35 | //! | |
| 36 | //! @tparam M | |
| 37 | //! The tag (a `Monad`) of the monads in the tapped monadic chain. | |
| 38 | //! | |
| 39 | //! @param f | |
| 40 | //! A function to be executed inside a monadic chain. It will be called | |
| 41 | //! as `f(x)`, where `x` is a value inside the previous monad in the | |
| 42 | //! chain. The result of `f` is always discarded. | |
| 43 | //! | |
| 44 | //! | |
| 45 | //! Example | |
| 46 | //! ------- | |
| 47 | //! @include example/tap.cpp | |
| 48 | #ifdef BOOST_HANA_DOXYGEN_INVOKED | |
| 49 | template <typename M> | |
| 50 | constexpr auto tap = [](auto&& f) { | |
| 51 | return tag-dispatched; | |
| 52 | }; | |
| 53 | #else | |
| 54 | template <typename M, typename = void> | |
| 55 | struct tap_impl : tap_impl<M, when<true>> { }; | |
| 56 | ||
| 57 | template <typename M> | |
| 58 | struct tap_t; | |
| 59 | ||
| 60 | template <typename M> | |
| 61 | constexpr tap_t<M> tap{}; | |
| 62 | #endif | |
| 63 | BOOST_HANA_NAMESPACE_END | |
| 64 | ||
| 65 | #endif // !BOOST_HANA_FWD_TAP_HPP |