Re: [PATCH] seq.el: add seq-last for symmetry with seq-first

[Augusto Stoffel]

On Tue, 14 Mar 2023 at 22:04, Philip Kaludercic wrote:

>>> Maybe it's more useful to allow negative arguments in seq-elt?  Saying
>>> (seq-elt seq -1) isn't much more effort than (seq-last seq).
>>
>> I'm personally a bit ambiguous about negative indices.  It's nice for
>> those one liners when you need them, but they are quite confusing
>> especially if you switch between languages and they all implement them a
>> bit differently.
>
> The only language I am really familiar with is python, and what that
> effectively does is (mod i (length n)), what do other languages do?

In Python "abc"[-4] throws and error.  I don't think there's any choice
to be made here: If it were to support a negative index n, then
(seq-elt s n) should just return the (+ (length s) n)-th element, with
the usual treatment for out of bounds indices (whatever it is).

> Another thing that should be kept in mind that sequences can be streams
> (as provided by stream.el), and there doesn't have to be a final
> element.

What is the seq-length of an infinite stream?  And does asking for the
length of a stream consume it?  If so, then seq is an imperfect
abstraction for streams.