if_error.h

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#pragma once
 
#include "di/function/curry_back.h"
#include "di/function/invoke.h"
#include "di/meta/util.h"
#include "di/meta/vocab.h"
 
namespace di::vocab {
namespace detail {
    struct IfErrorFunction {
        template<concepts::Expected T, concepts::InvocableTo<void, meta::Like<T, meta::ExpectedError<T>>&> F>
        requires(concepts::DecayConstructible<T>)
        constexpr auto operator()(T&& expected, F&& function) const {
            if (!expected) {
                function::invoke(util::forward<F>(function), expected.error());
            }
            return util::forward<T>(expected);
        }
 
        template<concepts::Expected T, typename F>
        requires(concepts::DecayConstructible<T> && concepts::LanguageVoid<meta::ExpectedError<T>>)
        constexpr auto operator()(T&& expected, F&&) const {
            return util::forward<T>(expected);
        }
    };
}
 
constexpr inline auto if_error = function::curry_back(detail::IfErrorFunction {}, meta::c_<2ZU>);
}
 
namespace di {
using vocab::if_error;
}