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Forward declares `boost::hana::adjust_if`.

@copyright Louis Dionne 2013-2017
Distributed under the Boost Software License, Version 1.0.
(See accompanying file or copy at


#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>

namespace boost { namespace hana {
    //! Apply a function on all the elements of a structure satisfying a predicate.
    //! @ingroup group-Functor
    //! Given a Functor, a predicate `pred` and a function `f`, `adjust_if`
    //! will _adjust_ the elements of the Functor that satisfy the predicate
    //! with the function `f`. In other words, `adjust_if` will return a new
    //! Functor equal to the original one, except that the elements satisfying
    //! the predicate will be transformed with the given function. Elements
    //! for which the predicate is not satisfied are left untouched, and they
    //! are kept as-is in the resulting Functor.
    //! Signature
    //! ---------
    //! Given a `Functor` `F` and a `Logical` `Bool`, the signature is
    //! \f$
    //!     \mathtt{adjust\_if} : F(T) \times (T \to Bool) \times (T \to T) \to F(T)
    //! \f$
    //! @param xs
    //! The structure to adjust with `f`.
    //! @param pred
    //! A function called as `pred(x)` for each element of the Functor,
    //! and returning whether `f` should be applied on that element.
    //! @param f
    //! A function called as `f(x)` on the element(s) of the Functor that
    //! satisfy the predicate.
    //! Example
    //! -------
    //! @include example/adjust_if.cpp
    constexpr auto adjust_if = [](auto&& xs, auto const& pred, auto const& f) {
        return tag-dispatched;
    template <typename Xs, typename = void>
    struct adjust_if_impl : adjust_if_impl<Xs, when<true>> { };

    struct adjust_if_t {
        template <typename Xs, typename Pred, typename F>
        constexpr auto operator()(Xs&& xs, Pred const& pred, F const& f) const;

    BOOST_HANA_INLINE_VARIABLE constexpr adjust_if_t adjust_if{};
}} // end namespace boost::hana