Hi Robert,
>> One of the main problem here is that PEP 335 "Overloadable Boolean
>> Operators"
>> is not yet accepted. So right now there is no way to implement a three
>> state
>> logic, is there one ? If not, Is there a way we can push on python dev to
>> have
>> this PEP accepted ?
>
> It's unclear how this would work with the current short-circuit semantics
> of and and or.
Here are the truth table in MuPAD, all computed in short-circuit semantics.
>> bool3 := [FALSE, UNKNOWN, TRUE];
>> print ([(a and b) $ b in bool3]) $ a in bool3
[FALSE, FALSE, FALSE]
[FALSE, UNKNOWN, UNKNOWN]
[FALSE, UNKNOWN, TRUE]
>> print ([(a or b) $ b in bool3]) $ a in bool3
[FALSE, UNKNOWN, TRUE]
[UNKNOWN, UNKNOWN, TRUE]
[TRUE, TRUE, TRUE]
To have a precise short-circuit semantic. Consider that
False < Unknown < True
and define "and" = "min" and "or" = "max". The natural short-circuit
evaluation for min and max works as expected.
in preudocode:
if a is False: return False
else if a <= b: return a
else: return b
in preudocode:
if a is True : return True
else if a >= b: return a
else: return b
> However, if Unknown was returned such that bool(Unknown) was False, then the
> information would be preserved at least a little bit longer.
This could indeed be a fallback.
Cheers,
Florent
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