Re: [boost] Ternary logic programming

Subject: Re: [boost] Ternary logic programming
From: Rob Stewart (rob.stewart_at_[hidden])
Date: 2015-07-05 06:10:37

On July 5, 2015 5:44:58 AM EDT, Bjorn Reese <breese_at_[hidden]> wrote:
> 
> We could regard Empty as "I don't care". You may have a set of these
> variables, and you have to take action at some point even if all the
> variables have not become Value or Error so far, but you still want
> an indication if none have been set. In that case, you get:
> 
>     Empty && Error => Error
>     Empty && Value => Value
>     Empty && Empty => Empty
> 
>     Empty || Error => Error
>     Empty || Value => Value
>     Empty || Empty => Empty

That might be legitimate, and even useful, but doesn't it seem wrong that && and || yield the same results?

> Another alternative is to regard Empty as "I do care", which makes it
> behave like a NaN-like construct. This makes Empty "sticky"; if only
> one of the variables is Empty then the entire condition should be
> Empty:
> 
>     Empty && Error => Empty
>     Empty && Value => Empty
>     Empty && Empty => Empty
> 
>     Empty || Error => Empty
>     Empty || Value => Empty
>     Empty || Empty => Empty

NaN is always a source of confusion at first, so that pattern may be a source of issues, too. It doesn't help that && and || behave alike, though for different reasons than before.

> I agree that "I may care" looks fine for futures, but since Niall is
> aiming for general framework of monads, I am less convinced that this
> approach is best approach for all. It may very well be that different
> monads require different three-valued logic.

If the empty state is called the same thing in each case, different behavior will be confusing. Otherwise, I agree that forcing everything into the same pattern may be more trouble than it's worth.

___
Rob

(Sent from my portable computation engine)