[squeak-dev] [bug] xor: is this an old bug or what?

Keith Hodges wrote:

true xor: [ true ]  => true

I don't think it's a bug. Unless you want to sprinkle lots of code with type tests, the result is whatever the implementation says, in this case:

xor: aBoolean
       "Exclusive OR. Answer true if the receiver is not equivalent to aBoolean."

       ^(self == aBoolean) not

true xor: [ true ]  => true

discuss

Keith

1) #xor: should not take a block as argument.
Rationale:
- the block would ALWAYS have to be evaluated
- block arguments are either for repeated evaluation, or condiitonal evaluation...
This is not the case here, so the need of a block argument vanishes...

2) #xor: argument should be a Boolean, but here no guard is made.

We could eventually add:
   aBoolean isBoolean ifFalse; [self error: 'xor must take a boolean as argument']

Or if we want to later extend to some other class:
True>>xor: aBoolean
    ^aBoolean xorTrue
False>>xor: aBoolean
    ^aBoolean xorFalse
True>>xorTrue
    ^false
True>>xorFalse
    ^self
False>>xorTrue
    ^true
False>>xorFalse
    ^self

MySymbolicBooleanClass>>xorTrue
    ^SymbolicXorOperation with: self with: true

I'm not sure whether we need to protect or not...
I tend to avoid over-protecting code. As long as a debugger opens, I claim protecting is void...

But here, I don't like a false program proceeding without a Notifier....

Nicolas

2009/3/24 Keith Hodges <[hidden email]>

true xor: [ true ]  => true

discuss

Keith

>>>>> "Eliot" == Eliot Miranda <[hidden email]> writes:

Eliot> I think just

True> xor: aBoolean
Eliot>     ^aBoolean not

False> xor: aBoolean
Eliot>     ^aBoolean

Eliot> and then leave subsequent usage to catch possible type errors; e.g. (false
Eliot> xor: #blah) ifTrue: ... will raise a mustBeBoolean error.

What I don't like about this is that the right operand doesn't get a chance to
"boolify" itself, or define its own xor logic.  The double-dispatch versions
were a lot better at that.

--
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<[hidden email]> <URL:http://www.stonehenge.com/merlyn/>
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On Tue, 24 Mar 2009 23:01:45 +0100, Eliot Miranda wrote:

On Tue, Mar 24, 2009 at 1:36 PM, Randal L. Schwartz wrote:

>>>>> "Eliot" == Eliot Miranda <[hidden email]> writes:

Eliot> I think just

True> xor: aBoolean
Eliot>     ^aBoolean not

False> xor: aBoolean
Eliot>     ^aBoolean

Eliot> and then leave subsequent usage to catch possible type errors; e.g.
(false
Eliot> xor: #blah) ifTrue: ... will raise a mustBeBoolean error.

What I don't like about this is that the right operand doesn't get a chance
to
"boolify" itself, or define its own xor logic.  The double-dispatch
versions
were a lot better at that.

If you want to do that you could implement it as

False> xor: aBoolean
   ^aBoolean not not

This is, by Randal E. Bryant (who's using Shannon's expansion), equivalent to

 ^ aBoolean ifTrue: [true] ifFalse: [false]

which the Squeak inlining compiler's magic + decompiler transforms to

 ^ aBoolean and: [true]

When implemented in one of these two forms, a comment can explain that it's a not not implementation.

but I'd argue that isn't necessary. The old code didn't type check and
we've lived with it for a loooong time.

I think that Randal makes the point; especially when the result is stored for later.

The other boolean messages do implicit (by the inlining compiler) check their argument for #isBoolean, that's what should be done by #xor: as well.

But in any case, the not is a form of double-dispatch.  It just points out
that one can use not instead of xorTrue and is more comprehensible because "not" is familiar.

--
Randal L. Schwartz - Stonehenge Consulting Services, Inc. - +1 503 777 0095
<[hidden email]> <URL:http://www.stonehenge.com/merlyn/>
Smalltalk/Perl/Unix consulting, Technical writing, Comedy, etc. etc.
See http://methodsandmessages.vox.com/ for Smalltalk and Seaside
discussion

--
"If at first, the idea is not absurd, then there is no hope for it". Albert Einstein