bind.hpp

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// Copyright (C) 2020-2024 Jonathan Müller and lexy contributors
// SPDX-License-Identifier: BSL-1.0
 
#ifndef LEXY_CALLBACK_BIND_HPP_INCLUDED
#define LEXY_CALLBACK_BIND_HPP_INCLUDED
 
#include <lexy/_detail/tuple.hpp>
#include <lexy/callback/base.hpp>
 
//=== placeholder details ===//
namespace lexy::_detail
{
struct placeholder_base
{};
template <typename T>
constexpr auto is_placeholder = std::is_base_of_v<placeholder_base, T>;
 
template <typename BoundArg, typename State, typename... Args>
constexpr decltype(auto) expand_bound_arg(const BoundArg& bound, State& state,
                                          const _detail::tuple<Args...>& actual_args)
{
    if constexpr (is_placeholder<std::decay_t<BoundArg>>)
        return bound(state, actual_args);
    else
        return bound;
}
 
struct all_values_placeholder
{};
 
struct no_bind_state
{};
 
template <std::size_t Idx, typename Fn, typename... BoundArgs, typename State,
          typename... ActualArgs, std::size_t... ActualIdx, typename... ProducedArgs>
constexpr decltype(auto) _invoke_bound(Fn&& fn, const _detail::tuple<BoundArgs...>& bound_args,
                                       State&                               state,
                                       const _detail::tuple<ActualArgs...>& actual_args,
                                       _detail::index_sequence<ActualIdx...>,
                                       ProducedArgs&&... produced_args)
{
    if constexpr (Idx == sizeof...(BoundArgs))
    {
        (void)bound_args;
        (void)state;
        (void)actual_args;
        return LEXY_FWD(fn)(LEXY_FWD(produced_args)...);
    }
    else
    {
        using bound_arg_t
            = std::decay_t<typename _detail::tuple<BoundArgs...>::template element_type<Idx>>;
        if constexpr (std::is_same_v<bound_arg_t, all_values_placeholder>)
        {
            return _invoke_bound<Idx + 1>(LEXY_FWD(fn), bound_args, state, actual_args,
                                          actual_args.index_sequence(), LEXY_FWD(produced_args)...,
                                          // Expand to all actual arguments.
                                          LEXY_FWD(actual_args.template get<ActualIdx>())...);
        }
        else
        {
            return _invoke_bound<Idx + 1>(LEXY_FWD(fn), bound_args, state, actual_args,
                                          actual_args.index_sequence(), LEXY_FWD(produced_args)...,
                                          // Expand the currently bound argument
                                          expand_bound_arg(bound_args.template get<Idx>(), state,
                                                           actual_args));
        }
    }
}
 
template <typename Fn, typename... BoundArgs, std::size_t... Idx, typename State,
          typename... Args>
constexpr decltype(auto) invoke_bound(Fn&&                                fn, //
                                      const _detail::tuple<BoundArgs...>& bound_args,
                                      _detail::index_sequence<Idx...>, //
                                      State& state, Args&&... args)
{
    auto actual_args = _detail::forward_as_tuple(LEXY_FWD(args)...);
    if constexpr ((_detail::is_decayed_same<BoundArgs, _detail::all_values_placeholder> || ...))
    {
        // If we're having the placeholder we need to recursively expand every argument.
        return _invoke_bound<0>(LEXY_FWD(fn), bound_args, state, actual_args,
                                actual_args.index_sequence());
    }
    else
    {
        // If we're not having the all_values_placeholder, every placeholder expands to a single
        // argument. We can thus expand it easily by mapping each value of bound args to the actual
        // argument.
        return LEXY_FWD(fn)(
            expand_bound_arg(bound_args.template get<Idx>(), state, actual_args)...);
    }
}
} // namespace lexy::_detail
 
