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Re: 7 logical-xor implementations in source tree

From: Alan Mackenzie
Subject: Re: 7 logical-xor implementations in source tree
Date: Tue, 30 Jul 2019 09:36:51 +0000
User-agent: Mutt/1.10.1 (2018-07-13)

Hello, Marcin.

On Sun, Jul 28, 2019 at 21:43:19 +0200, Marcin Borkowski wrote:

> On 2019-07-28, at 10:04, Alan Mackenzie <address@hidden> wrote:

> > Hello, Philippe.

> > On Sun, Jul 28, 2019 at 09:09:01 +0200, Philippe Schnoebelen wrote:
> >> On 2019/07/25 14:07, Mattias Engdegård wrote:
> >> > 25 juli 2019 kl. 01.44 skrev Basil L. Contovounesios <address@hidden>:

> >> > bool-equal, bool-equiv, bool=, bool-eq are all fine as far as I'm 
> >> > concerned. `xnor' and `nxor', not so much.
> >> > Racket has `boolean=?', but presumably it only copes with #t/#f.
> >> > I'll be using `equiv' as placeholder below for brevity.

> >> I like the name `iff' for this function.

> > No, please don't use the name `iff' here.  In mathematical circles, iff
> > means "if and only if", and has done for many decades/several centuries.
> > Introducing it into Emacs with a radically different meaning will be
> > jarring in the extreme to anybody with a maths background.

> Out of curiosity: how is that a "radically different meaning"?  I assume
> that we are talking about a function `iff' such that
> (iff nil nil) evaluates to t
> (iff nil <non-nil>) evaluates to nil
> (iff <non-nil> nil) evaluates to nil
> (iff <non-nil> <non-nil>) evaluates to t (or perhaps the latter
> <non-nil>)

Er, it's not radically different.  My brain seems to have been switched
off when I wrote my last post.  Apologies.

Less importantly, I don't like iff being used in this way.  I'm not sure
why.  Maybe it's because I've been used to iff applying solely to TRUE
and FALSE.  Maybe it's that I've been used to iff declaring a
proposition, rather than being something to be calculated.

> This could of course be generalized to n arguments, though I'm not sure
> whether anyone would want that (as with xor, there is more than one
> "natural" way to do that).

> If so, this is precisely the meaning we are talking about, no?

> Also, Wikipedia claims that "iff" is relatively new (the fifties), btw.

Well, I think that counts as "many decades", even if not "several
centuries".  It was certainly in widespread use in the 1970s.

> Best,

> --
> Marcin Borkowski
> http://mbork.pl

Alan Mackenzie (Nuremberg, Germany).

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