1 /*! 2 @file 3 Forward declares `boost::hana::chain`. 4 5 @copyright Louis Dionne 2013-2017 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_CHAIN_HPP 11 #define BOOST_HANA_FWD_CHAIN_HPP 12 13 #include <boost/hana/config.hpp> 14 #include <boost/hana/core/when.hpp> 15 16 17 BOOST_HANA_NAMESPACE_BEGIN 18 //! Feed a monadic value into a monadic computation. 19 //! @ingroup group-Monad 20 //! 21 //! Given a monadic value and a monadic function, `chain` feeds the 22 //! monadic value into the function, thus performing some Monad-specific 23 //! effects, and returns the result. An implementation of `chain` must 24 //! satisfy 25 //! @code 26 //! chain(xs, f) == flatten(transform(xs, f)) 27 //! @endcode 28 //! 29 //! 30 //! Signature 31 //! --------- 32 //! For a monad `M`, given a monadic value of type `M(A)` and a monadic 33 //! function @f$ f : A \to M(B) @f$, `chain` has the signature 34 //! @f$ 35 //! \mathtt{chain} : M(A) \times (A \to M(B)) \to M(B) 36 //! @f$. 37 //! 38 //! @param xs 39 //! A monadic value to be fed to the function `f`. 40 //! 41 //! @param f 42 //! A function taking a normal value in the `xs` structure, and returning 43 //! a monadic value. This function is called as `f(x)`, where `x` is an 44 //! element of the structure `xs`. 45 //! 46 //! 47 //! Example 48 //! ------- 49 //! @include example/chain.cpp 50 #ifdef BOOST_HANA_DOXYGEN_INVOKED 51 constexpr auto chain = [](auto&& xs, auto&& f) -> decltype(auto) { 52 return tag-dispatched; 53 }; 54 #else 55 template <typename M, typename = void> 56 struct chain_impl : chain_impl<M, when<true>> { }; 57 58 struct chain_t { 59 template <typename Xs, typename F> 60 constexpr decltype(auto) operator()(Xs&& xs, F&& f) const; 61 }; 62 63 constexpr chain_t chain{}; 64 #endif 65 BOOST_HANA_NAMESPACE_END 66 67 #endif // !BOOST_HANA_FWD_CHAIN_HPP 68