| 1 | /*! |
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| 2 | @file |
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| 3 | Forward declares `boost::hana::monadic_compose`. |
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| 4 | |
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| 5 | Copyright Louis Dionne 2013-2022 |
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| 6 | Distributed under the Boost Software License, Version 1.0. |
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| 7 | (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) |
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| 8 | */ |
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| 9 | |
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| 10 | #ifndef BOOST_HANA_FWD_MONADIC_COMPOSE_HPP |
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| 11 | #define BOOST_HANA_FWD_MONADIC_COMPOSE_HPP |
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| 12 | |
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| 13 | #include <boost/hana/config.hpp> |
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| 14 | #include <boost/hana/core/when.hpp> |
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| 15 | |
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| 16 | |
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| 17 | namespace boost { namespace hana { |
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| 18 | //! Composition of monadic functions. |
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| 19 | //! @ingroup group-Monad |
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| 20 | //! |
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| 21 | //! Given two monadic functions `f` and `g`, `monadic_compose` returns |
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| 22 | //! a new function equivalent to the composition of `f` with `g`, except |
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| 23 | //! the result of `g` is `chain`ed into `f` instead of simply passed to |
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| 24 | //! it, as with normal composition. `monadic_compose` satisfies |
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| 25 | //! @code |
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| 26 | //! monadic_compose(f, g)(x) == chain(g(x), f) |
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| 27 | //! @endcode |
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| 28 | //! |
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| 29 | //! |
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| 30 | //! @note |
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| 31 | //! Unlike `compose`, `monadic_compose` does not generalize nicely to |
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| 32 | //! arities higher than one. Hence, only unary functions may be used |
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| 33 | //! with `monadic_compose`. |
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| 34 | //! |
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| 35 | //! |
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| 36 | //! Signature |
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| 37 | //! --------- |
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| 38 | //! Given a `Monad` `M` and two functions @f$ f : B \to M(C) @f$ and |
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| 39 | //! @f$ g : A \to M(B) @f$, the signature is |
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| 40 | //! @f$ |
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| 41 | //! \mathtt{monadic\_compose} |
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| 42 | //! : (B \to M(C)) \times (A \to M(B)) \to (A \to M(C)) |
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| 43 | //! @f$. |
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| 44 | //! |
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| 45 | //! @param f |
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| 46 | //! A monadic function with signature @f$ B \to M(C) @f$. |
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| 47 | //! |
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| 48 | //! @param g |
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| 49 | //! A monadic function with signature @f$ A \to M(B) @f$. |
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| 50 | //! |
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| 51 | //! |
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| 52 | //! @note |
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| 53 | //! This method is not tag-dispatched, so it can't be customized directly. |
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| 54 | //! |
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| 55 | //! |
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| 56 | //! Example |
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| 57 | //! ------- |
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| 58 | //! @include example/monadic_compose.cpp |
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| 59 | #ifdef BOOST_HANA_DOXYGEN_INVOKED |
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| 60 | constexpr auto monadic_compose = [](auto&& f, auto&& g) { |
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| 61 | return [perfect-capture](auto&& x) -> decltype(auto) { |
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| 62 | return hana::chain(forwarded(g)(forwarded(x)), forwarded(f)); |
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| 63 | }; |
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| 64 | }; |
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| 65 | #else |
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| 66 | struct monadic_compose_t { |
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| 67 | template <typename F, typename G> |
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| 68 | constexpr auto operator()(F&& f, G&& g) const; |
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| 69 | }; |
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| 70 | |
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| 71 | BOOST_HANA_INLINE_VARIABLE constexpr monadic_compose_t monadic_compose{}; |
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| 72 | #endif |
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| 73 | }} // end namespace boost::hana |
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| 74 | |
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| 75 | #endif // !BOOST_HANA_FWD_MONADIC_COMPOSE_HPP |
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| 76 | |
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