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
