68 lines
2.0 KiB
C++
68 lines
2.0 KiB
C++
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/*!
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@file
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Forward declares `boost::hana::chain`.
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@copyright Louis Dionne 2013-2016
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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*/
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#ifndef BOOST_HANA_FWD_CHAIN_HPP
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#define BOOST_HANA_FWD_CHAIN_HPP
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#include <boost/hana/config.hpp>
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#include <boost/hana/core/when.hpp>
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BOOST_HANA_NAMESPACE_BEGIN
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//! Feed a monadic value into a monadic computation.
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//! @ingroup group-Monad
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//!
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//! Given a monadic value and a monadic function, `chain` feeds the
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//! monadic value into the function, thus performing some Monad-specific
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//! effects, and returns the result. An implementation of `chain` must
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//! satisfy
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//! @code
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//! chain(xs, f) == flatten(transform(xs, f))
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//! @endcode
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//!
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//!
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//! Signature
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//! ---------
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//! For a monad `M`, given a monadic value of type `M(A)` and a monadic
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//! function @f$ f : A \to M(B) @f$, `chain` has the signature
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//! @f$
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//! \mathtt{chain} : M(A) \times (A \to M(B)) \to M(B)
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//! @f$.
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//!
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//! @param xs
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//! A monadic value to be fed to the function `f`.
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//!
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//! @param f
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//! A function taking a normal value in the `xs` structure, and returning
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//! a monadic value. This function is called as `f(x)`, where `x` is an
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//! element of the structure `xs`.
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//!
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//!
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//! Example
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//! -------
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//! @include example/chain.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto chain = [](auto&& xs, auto&& f) -> decltype(auto) {
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return tag-dispatched;
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};
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#else
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template <typename M, typename = void>
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struct chain_impl : chain_impl<M, when<true>> { };
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struct chain_t {
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template <typename Xs, typename F>
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constexpr decltype(auto) operator()(Xs&& xs, F&& f) const;
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};
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constexpr chain_t chain{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_CHAIN_HPP
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