From: <tomas@tuxteam.de>
To: emacs-devel@gnu.org
Subject: Re: lists, plain conses and length
Date: Fri, 4 Sep 2020 10:53:30 +0200 [thread overview]
Message-ID: <20200904085330.GB25869@tuxteam.de> (raw)
In-Reply-To: <875z8ugtpg.fsf@gnus.org>
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On Fri, Sep 04, 2020 at 04:29:47AM +0200, Lars Ingebrigtsen wrote:
> "T.V Raman" <raman@google.com> writes:
>
> > (length a)=> Debugger entered--Lisp error: (wrong-type-argument listp 2)
> >
> > Q: how does one distinguish a cons from a list.
> > The error message for length may need fixing since it says the
> > argument passed to it is not a list (it's correct) but then listp is wrong?
>
> `length' (and many other functions) require what's called a "proper
> list", which simply means that the cdr of the last cons cell is nil.
>
> (length '(1 2 . 3))
> -> (wrong-type-argument listp 3)
>
> The predicate to use here is `proper-list-p'.
Note that `proper-list-p' has to run down the whole list to make
sure. For short lists, that's fine.
Cheers
- t
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prev parent reply other threads:[~2020-09-04 8:53 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-04 2:23 lists, plain conses and length T.V Raman
2020-09-04 2:26 ` Stefan Monnier
2020-09-04 14:19 ` T.V Raman
2020-09-04 2:29 ` Lars Ingebrigtsen
2020-09-04 8:53 ` tomas [this message]
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