mirror of
https://github.com/matrix-construct/construct
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73 lines
2.6 KiB
C++
73 lines
2.6 KiB
C++
// Matrix Construct
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//
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// Copyright (C) Matrix Construct Developers, Authors & Contributors
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// Copyright (C) 2016-2019 Jason Volk <jason@zemos.net>
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//
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// Permission to use, copy, modify, and/or distribute this software for any
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// purpose with or without fee is hereby granted, provided that the above
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// copyright notice and this permission notice is present in all copies. The
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// full license for this software is available in the LICENSE file.
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#pragma once
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#define HAVE_IRCD_CTX_TRIT_H
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namespace ircd::ctx
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{
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struct trit;
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}
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namespace ircd
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{
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using ctx::trit;
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}
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/// Implementation of a tribool with custom semantics in a stackful coroutine
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/// environment. Tribools have three states: true, false, and unknown. Our goal
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/// here is to integrate this "unknown" property with the context-switching
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/// system. In a nutshell, when a `trit` in an unknown state is sampled, the
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/// context blocks until the `trit` leaves the unknown state. This action is
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/// based on the existing promise/future system and waiting for the known state
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/// can be thought of as a `future<bool>::get()`. The value of this endeavor
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/// though is greater than mere syntactic sugar for future<bool>.
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/// ```
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/// if(trit == true) // context blocks here until the value is known
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/// ... // branch taken if value==true once known.
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///
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/// ```
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///
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/// Overloaded logic operators make it possible to optimize
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/// conditional predicates by employing Kleene's strong logic of indeterminacy.
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/// This allows us to optimize these predicates for I/O rather than
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/// computation. In the example below, traditionally under boolean logic we
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/// evaluate trit[0] first, and if it's false, short-circuit evaluation elides
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/// observing trit[1]:
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/// ```
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/// if(trit[0] && trit[1])
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/// ...
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///
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/// ```
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/// Under the trilean logic in our system, we first test if trit[0] is known,
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/// because if it isn't, we can test if trit[1] is knowably false to conclude
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/// the predicate. The benefit is seen when these objects represent the result
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/// of latent asynchronous operations; head-of-line blocking is avoided in
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/// this case because any false value can abrogate any further blocking.
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///
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struct ircd::ctx::trit
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{
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/// The boolean value state of true or false. This value is undefined when
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/// `unknown` is true
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bool value;
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/// Whether the boolean value is determinable/determined or not. We name
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/// this negatively such that any zero pre-initialization creates a known
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/// false value without requiring any code to be executed.
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bool unknown;
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trit();
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};
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inline
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ircd::ctx::trit::trit()
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:value{false}
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,unknown{false}
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{}
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