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+/*=============================================================================
+    Copyright (c) 2014 Paul Fultz II
+    tap.h
+    Distributed under the Boost Software License, Version 1.0. (See accompanying
+    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+==============================================================================*/
+
+#ifndef BOOST_HOF_GUARD_FUNCTION_TAP_H
+#define BOOST_HOF_GUARD_FUNCTION_TAP_H
+
+/// tap
+/// ===
+/// 
+/// Description
+/// -----------
+/// 
+/// The `tap` function invokes a function on the first argument passed in and
+/// then returns the first argument. This is useful in a chain of pipable
+/// function to perform operations on intermediate results. As a result, this
+/// function is [`pipable`](/include/boost/hof/pipable).
+/// 
+/// Synopsis
+/// --------
+/// 
+///     template<class T, class F>
+///     pipable constexpr T tap(T&& x, const F& f);
+/// 
+/// Requirements
+/// ------------
+/// 
+/// F must be:
+/// 
+/// * [UnaryInvocable](UnaryInvocable)
+/// 
+/// Example
+/// -------
+/// 
+///     #include <boost/hof.hpp>
+///     #include <cassert>
+///     #include <iostream>
+///     using namespace boost::hof;
+/// 
+///     struct sum_f
+///     {
+///         template<class T, class U>
+///         T operator()(T x, U y) const
+///         {
+///             return x+y;
+///         }
+///     };
+/// 
+///     const pipable_adaptor<sum_f> sum = {};
+///     int main() {
+///         // Prints 3
+///         int r = 1 | sum(2) | tap([](int i) { std::cout << i; }) | sum(2);
+///         assert(r == 5);
+///     }
+/// 
+
+#include <boost/hof/pipable.hpp>
+#include <boost/hof/apply.hpp>
+#include <boost/hof/detail/static_const_var.hpp>
+
+namespace boost { namespace hof { namespace detail {
+
+struct tap_f
+{
+    template<class T, class F>
+    constexpr T operator()(T&& x, const F& f) const
+    BOOST_HOF_RETURNS_DEDUCE_NOEXCEPT((boost::hof::apply(f, x), BOOST_HOF_FORWARD(T)(x)))
+    {
+        return boost::hof::apply(f, x), BOOST_HOF_FORWARD(T)(x);
+    }
+};
+
+}
+
+BOOST_HOF_DECLARE_STATIC_VAR(tap, pipable_adaptor<detail::tap_f>);
+
+
+}} // namespace boost::hof
+
+#endif