From: "Pascal J. Bourguignon" <pjb@informatimago.com>
To: help-gnu-emacs@gnu.org
Subject: Re: How to mapcar or across a list?
Date: Thu, 16 Jul 2015 00:28:56 +0200 [thread overview]
Message-ID: <87oajdkqc7.fsf@kuiper.lan.informatimago.com> (raw)
In-Reply-To: mailman.6977.1436998965.904.help-gnu-emacs@gnu.org
Emanuel Berg <embe8573@student.uu.se> writes:
> 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
Nope.
It's always a bad idea to use eval, because eval evalutes in the nil
environment, not in the local lexical envrionment.
Your example works because those lists are literal, but then you don't
need or to know the answer.
In pratice, translating:
(flet ((some-complex-predicate (x) …))
(let ((some-data (generate-some-data)))
(some (function some-complex-predicate) some-data)))
to:
(flet ((some-complex-predicate (x) …))
(let ((some-data (generate-some-data)))
(eval `(or ,@(mapcar (lambda (data)
`(some-complex-predicate ',data))
some-data)))))
fails lamentably.
It is also very slow, since it makes you build a new list,
and it makes you interpret, or worse, compile, some new code!
--
__Pascal Bourguignon__ http://www.informatimago.com/
“The factory of the future will have only two employees, a man and a
dog. The man will be there to feed the dog. The dog will be there to
keep the man from touching the equipment.” -- Carl Bass CEO Autodesk
next prev parent reply other threads:[~2015-07-15 22:28 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
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 [this message]
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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