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|---|---|---|
| 7c673cae FG |
1 | /*! |
| 2 | @file | |
| 3 | Forward declares `boost::hana::monadic_compose`. | |
| 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_MONADIC_COMPOSE_HPP | |
| 11 | #define BOOST_HANA_FWD_MONADIC_COMPOSE_HPP | |
| 12 | ||
| 13 | #include <boost/hana/config.hpp> | |
| 14 | #include <boost/hana/core/when.hpp> | |
| 15 | ||
| 16 | ||
| 17 | BOOST_HANA_NAMESPACE_BEGIN | |
| 18 | //! Composition of monadic functions. | |
| 19 | //! @ingroup group-Monad | |
| 20 | //! | |
| 21 | //! Given two monadic functions `f` and `g`, `monadic_compose` returns | |
| 22 | //! a new function equivalent to the composition of `f` with `g`, except | |
| 23 | //! the result of `g` is `chain`ed into `f` instead of simply passed to | |
| 24 | //! it, as with normal composition. `monadic_compose` satisfies | |
| 25 | //! @code | |
| 26 | //! monadic_compose(f, g)(x) == chain(g(x), f) | |
| 27 | //! @endcode | |
| 28 | //! | |
| 29 | //! | |
| 30 | //! @note | |
| 31 | //! Unlike `compose`, `monadic_compose` does not generalize nicely to | |
| 32 | //! arities higher than one. Hence, only unary functions may be used | |
| 33 | //! with `monadic_compose`. | |
| 34 | //! | |
| 35 | //! | |
| 36 | //! Signature | |
| 37 | //! --------- | |
| 38 | //! Given a `Monad` `M` and two functions @f$ f : B \to M(C) @f$ and | |
| 39 | //! @f$ g : A \to M(B) @f$, the signature is | |
| 40 | //! @f$ | |
| 41 | //! \mathtt{monadic\_compose} | |
| 42 | //! : (B \to M(C)) \times (A \to M(B)) \to (A \to M(C)) | |
| 43 | //! @f$. | |
| 44 | //! | |
| 45 | //! @param f | |
| 46 | //! A monadic function with signature @f$ B \to M(C) @f$. | |
| 47 | //! | |
| 48 | //! @param g | |
| 49 | //! A monadic function with signature @f$ A \to M(B) @f$. | |
| 50 | //! | |
| 51 | //! | |
| 52 | //! @note | |
| 53 | //! This method is not tag-dispatched, so it can't be customized directly. | |
| 54 | //! | |
| 55 | //! | |
| 56 | //! Example | |
| 57 | //! ------- | |
| 58 | //! @include example/monadic_compose.cpp | |
| 59 | #ifdef BOOST_HANA_DOXYGEN_INVOKED | |
| 60 | constexpr auto monadic_compose = [](auto&& f, auto&& g) { | |
| 61 | return [perfect-capture](auto&& x) -> decltype(auto) { | |
| 62 | return hana::chain(forwarded(g)(forwarded(x)), forwarded(f)); | |
| 63 | }; | |
| 64 | }; | |
| 65 | #else | |
| 66 | struct monadic_compose_t { | |
| 67 | template <typename F, typename G> | |
| 68 | constexpr auto operator()(F&& f, G&& g) const; | |
| 69 | }; | |
| 70 | ||
| 71 | constexpr monadic_compose_t monadic_compose{}; | |
| 72 | #endif | |
| 73 | BOOST_HANA_NAMESPACE_END | |
| 74 | ||
| 75 | #endif // !BOOST_HANA_FWD_MONADIC_COMPOSE_HPP |