Tristate - wanna?

[Alex Parrill via Digitalmars-d]

On Saturday, 26 March 2016 at 22:11:53 UTC, Nordlöw wrote:
> On Saturday, 26 October 2013 at 15:41:32 UTC, Andrei 
> Alexandrescu wrote:
>> While messing with std.allocator I explored the type below. I 
>> ended up not using it, but was surprised that implementing it 
>> was quite nontrivial. Should we add it to stdlib?
>
> I can think of many variants of for this. What about
>
>     { yes, // 1 chance
>       no, // 0 chance
>       likely, // > 1/2 chance
>       unlikely, // < 1/2 chance
>       unknown // any chance
>     }
>
> ?
>
> Partial implementation at
>
> https://github.com/nordlow/justd/blob/master/fuzzy.d#L15
>
> :)

If we're going down that route, might as well use state tables. 
With CTFE + templates, you could possibly do something like this:


	immutable State[] StateOrTable = ParseStateTable!q{
	        | yes | no       | likely | unlikely | unknown
	------------------------------------------------------
	yes     | yes | yes      | yes    | yes      | yes
	no      | yes | no       | likely | unlikely | unknown
	likely  | yes | likely   | likely | likely   | likely
	unlikely| yes | unlikely | likely | unlikely | unknown
	unknown | yes | unknown  | likely | unknwon  | unknown
	};

	State opBinary(string op)(State other)
	if(op == "||") {
		return StateOrTable[this.value*NumStates+other.value];
	}

Though I see issues with having a generic n-state value template 
and also rewriting `a != b` to `!(a == b)`; I suspect that there 
may be some class of values where the two are not equivalent.