Re: [boost] Ternary logic programming

Subject: Re: [boost] Ternary logic programming
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2015-07-02 19:45:08

On 3/07/2015 08:54, Andrzej Krzemienski wrote:
> A mongel monad has three states. But there are many things out there that
> have three states, and it does not mean they have to be mapped on a tribool
> type. For instance, a file can be empty or can contain some data or be
> non-existent. But why would I ever think of mapping it onto a tribool?

This instantly reminded me of this:
   http://thedailywtf.com/articles/What_Is_Truth_0x3f_

> The reason people choose to use Boolean values is because at some point
> they need Boolean arithmetic. Unless I am missing something, you need not
> exercise the Boolean algebra on the state of the result of the asynchronous
> operation. It seams that the conceptual mapping of the state of the monad
> onto a Boolean type brings no value to the programmers that will be
> launching asynchronous tasks.

I can see some logic in the mapping {has_value() => true, 
has_error/exception() => false, !ready() => indeterminate}.  However 
from the sounds of it this is not the mapping that is in the current 
code (I haven't looked too closely).

I can also see some reasonable argument for {ready() => true, !ready() 
=> false}, with no third possibility.

(In terms of boolean arithmetic, both of the above are still potentially 
useful if eg. a bit of code wants to test if three futures are all ready 
[without waiting for them] it could use a && b && c... but the brevity 
could be confusing if you don't know *which* of those mappings were used.)

For the avoidance of confusion, I'm inclined to agree with Andrzej, that 
at least initially it's probably better not to attempt to provide any 
sort of bool/tribool implicit mapping, and force the user to be explicit 
about what question they're asking.  Such things can be added later if 
it turns out that there is a clear consensus on what it should actually 
mean.