promise.hpp
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promise.hpp
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	/*
	* This file is part of libkazv.
	* SPDX-FileCopyrightText: 2021 Tusooa Zhu <tusooa@kazv.moe>
	* SPDX-License-Identifier: AGPL-3.0-or-later
	*/

	#pragma once

	static_assert(false, "This is an interface, not an actual class in the library.");

	/**
	* A Promise represents an asynchronous result that can be fetched later.
	*
	* @warning This is an interface, not an actual class in the library.
	*
	* A Promise is called resolved when the result becomes available.
	*
	* In libkazv docs, the result type is usually EffectStatus.
	*/
	template<class T>
	class Promise
	{
	public:
	/**
	* The type of the value this Promise will hold.
	*/
	using DataT = T;

	/**
	* Specify something to do after this Promise resolves.
	*
	* @param f the function to run. It should take a DataT as the only argument.
	*
	* @return A Promise.
	* Assume `result` is the result this Promise resolves to.
	* If `f(result)` returns void, it resolves to `DataT()` after `f` returns.
	* If `f(result)` returns a Promise, it resolves to what the Promise will resolve to,
	* after that Promise resolves.
	* Otherwise it resolves to what `f(result)` returns, implicitly converted to `DataT`,
	* after that function returns.
	*/
	template<class Func>
	Promise then(Func &&f);
	};