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Subject: Re: [boost] Ternary logic programming
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2015-07-04 11:22:28

On 3 Jul 2015 at 11:45, Bjorn Reese wrote:

> > Out of interest, what do you think of my free function ternary logic
> > programming:
> So far, the discussion has focussed on how the ternary operators should
> behave in if-statements. In that case, I tend to agree with Lee's enum
> and switch suggestion, which also extends well into multi-variate logic.

My tribool allows switch and enum if you want it. It's just it
requires a bit more typing, and I dislike typing :)

> If we want to use the ternary logic in if-statements, then we should
> address composite conditions. For instance, what should the following
> evaluate to?
> if (Empty && Value) {}
> if (Empty && Error) {}
> if (Empty && Empty) {}
> The question really boils done to what influence Empty has. Do we want
> it to influence the results, or should it act like a "don't care" value?

That's exactly the nub of the problem. Which ought to be dominant
over the other, empty or errored? i.e. is it:

Empty < Errored < Valued

i.e. Empty && Errored = Empty


Errored < Empty < Valued

i.e. Empty && Errored = Errored.

Me personally, I chose the Kleene logic because I felt Empty is like
NaN in floating point, so it is always dominant.

And I suppose we can retain that, even with Errored => False, Empty
=> Unknown, with the appropriate truth tables.

The tricky part is which is the best design? I'm not sure if that's
answerable from a purely top down approach.


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