Do you like bounded integrals?

[Andrei Alexandrescu via Digitalmars-d]

On 08/24/2016 05:05 AM, Lodovico Giaretta wrote:
> On Wednesday, 24 August 2016 at 00:40:15 UTC, Andrei Alexandrescu wrote:
>> That wouldn't work for e.g. NaN. A NaN wants to "steal" a value but
>> only if that's not available. It's complicated.
>
> Ok, I get your point. WithNaN should not waste the "official range" when
> there is a huge wasted representable range that can be exploited to
> choose the special NaN value.

Yes, thanks for divining the meaning from the poor explanation.

> While this would be a very good thing, it
> poses a problem.
>
> If a hook is allowed to special-case values outside the official range,
> then it may happen that composing two hooks, they both special-case the
> same value, leading to wrong results.
> For example, in some statistical oriented environments, the special
> value NA (not available), very similar to NaN, is used to represent
> missing input data; a WithNA hook may decide to use the same policy used
> by WithNaN to reserve its NA value, causing corruption, as the same
> value has two meanings.

Yes, that is indeed a potential problem.

> So, the first solution is that hooks should always reserve special
> values from the edges of the official range, and expose to the higher
> layer a correctly reduced official range.
>
> The second solution is having two ranges: the "official range" and the
> "usable range", where the usable range is the full representable range
> minus the special values already used. Hooks must take special values
> from the usable range and reduce it accordingly, while leaving the
> official range intact.
>
> The third (more complex) solution, which leaves the official range
> intact, is that hooks should take special values from wherever outside
> the official range and in some way communicate to the higher level which
> values are already taken. This is not impossible nor conceptually
> difficult, but it may not be worth.

The fourth solution is to document hooks appropriately and acknowledge 
the fact (which is already true) that not all hooks can work together.


Andrei