build/_deps/metal-src/include/metal/map/at_key.hpp

#ifndef METAL_MAP_AT_KEY_HPP
#define METAL_MAP_AT_KEY_HPP

#include "../config.hpp"

namespace metal {
/// \cond
namespace detail {
    template <class seq, class key>
    struct _at_key;
}
/// \endcond

/// \ingroup map
///
/// ### Description
/// Retrieves the value associated with some key in a \map.
///
/// ### Usage
/// For any \map `m` and \value `k`
/// \code
///     using result = metal::at_key<m, k>;
/// \endcode
///
/// \returns: \value
/// \semantics:
///     If `m` associates keys `k_1, ..., k, ..., k_n` to values
///     `v_1, ..., v, ..., v_n`, then
///     \code
///         using result = v;
///     \endcode
///
/// ### Example
/// \snippet map.cpp at_key
///
/// ### See Also
/// \see map, has_key, insert_key, erase_key
template <class seq, class key>
using at_key = typename detail::_at_key<seq, key>::type;
}

#include "../detail/declptr.hpp"
#include "../list/list.hpp"
#include "../number/number.hpp"
#include "../value/distinct.hpp"
#include "../value/value.hpp"

namespace metal {
/// \cond
namespace detail {
    template <class, class = true_>
    struct table {
    };

    template <class... keys, class... vals>
    struct table<list<list<keys, vals>...>, distinct<list<keys, vals>...>>
        : list<keys, vals>... {
    };

    template <>
    struct table<list<>> {
    };

    template <class key, class val>
    value<val> lookup(list<key, val>*);

    template <class>
    value<> lookup(...);

    template <class seq, class key>
    struct _at_key : decltype(lookup<key>(declptr<table<seq>>())) {
    };
}
/// \endcond
}

#endif