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/*!
@file
Forward declares `boost::hana::span`.
Copyright Louis Dionne 2013-2022
Distributed under the Boost Software License, Version 1.0.
(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
*/
#ifndef BOOST_HANA_FWD_SPAN_HPP
#define BOOST_HANA_FWD_SPAN_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
#include <boost/hana/detail/nested_by_fwd.hpp>
namespace boost { namespace hana {
//! Returns a `Product` containing the longest prefix of a sequence
//! satisfying a predicate, and the rest of the sequence.
//! @ingroup group-Sequence
//!
//! The first component of the returned `Product` is a sequence for which
//! all elements satisfy the given predicate. The second component of the
//! returned `Product` is a sequence containing the remainder of the
//! argument. Both or either sequences may be empty, depending on the
//! input argument. More specifically,
//! @code
//! span(xs, predicate) == make_pair(take_while(xs, predicate),
//! drop_while(xs, predicate))
//! @endcode
//! except that `make_pair` may be an arbitrary `Product`.
//!
//!
//! Signature
//! ---------
//! Given a `Sequence` `S(T)`, a `Logical` `Bool` and a predicate
//! \f$ T \to Bool \f$, `span` has the following signature:
//! \f[
//! \mathtt{span} : S(T) \times (T \to Bool) \to S(T) \times S(T)
//! \f]
//!
//! @param xs
//! The sequence to break into two parts.
//!
//! @param predicate
//! A function called as `predicate(x)`, where `x` is an element of the
//! sequence, and returning a `Logical. In the current implementation of
//! the library, `predicate` has to return a compile-time `Logical`.
//!
//!
//! Syntactic sugar (`span.by`)
//! ---------------------------
//! `span` can be called in an alternate way, which provides a nice syntax
//! in some cases where the predicate is short:
//! @code
//! span.by(predicate, xs) == span(xs, predicate)
//! span.by(predicate) == span(-, predicate)
//! @endcode
//!
//! where `span(-, predicate)` denotes the partial application of
//! `span` to `predicate`.
//!
//!
//! Example
//! -------
//! @include example/span.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto span = [](auto&& xs, auto&& predicate) {
return tag-dispatched;
};
#else
template <typename S, typename = void>
struct span_impl : span_impl<S, when<true>> { };
struct span_t : detail::nested_by<span_t> {
template <typename Xs, typename Pred>
constexpr auto operator()(Xs&& xs, Pred&& pred) const;
};
BOOST_HANA_INLINE_VARIABLE constexpr span_t span{};
#endif
}} // end namespace boost::hana
#endif // !BOOST_HANA_FWD_SPAN_HPP
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