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 Hu Chunming
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/*!
@file
Forward declares `boost::hana::tap`.
Copyright Louis Dionne 2013-2022
Distributed under the Boost Software License, Version 1.0.
(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
*/
#ifndef BOOST_HANA_FWD_TAP_HPP
#define BOOST_HANA_FWD_TAP_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
namespace boost { namespace hana {
//! Tap inside a monadic chain.
//! @ingroup group-Monad
//!
//! Given a function `f`, `tap<M>` returns a new function which performs
//! `f` on its argument and then returns the argument lifted in the `M`
//! `Monad`. Combined with the property that `chain(m, lift<M>) == m`,
//! this provides a way of executing an action inside a monadic chain
//! without influencing its overall result. This is useful to e.g. insert
//! debug statements or perform actions that are not tied to the chain but
//! that need to be executed inside of it.
//!
//! @note
//! Since C++ is not a pure language, it is possible to perform side
//! effects inside the `f` function. Actually, side effects are the
//! only reason why one might want to use `tap`. However, one should
//! not rely on the side effects being done in any specific order.
//!
//!
//! @tparam M
//! The tag (a `Monad`) of the monads in the tapped monadic chain.
//!
//! @param f
//! A function to be executed inside a monadic chain. It will be called
//! as `f(x)`, where `x` is a value inside the previous monad in the
//! chain. The result of `f` is always discarded.
//!
//!
//! Example
//! -------
//! @include example/tap.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
template <typename M>
constexpr auto tap = [](auto&& f) {
return tag-dispatched;
};
#else
template <typename M, typename = void>
struct tap_impl : tap_impl<M, when<true>> { };
template <typename M>
struct tap_t {
template <typename F>
constexpr auto operator()(F&& f) const;
};
template <typename M>
BOOST_HANA_INLINE_VARIABLE constexpr tap_t<M> tap{};
#endif
}} // end namespace boost::hana
#endif // !BOOST_HANA_FWD_TAP_HPP
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