On Wed, 14 Jan 2015 23:23:54 -0800
Andrew Robinson <andr...@r3dsolutions.com> wrote:
>
> > Boolean algebra has two values: true and false, or 1 and 0, or humpty
> > and dumpty, or whatever you like to call them.
> You're speaking to an Electrical engineer. I know there are 10 kinds of
> people, those who know binary and those who don't. But you're way off
> base if you think most industrial quality code for manipulating boolean
> logic uses (or even can be restricted to use) only two values and still
> accomplish their tasks correctly and within a finite amount of time.
>
[snip]
Absolutely it does and can. You store anything that's non-boolean in a
non-boolean value, and keep it as fuzzy as you like. But at the end of
the day, an if statement has no "kinda". You do, or you don't. 1 or
0. And so you must ultimately resolve to a Boolean decision.
> >
> > Can you name any other language that *does* allow subclassing of
> > booleans or creation of new boolean values?
> Yes. Several off the top of my head -- and I have mentioned these
> before. They generally come with the extra subclasses pre-created and
> the user doesn't get to create the classes, but only use them; none the
> less -- they have more than two values with which to do logic equations
> with.
>
> VHDL, Verilog, HDL, Silos III, and there are IEEE variants also.
> C/C++ historically allowed you to do it with instances included,
> although I am not sure it still does.
>
Incorrect, at least in VHDL. If I've got "signal x : boolean;", then x
is defined on the range (true, false). I can ask VHDL "if x then; ...;
end if;"
What you're talking about is not at all a subclass of boolean, it's a
std_logic. It's a separate enumerated type, with a set of resolution
rules defined in a function the source of which is available in
std_logic_1164.vhd. And according to the strict rules of VHDL (up
until VHDL2008 decided to forfeit some purity for simplicity's), you
can't simply have:
signal x: std_logic
...
if x then
You have to ask
if x = '1'
or
if (x = '1') or (x = 'H')
Because you are comparing one value of an enumerated type against
others, the result of that '=' operation being, in fact, a boolean,
defined again on the range (true, false).
> [snip]
>
> We've discovered that we live in a quantum-mechanical universe -- yet
> people still don't grasp the pragmatic issue that basic logic can be
> indeterminate at least some of the time ?!
>
But actions can't be. You're not asking the software about it's
feelings, you're telling it to follow a defined sequence of
instructions. Do this, or don't do this.
> I don't know what you mean about composition vs. sub-classing.
> Would you care to show an example of how it would solve the problem and
> still allow hierarchical sorting ?
>
> I don't see how you can get pre-existing python functions (like sort,
> max, and min) to accept a complex bool value and have that value
> automatically revert to a True or False in an intelligent manner without
> overloading the operators defined in some class -- somewhere -- to
> operate on the data the class contains.
>
> How do you intend to achieve that with this -- composition -- thing you
> mentioned ?
>
>
You'd do it something like this.
class _clogic(object):
"""Represents 'continuous' logic. For any given instance,
there is a threshold value between 0 and 1 that delineates True from
False, with 0 being entirely False and 1 being entirely True.
"""
def __init__(self, value, threshold=0.5):
self.value = value
self.threshold = threshold
def __bool__(self):
return value > threshold
__nonzero__ = __bool__
def __eq__(self, other):
if other in (True, False):
return bool(self) == other
try:
return self.value == other.value
except AttributeError:
return self.value == other
def __and__(self, other):
return self.__class__(
min(self.value, other.value),
self.threshold)
def __or__(self, other):
return self.__class__(
max(self.value, other.value),
self.threshold)
No need to subclass bool, you just use the __bool__ (or __nonzero__)
methods to say "Alright, well when you do finally have to make a
decision on this thing, here's how you make it. And obviously, you can
add on as many additional data members to carry additional information
as your heart desires.
--
Rob Gaddi, Highland Technology -- www.highlandtechnology.com
Email address domain is currently out of order. See above to fix.
--
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