86 lines
2.6 KiB
C++
86 lines
2.6 KiB
C++
/*!
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@file
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Forward declares `boost::hana::unfold_left`.
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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_UNFOLD_LEFT_HPP
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#define BOOST_HANA_FWD_UNFOLD_LEFT_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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//! Dual operation to `fold_left` for sequences.
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//! @ingroup group-Sequence
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//!
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//! While `fold_left` reduces a structure to a summary value from the left,
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//! `unfold_left` builds a sequence from a seed value and a function,
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//! starting from the left.
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//!
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//!
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//! Signature
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//! ---------
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//! Given a `Sequence` `S`, an initial value `state` of tag `I`, an
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//! arbitrary Product `P` and a function \f$ f : I \to P(I, T) \f$,
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//! `unfold_left<S>` has the following signature:
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//! \f[
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//! \mathtt{unfold\_left}_S : I \times (I \to P(I, T)) \to S(T)
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//! \f]
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//!
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//! @tparam S
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//! The tag of the sequence to build up.
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//!
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//! @param state
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//! An initial value to build the sequence from.
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//!
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//! @param f
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//! A function called as `f(state)`, where `state` is an initial value,
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//! and returning
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//! 1. `nothing` if it is done producing the sequence.
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//! 2. otherwise, `just(make<P>(state, x))`, where `state` is the new
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//! initial value used in the next call to `f`, `x` is an element to
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//! be appended to the resulting sequence, and `P` is an arbitrary
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//! `Product`.
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//!
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//!
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//! Fun fact
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//! ---------
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//! In some cases, `unfold_left` can undo a `fold_left` operation:
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//! @code
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//! unfold_left<S>(fold_left(xs, state, f), g) == xs
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//! @endcode
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//!
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//! if the following holds
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//! @code
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//! g(f(x, y)) == just(make_pair(x, y))
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//! g(state) == nothing
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//! @endcode
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//!
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//!
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//! Example
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//! -------
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//! @include example/unfold_left.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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template <typename S>
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constexpr auto unfold_left = [](auto&& state, auto&& f) {
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return tag-dispatched;
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};
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#else
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template <typename S, typename = void>
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struct unfold_left_impl : unfold_left_impl<S, when<true>> { };
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template <typename S>
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struct unfold_left_t;
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template <typename S>
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constexpr unfold_left_t<S> unfold_left{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_UNFOLD_LEFT_HPP
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