Oops, this should have been redirected to perl6-language@perl.org, so I'm doing that now.
--- Trewth Seeker <[EMAIL PROTECTED]> wrote: > Date: Fri, 6 May 2005 13:15:37 -0700 (PDT) > From: Trewth Seeker <[EMAIL PROTECTED]> > Subject: Re: Pugs 6.2.0 released. > To: "Mark A. Biggar" <[EMAIL PROTECTED]> > CC: perl6-compiler@perl.org > > > --- "Mark A. Biggar" <[EMAIL PROTECTED]> wrote: > > [EMAIL PROTECTED] wrote: > > > > > I see here another case of a common erroneous approach to > > > problem-solving. People are trying to enumerate definitions > to > > impose > > > on something, rather than starting with the thing at hand and > > > exhausting any clues it may provide before moving on. This can > > lead to > > > serious and, in hindsight, embarrassing mistakes. > > > > > > In this case, we are dealing with '^^', a meaningless > > unpronounceable > > > symbol. Oh, but wait ... we also spell it 'xor', which I > suppose > > is > > > often pronounced "ex or", which might be the source of the > > difficulty. > > > Because 'xor' stands for ... ... 'exclusive or'. Exclusive? > > It's not > > > hard to figure out what that means. Here are some of the > > relevant > > > dictionary definitions: > > > > > > Not allowing something else; incompatible: mutually > exclusive > > > conditions > > > Not accompanied by others; single or sole > > > > > > So what does that say about proposing that xor(p1,p2,...) is > true > > if an > > > odd number of p[i] are true? Other than that people should > > pronounce > > > these operators out loud more often? > > > > > > Clearly, xor is true iff *exactly* one of its arguments is > true, > > and of > > > course it should return that argument (or bool::false if no > > argument is > > > true). > > > > > > That should now be so blatantly obvious that everyone should be > > > embarrassed for not having seen it immediately. But don't run > > from > > > embarrassment (or become defensive and attack the messenger) -- > > it's a > > > powerful tool (which is why we evolved to have it). It should > > cause > > > one to question one's thought processes and consider how to > > improve > > > upon them, which is all to the good, isn't it? > > > > Except that xor or ^^ is only a binary operation, there is no > > "xor(p1,p2,...)", only "p1 xor p2 xor ..." which can really only > be > > > > understood if you add () to disambiguate the order that the > binary > > ops > > are performed. Fortunately, xor is associative so it doesn't > > matter how > > you add the (), you get the same answer. Try it out, you will > > discover > > that "p1 xor p2 xor ..." is true iff an odd number of the p's are > > true. > > As long as you build "p1 xor p2 xor ..." out of binary xor ops > > that is > > the result you get. Computing parity is much more common that > your > > > > multi-arg operation. Besides, all the literature about logic and > > > circuit design define "p1 xor p2 xor ..." in terms of binary xor, > > so > > your trying to buck hundreds of years of consensus. > > Sorry, but you're quite wrong here, because that literature applies > to an associative operator, which Perl's xor is not. ((1 xor 2) > xor > 3) == 3, while (1 xor (2 xor 3)) == 1. I again ask that you pay > more > attention to the thing you're dicussing, rather than to simply > generate stuff out of your own head, so as to avoid trivial > embarrassing error. You wrote q(Try it out, you will discover that > "p1 xor p2 xor ..." is true iff an odd number of the p's are true) > -- > this fails on two counts; 1) it begs the question, which was how > "p1 > xor p2 xor p3" should be evaluated -- it can only be "tried out" if > that answer has already been decided upon; and it ignores the > context of the discussion in which it has been noted that the value > of "p1 xor p2" is either p1 or p2 or bool::false, but not "true". > > The literature on logic design treats n-ary exclusive xor as > modular > arithmetic, which is natural to do since it is an associative > operator. But it provides no guidance as to the value of "1 xor 2 > xor 3", nor do your remarks, starting out with your claim that > there > is no "xor(p1,p2,...)". The language is being defined, so such > claims are meaningless; there is no reason why xor cannot be an > n-ary > function. My position is that it should either have its linguistic > meaning, or expressions of the form "p1 xor p2 xor p3" should be > disallowed, since there is no other natural meaning for it. > bool::true is not an acceptable result (unless one of p1, p2, or p3 > is bool::true), and simply picking one or the other of p1, p2, and > p3 > when they are all true is arbitrary. All of which was obvious if > you > were to follow my advice about problem solving. > > -- ts > > > > Yahoo! Mail > Stay connected, organized, and protected. Take the tour: > http://tour.mail.yahoo.com/mailtour.html > > __________________________________ Yahoo! Mail Mobile Take Yahoo! Mail with you! Check email on your mobile phone. http://mobile.yahoo.com/learn/mail
