89 lines
3.0 KiB
C++
89 lines
3.0 KiB
C++
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/*!
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@file
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Forward declares `boost::hana::fold_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_FOLD_LEFT_HPP
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#define BOOST_HANA_FWD_FOLD_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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//! Left-fold of a structure using a binary operation and an optional
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//! initial reduction state.
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//! @ingroup group-Foldable
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//!
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//! `fold_left` is a left-associative fold using a binary operation.
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//! Given a structure containing `x1, ..., xn`, a function `f` and
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//! an optional initial state, `fold_left` applies `f` as follows
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//! @code
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//! f(... f(f(f(x1, x2), x3), x4) ..., xn) // without state
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//! f(... f(f(f(f(state, x1), x2), x3), x4) ..., xn) // with state
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//! @endcode
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//!
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//! When the structure is empty, two things may arise. If an initial
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//! state was provided, it is returned as-is. Otherwise, if the no-state
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//! version of the function was used, an error is triggered. When the
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//! stucture contains a single element and the no-state version of the
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//! function was used, that single element is returned as is.
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//!
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//!
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//! Signature
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//! ---------
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//! Given a `Foldable` `F` and an optional initial state of tag `S`,
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//! the signatures for `fold_left` are
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//! \f[
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//! \mathtt{fold\_left} : F(T) \times S \times (S \times T \to S) \to S
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//! \f]
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//!
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//! for the variant with an initial state, and
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//! \f[
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//! \mathtt{fold\_left} : F(T) \times (T \times T \to T) \to T
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//! \f]
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//!
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//! for the variant without an initial state.
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//!
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//! @param xs
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//! The structure to fold.
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//!
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//! @param state
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//! The initial value used for folding.
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//!
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//! @param f
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//! A binary function called as `f(state, x)`, where `state` is the
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//! result accumulated so far and `x` is an element in the structure.
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//! For left folds without an initial state, the function is called as
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//! `f(x1, x2)`, where `x1` and `x2` are elements of the structure.
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//!
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//!
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//! Example
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//! -------
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//! @include example/fold_left.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto fold_left = [](auto&& xs[, auto&& state], 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 T, typename = void>
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struct fold_left_impl : fold_left_impl<T, when<true>> { };
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struct fold_left_t {
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template <typename Xs, typename State, typename F>
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constexpr decltype(auto) operator()(Xs&& xs, State&& state, F&& f) const;
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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 fold_left_t fold_left{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_FOLD_LEFT_HPP
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