From: Drew Adams <drew.adams@oracle.com>
To: Emanuel Berg <incal@dataswamp.org>,
"help-gnu-emacs@gnu.org" <help-gnu-emacs@gnu.org>
Subject: RE: [External] : Re: Setting up abbrev
Date: Mon, 1 Aug 2022 16:36:23 +0000 [thread overview]
Message-ID: <SJ0PR10MB548892C62DB800E5326EEB1EF39A9@SJ0PR10MB5488.namprd10.prod.outlook.com> (raw)
In-Reply-To: <87zggoatct.fsf@dataswamp.org>
> >> Only a pair is a list as well ...
> >
> > Yes. But a list isn't a pair. ;-)
>
> Every pair is a list but every list isn't a pair ...
|
not every list is a pair
"Not every X is" = "some X isn't" != "every X isn't"
> Well, maybe one can think of a pair without order, i.e.
> a set of two items?
>
> The _ordered_ pair is the universal data structure that can
> express anything then. The list is equally universal but the
> ordered pair is the minimal working example, to speak with the
> LaTeX guys ...
Yes, ordered pair. That's a cons.
> > As for the (a . b) _notation_: Why not? JSON uses `:'.
> > Prolog uses `|'. Doesn't matter much what notation you
> > choose. OOPs often use a dot notation for method
> > application. Six of one; half a dozen of another...
>
> Okay, but I still don't understand the benefit of using it in
> practice compared to the list (a b)?
Do you mean use the _notation_? I guess you instead
mean use a dotted list: a list whose last cdr is a
non-nil atom.
If so, the answer is that in general you do want to
use a true/proper list (last cdr is nil), and NOT a
dotted list. For many reasons, not least of which
is using a function that needs to traverse all list
elements (e.g. `length', mapping/sequence functions).
You can use a dotted list in these cases:
1. You want to save conses (not create so many), in
a context where you're not going to be needing to
use the cons as a list (mapping etc.), or a context
where you know you'll only ever want an atom cdr.
2. You want/need, in effect, to have a backwards
key-value pair, (list-value . atom-key). You have
a list to use for most purposes (ignoring the last
cdr), and you have an atom (the cdr) for a few rare
purposes, as a kind of label for the list.
#2 is usually the result of trying to adapt to
legacy code that expects to treat just a list (and
doesn't need to follow it to its end). IOW, #2 is
typically an ugly hack. For new code you'd instead
just put the atom first: (atom-key . list-value).
next prev parent reply other threads:[~2022-08-01 16:36 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-07-31 12:23 Setting up abbrev wilnerthomas--- via Users list for the GNU Emacs text editor
2022-07-31 13:08 ` Jean Louis
2022-08-01 3:06 ` Emanuel Berg
2022-08-01 6:04 ` Jean Louis
2022-08-01 7:42 ` Emanuel Berg
[not found] ` <YuZ+ucBrwA9vOR/F@protected.localdomain-N8JMuQk----2>
2022-07-31 15:35 ` wilnerthomas--- via Users list for the GNU Emacs text editor
2022-07-31 16:40 ` [External] : " Drew Adams
2022-08-01 6:21 ` Jean Louis
2022-08-01 6:05 ` Jean Louis
2022-07-31 20:02 ` kf
2022-08-01 3:03 ` Emanuel Berg
2022-08-01 10:41 ` uzibalqa
2022-08-01 11:39 ` Emanuel Berg
2022-08-01 14:56 ` [External] : " Drew Adams
2022-08-01 15:14 ` Emanuel Berg
2022-08-01 16:36 ` Drew Adams [this message]
2022-08-01 16:57 ` wilnerthomas--- via Users list for the GNU Emacs text editor
2022-08-01 18:36 ` Emanuel Berg
2022-08-01 19:20 ` Drew Adams
2022-08-01 16:56 ` tomas
2022-08-01 18:40 ` Emanuel Berg
2022-08-01 20:37 ` tomas
2022-08-01 11:33 ` carlmarcos--- via Users list for the GNU Emacs text editor
2022-08-01 11:42 ` Emanuel Berg
2022-08-01 12:54 ` carlmarcos--- via Users list for the GNU Emacs text editor
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