//=== placeholders ===//
namespace lexy
{
constexpr auto values = _detail::all_values_placeholder{};
 
struct nullopt;
 
struct _default
{
    template <typename T, typename = std::enable_if_t<std::is_default_constructible_v<T>>>
    constexpr operator T() const noexcept
    {
        return T();
    }
};
 
template <std::size_t N, typename T, typename Fn>
struct _nth_value : _detail::placeholder_base // fallback + map
{
    LEXY_EMPTY_MEMBER T  _fallback;
    LEXY_EMPTY_MEMBER Fn _fn;
 
    template <typename State, typename... Args>
    LEXY_FORCE_INLINE constexpr decltype(auto) operator()(State&,
                                                          const _detail::tuple<Args...>& args) const
    {
        if constexpr (N > sizeof...(Args))
            return _fallback; // Argument is missing.
        else
        {
            using arg_t = typename _detail::tuple<Args...>::template element_type<N - 1>;
            if constexpr (_detail::is_decayed_same<arg_t, nullopt>)
                return _fallback; // Argument is nullopt.
            else
                return _detail::invoke(_fn, args.template get<N - 1>());
        }
    }
};
template <std::size_t N, typename T>
struct _nth_value<N, T, void> : _detail::placeholder_base // fallback only
{
    LEXY_EMPTY_MEMBER T _fallback;
 
    template <typename State, typename... Args>
    LEXY_FORCE_INLINE constexpr decltype(auto) operator()(State&,
                                                          const _detail::tuple<Args...>& args) const
    {
        if constexpr (N > sizeof...(Args))
            return _fallback; // Argument is missing.
        else
        {
            using arg_t = typename _detail::tuple<Args...>::template element_type<N - 1>;
            if constexpr (_detail::is_decayed_same<arg_t, nullopt>)
                return _fallback; // Argument is nullopt.
            else
                return args.template get<N - 1>();
        }
    }
 
    template <typename Fn>
    constexpr auto map(Fn&& fn) const
    {
        return _nth_value<N, T, std::decay_t<Fn>>{{}, _fallback, LEXY_FWD(fn)};
    }
};
template <std::size_t N, typename Fn>
struct _nth_value<N, void, Fn> : _detail::placeholder_base // map only
{
    LEXY_EMPTY_MEMBER Fn _fn;
 
    template <typename State, typename... Args>
    LEXY_FORCE_INLINE constexpr decltype(auto) operator()(State&,
                                                          const _detail::tuple<Args...>& args) const
    {
        static_assert(N <= sizeof...(Args), "not enough arguments for nth_value<N>");
        return _detail::invoke(_fn, args.template get<N - 1>());
    }
 
    template <typename Arg>
    constexpr auto operator||(Arg&& fallback) const
    {
        return _nth_value<N, std::decay_t<Arg>, Fn>{{}, LEXY_FWD(fallback), _fn};
    }
    template <typename Arg>
    constexpr auto or_(Arg&& fallback) const
    {
        return *this || LEXY_FWD(fallback);
    }
 
    constexpr auto or_default() const
    {
        return *this || _default{};
    }
};
template <std::size_t N>
struct _nth_value<N, void, void> : _detail::placeholder_base
{
    // I'm sorry, but this is for consistency with std::bind.
    static_assert(N > 0, "values are 1-indexed");
 
    template <typename State, typename... Args>
    LEXY_FORCE_INLINE constexpr decltype(auto) operator()(State&,
                                                          const _detail::tuple<Args...>& args) const
    {
        static_assert(N <= sizeof...(Args), "not enough arguments for nth_value<N>");
        return args.template get<N - 1>();
    }
 
    template <typename Arg>
    constexpr auto operator||(Arg&& fallback) const
    {
        return _nth_value<N, std::decay_t<Arg>, void>{{}, LEXY_FWD(fallback)};
    }
    template <typename Arg>
    constexpr auto or_(Arg&& fallback) const
    {
        return *this || LEXY_FWD(fallback);
    }
 
    constexpr auto or_default() const
    {
        return *this || _default{};
    }
 
    template <typename Fn>
    constexpr auto map(Fn&& fn) const
    {
        return _nth_value<N, void, std::decay_t<Fn>>{{}, LEXY_FWD(fn)};
    }
};
 
template <std::size_t N>
constexpr auto nth_value = _nth_value<N, void, void>{};
 
inline namespace placeholders
{
    constexpr auto _1 = nth_value<1>;
    constexpr auto _2 = nth_value<2>;
    constexpr auto _3 = nth_value<3>;
    constexpr auto _4 = nth_value<4>;
    constexpr auto _5 = nth_value<5>;
    constexpr auto _6 = nth_value<6>;
    constexpr auto _7 = nth_value<7>;
    constexpr auto _8 = nth_value<8>;
} // namespace placeholders
 
template <typename Fn>
struct _parse_state : _detail::placeholder_base
{
    LEXY_EMPTY_MEMBER Fn _fn;
 
