Hi Vincent,
sorry for my previous post, as you discussed the example already.
On 2016-11-15, Vincent Delecroix <20100.delecr...@gmail.com> wrote:
> There is still a semantic problem to solve the following
>
> sage: Unknown or False
> False
No. Unknown or False is Unknown.
The shortcuts would be
False and x = False
True and x = x
True or x = True
False or x = x
Unfortunately I don't see a shortcut when the first item is Unknown, for
"and" resp. "or".
But there is a further shortcut for xor:
Unknown xor x = Unknown
The shortcut
False xor x = x
follows from the and/or shortcuts, hence, is not new.
I believe "True xor x = not x" is not a shortcut, since the result is
obtained from an *operation* applied to the second argument.
Hence, implementing a shortcut for __and__ determines all shortcuts needed
for bools, but you need to implement a shortcut in yet another method
(namely __xor__) to determine further shortcuts needed for trools.
> Python is using __nonzero__ to deal with these or/and/not operators.
> This method __nonzero__ implicitly defines the conversion to booleans.
> And Python sets
>
> x or y := x if bool(x) is True and y otherwise
Is it not calling x.__or__(y), which could be overridden?
> It might be changed to
>
> x or y :=
> x if trool(x) is True
> y if trool(x) is False
> x if trool(x) is Unknown and trool(y) is not True
> y if trool(x) is Unknown and trool(y) is True
>
> I guess it is what you called "shortcut". In this form it is a max
> operation with True -> 1, Unknown -> 1/2, False -> 0.
I guess a shortcut means: If the first argument is suitable, then
the value of the expression either is a constant (True/False/Unknown)
or is equal to the second argument. If you need to evaluate trool(y)
or not(y) to evaluate the expression, then it is not a shortcut, IMHO.
Cheers,
Simon
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