From: Juri Linkov <juri@linkov.net>
To: "Basil L. Contovounesios" <contovob@tcd.ie>
Cc: "Mattias Engdegård" <mattiase@acm.org>,
"Andy Moreton" <andrewjmoreton@gmail.com>,
emacs-devel@gnu.org
Subject: Re: 7 logical-xor implementations in source tree
Date: Fri, 26 Jul 2019 00:46:22 +0300 [thread overview]
Message-ID: <87k1c523a5.fsf@mail.linkov.net> (raw)
In-Reply-To: <8736ixni7q.fsf@tcd.ie> (Basil L. Contovounesios's message of "Tue, 23 Jul 2019 11:44:25 +0100")
> That depends on the preferred semantics, right? What semantics are you
> suggesting? Judging from online discussions I've come across, there's
> more than one way to skin this cat.
Shouldn't xor have an associative property:
(xor a b c) ≡ (xor a (xor b c)) ≡ (xor (xor a b) c)
> OTOH, a definition like the following:
>
> (defsubst xor (cond1 cond2)
> (cond ((not cond1) cond2)
> ((not cond2) cond1)))
>
> not only performs simple logical xor more efficiently, but also returns
> whichever of the two arguments is non-nil (though I can't imagine when
> this last detail would ever be useful).
Returning a non-nil argument as-is is a useful property.
Most implementations lack this property, so better to use:
(unless (and a b) (or a b))
But the implementation with 'cond' is nice too.
Another useful property is to evaluate arguments only once
that currently is not the case:
(defvar a 0)
(defvar b 0)
(defmacro xor (a b) `(cond ((not ,a) ,b) ((not ,b) ,a)))
(xor (setq a (1+ a)) nil)
⇒ 2
But I don't know if an implementation with such property
is possible at all (without using temporary variables).
next prev parent reply other threads:[~2019-07-25 21:46 UTC|newest]
Thread overview: 51+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-22 18:48 7 logical-xor implementations in source tree Oleh Krehel
2019-07-22 21:47 ` Basil L. Contovounesios
2019-07-24 22:17 ` Basil L. Contovounesios
2019-07-24 23:15 ` Stefan Monnier
2019-07-24 23:44 ` Basil L. Contovounesios
2019-07-25 12:07 ` Mattias Engdegård
2019-07-25 17:28 ` Paul Eggert
2019-07-25 17:46 ` Mattias Engdegård
2019-07-28 7:09 ` Philippe Schnoebelen
2019-07-28 8:04 ` Alan Mackenzie
2019-07-28 19:43 ` Marcin Borkowski
2019-07-30 9:36 ` Alan Mackenzie
2019-07-30 10:57 ` Philippe Schnoebelen
2019-07-30 11:28 ` Andy Moreton
2019-07-30 12:34 ` Stefan Monnier
2019-07-30 14:25 ` Barry Fishman
2019-07-31 3:16 ` Richard Stallman
2019-07-31 15:20 ` Barry Fishman
2019-07-31 15:42 ` Stefan Monnier
2019-07-31 20:22 ` Basil L. Contovounesios
2019-07-31 21:15 ` Michael Heerdegen
2019-07-31 22:28 ` Mattias Engdegård
2019-07-31 23:39 ` Basil L. Contovounesios
2019-08-02 10:29 ` Mattias Engdegård
2019-08-02 10:59 ` Basil L. Contovounesios
2019-08-06 11:58 ` Mattias Engdegård
2019-07-31 20:31 ` Michael Heerdegen
2019-07-31 21:38 ` Drew Adams
2019-07-31 22:03 ` Drew Adams
2019-07-31 22:05 ` Basil L. Contovounesios
2019-07-31 23:20 ` Drew Adams
2019-08-01 0:09 ` Basil L. Contovounesios
2019-07-31 17:12 ` Marcin Borkowski
2019-07-23 8:39 ` Andy Moreton
2019-07-23 9:08 ` Basil L. Contovounesios
2019-07-23 9:14 ` Mattias Engdegård
2019-07-23 10:26 ` Andy Moreton
2019-07-23 10:40 ` Andreas Schwab
2019-07-23 12:11 ` Andy Moreton
2019-07-23 13:20 ` Basil L. Contovounesios
2019-07-23 13:54 ` Andy Moreton
2019-07-24 22:21 ` Basil L. Contovounesios
2019-07-23 10:44 ` Basil L. Contovounesios
2019-07-23 11:01 ` Yuri Khan
2019-07-25 21:46 ` Juri Linkov [this message]
2019-07-23 11:24 ` Noam Postavsky
2019-07-23 12:38 ` Stefan Monnier
2019-07-23 16:41 ` Mattias Engdegård
2019-07-23 17:12 ` Eli Zaretskii
2019-07-23 17:48 ` Paul Eggert
2019-07-23 19:45 ` Stefan Monnier
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