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Re: Predicate for true lists
From: |
Basil L. Contovounesios |
Subject: |
Re: Predicate for true lists |
Date: |
Mon, 09 Jul 2018 01:38:53 +0300 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux) |
Drew Adams <address@hidden> writes:
> 2. You say this in the doc string of `proper-list-length':
>
> A proper list is neither circular nor dotted (i.e., its
> last cdr is nil).
>
> And your entry in the manual for that function:
>
> In addition to satisfying @code{listp}, a proper list is
> neither circular nor dotted.
>
> Fair enough, but it depends on what is meant by a "circular"
> list. Does it mean only a list whose last cdr shares list
> structure with some other of its cdr's? If so then what you
> say holds. And yes, I guess that's what most of us think of.
>
> But what of a list that has a finite number of elements,
> so it last cdr is nil, but one or more of whose elements
> is itself a circular list in the above sense? Is it in
> some sense "circular" because of its circular-list element?
No, because "proper", "circular", and "dotted" all refer to list
structure (CDRs/links), not contents (CARs).
> It should be enough to say that a "true", or "proper"
> list is one that has nil as its last cdr, without
> adding that this means non-"circular".
I think that both "last cdr is nil" and "neither dotted nor circular"
completely qualify a list as being proper, so I would prefer to be
explicit and state both definitions, say, in the docstring. If nothing
else, it makes the jargon "dotted" and "circular" more discoverable.
Unless my reasoning/understanding is mistaken, that is.
--
Basil
- Re: Predicate for true lists, (continued)
- Re: [Emacs-diffs] master babe0d4: Rearrange definition of zerop in subr.el, Karl Fogel, 2018/07/11
- Re: [Emacs-diffs] master babe0d4: Rearrange definition of zerop in subr.el, Basil L. Contovounesios, 2018/07/11
- Re: [Emacs-diffs] master babe0d4: Rearrange definition of zerop in subr.el, Paul Eggert, 2018/07/11
- Re: [Emacs-diffs] master babe0d4: Rearrange definition of zerop in subr.el, Basil L. Contovounesios, 2018/07/12
- Re: [Emacs-diffs] master babe0d4: Rearrange definition of zerop in subr.el, Basil L. Contovounesios, 2018/07/12
RE: Predicate for true lists, Drew Adams, 2018/07/06
Re: Predicate for true lists, Paul Eggert, 2018/07/06