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 <eliot.miranda at gmail.com> 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
>> <merlyn at stonehenge.com> <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