From: Emanuel Berg <embe8573@student.uu.se>
To: help-gnu-emacs@gnu.org
Subject: Re: How to mapcar or across a list?
Date: Thu, 16 Jul 2015 00:21:06 +0200 [thread overview]
Message-ID: <87fv4pukod.fsf@nl106-137-147.student.uu.se> (raw)
In-Reply-To: 87io9lmb4z.fsf@mbork.pl
Marcin Borkowski <mbork@mbork.pl> writes:
> so here's my problem: I have a list of Boolean
> values, and I want to `mapcar' an `or' across it
> (IOW, I want to test whether at least one of them is
> true). Of course, (apply #'or my-list) does not
> work. Of course, I can (cl-reduce (lambda (x y) (or
> x y)) my-list) -- but is there a better method?
"Better" I don't know, but how about using your
new-found skills with the backquote? (`or' itself
should be used, of course.)
(setq boolean-list-1 '(t nil nil nil t nil nil))
(setq boolean-list-2 '(nil nil nil nil nil nil nil))
(eval `(or ,@boolean-list-1)) ; t
(eval `(or ,@boolean-list-2)) ; nil
> BTW, my-list doesn't really exist: it is a result of
> `mapcar'ing a function taking some value and
> yielding a Boolean value, so bonus points if the
> method does not process the whole list.
Check. It is a consequence of using `or', which is
"short circuited" as it is called in other languages,
perhaps in Lisp too - I've read
"conditional evaluation" as well in AI books, but
perhaps that is something else.
Anyway:
(or t undefined) ; t
(or undefined t) ; error: (void-variable undefined) in eval
To verify, append "undefined" to "boolean-list-1"
(no error) - but, there will be an error (the same as
above) if appended to "boolean-list-2"!
... I suppose now would be a good time for you to hand
over them points!
--
underground experts united
http://user.it.uu.se/~embe8573
next prev parent reply other threads:[~2015-07-15 22:21 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-07-15 20:14 How to mapcar or across a list? Marcin Borkowski
2015-07-15 20:42 ` Rasmus
2015-07-15 20:55 ` Marcin Borkowski
2015-07-15 22:21 ` Emanuel Berg [this message]
2015-07-15 22:21 ` John Mastro
[not found] ` <mailman.6977.1436998965.904.help-gnu-emacs@gnu.org>
2015-07-15 22:28 ` Pascal J. Bourguignon
2015-07-15 22:36 ` Emanuel Berg
2015-07-15 22:57 ` never use `eval' (was: Re: How to mapcar or across a list?) Emanuel Berg
2015-07-15 23:27 ` John Mastro
2015-07-15 23:57 ` Emanuel Berg
2015-07-16 0:18 ` John Mastro
[not found] ` <mailman.6984.1437004760.904.help-gnu-emacs@gnu.org>
2015-07-16 0:04 ` Barry Margolin
[not found] ` <mailman.6982.1437001117.904.help-gnu-emacs@gnu.org>
2015-07-16 0:01 ` Barry Margolin
2015-07-16 1:13 ` Emanuel Berg
2015-07-17 0:55 ` Emanuel Berg
[not found] ` <mailman.7027.1437094623.904.help-gnu-emacs@gnu.org>
2015-07-17 1:39 ` never use `eval' Pascal J. Bourguignon
2015-07-17 2:16 ` Emanuel Berg
2015-07-17 8:36 ` Barry Margolin
2015-07-17 8:55 ` Pascal J. Bourguignon
2015-07-16 0:22 ` Pascal J. Bourguignon
2015-07-16 1:11 ` Emanuel Berg
[not found] <mailman.6968.1436991281.904.help-gnu-emacs@gnu.org>
2015-07-15 20:45 ` How to mapcar or across a list? Pascal J. Bourguignon
2015-07-15 21:02 ` Marcin Borkowski
2015-07-15 23:56 ` Barry Margolin
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