    template <typename State, typename... Args>
    constexpr decltype(auto) operator()(State& state, const _detail::tuple<Args...>&) const
    {
        static_assert(!std::is_same_v<State, _detail::no_bind_state>,
                      "lexy::parse_state requires that a state is passed to lexy::parse()");
        return _detail::invoke(_fn, state);
    }
};
template <>
struct _parse_state<void> : _detail::placeholder_base
{
    template <typename State, typename... Args>
    constexpr decltype(auto) operator()(State& state, const _detail::tuple<Args...>&) const
    {
        static_assert(!std::is_same_v<State, _detail::no_bind_state>,
                      "lexy::parse_state requires that a state is passed to lexy::parse()");
        return state;
    }
 
    template <typename Fn>
    constexpr auto map(Fn&& fn) const
    {
        return _parse_state<std::decay_t<Fn>>{{}, LEXY_FWD(fn)};
    }
};
 
constexpr auto parse_state = _parse_state<void>{};
} // namespace lexy
 
//=== bind ===//
namespace lexy
{
template <typename Callback, typename... BoundArgs>
struct _bound_cb
{
    LEXY_EMPTY_MEMBER Callback _callback;
    LEXY_EMPTY_MEMBER _detail::tuple<BoundArgs...> _bound_args;
 
    using return_type = typename Callback::return_type;
 
    template <typename State>
    struct _with_state
    {
        const _bound_cb& _bound;
        State&           _state;
 
        template <typename... Args>
        constexpr return_type operator()(Args&&... args) const&&
        {
            return _detail::invoke_bound(_bound._callback, _bound._bound_args,
                                         _bound._bound_args.index_sequence(), _state,
                                         LEXY_FWD(args)...);
        }
    };
 
    template <typename State>
    constexpr auto operator[](State& state) const
    {
        return _with_state<State>{*this, state};
    }
 
    template <typename... Args>
    constexpr return_type operator()(Args&&... args) const
    {
        auto state = _detail::no_bind_state{};
        return _detail::invoke_bound(_callback, _bound_args, _bound_args.index_sequence(), state,
                                     LEXY_FWD(args)...);
    }
};
 
template <typename Callback, typename... BoundArgs>
constexpr auto bind(Callback&& callback, BoundArgs&&... args)
{
    using bound = _bound_cb<std::decay_t<Callback>, std::decay_t<BoundArgs>...>;
    return bound{LEXY_FWD(callback), _detail::make_tuple(LEXY_FWD(args)...)};
}
} // namespace lexy
 
namespace lexy
{
template <typename Sink>
struct _sink_wrapper
{
    const Sink& _sink;
 
    template <typename... Args>
    constexpr auto operator()(Args&&... args)
    {
        return _sink.sink(LEXY_FWD(args)...);
    }
};
 
template <typename Sink, typename... BoundArgs>
struct _bound_sink
{
    LEXY_EMPTY_MEMBER Sink _sink;
    LEXY_EMPTY_MEMBER _detail::tuple<BoundArgs...> _bound;
 
    template <typename... Args>
    constexpr auto operator()(Args... args) const -> decltype(_sink(LEXY_FWD(args)...))
    {
        return _sink(LEXY_FWD(args)...);
    }
 
    template <bool Dummy = true,
              typename   = std::enable_if_t<(!_detail::is_placeholder<BoundArgs> && ... && Dummy)>>
    constexpr auto sink() const
    {
        auto state = _detail::no_bind_state{};
        return _detail::invoke_bound(_sink_wrapper<Sink>{_sink}, _bound, _bound.index_sequence(),
                                     state);
    }
 
    template <typename State>
    constexpr auto sink(State& state) const
    {
        return _detail::invoke_bound(_sink_wrapper<Sink>{_sink}, _bound, _bound.index_sequence(),
                                     state);
    }
};
 
template <typename Sink, typename... BoundArgs>
constexpr auto bind_sink(Sink&& sink, BoundArgs&&... args)
{
    static_assert(
        (!_detail::is_decayed_same<BoundArgs, _detail::all_values_placeholder> && ...),
        "lexy::values as a placeholder for bind_sink() doesn't make sense - there won't be any values");
    using bound = _bound_sink<std::decay_t<Sink>, std::decay_t<BoundArgs>...>;
    return bound{LEXY_FWD(sink), _detail::make_tuple(LEXY_FWD(args)...)};
}
} // namespace lexy
 
#endif // LEXY_CALLBACK_BIND_HPP_INCLUDED