Do you like bounded integrals?

[Lodovico Giaretta via Digitalmars-d]

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. 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.

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.