GNU bug report logs - #67456
[PATCH] seq.el: Add functions for mapping over subsequences

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Stefan Kangas <stefankangas <at> gmail.com>
Cc: tracker <at> debbugs.gnu.org
Subject: bug#67456: closed ([PATCH] seq.el: Add functions for mapping over
 subsequences)
Date: Tue, 25 Feb 2025 02:40:01 +0000

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From: Okamsn <okamsn <at> protonmail.com>
To: bug-gnu-emacs <at> gnu.org, Nicolas Petton <nicolas <at> petton.fr>
Subject: [PATCH] seq.el: Add functions for mapping over subsequences
Date: Sun, 26 Nov 2023 17:17:35 +0000

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Hello,

The attached features work like `cl-maplist` and `cl-mapl`, applying 
functions to a sequence and to the remaining parts of a sequence.  For 
example, `(seq-mapsub #'identity [1 2 3])` returns `([1 2 3] [2 3] [3])`.

The patch adds a `seq-mapsub`, `seq-dosub`, and a `seq-doseqsub`, 
similar to `seq-map`, `seq-do`, and `seq-doseq`, respectively.

I was looking for an equivalent for vectors of `cl-maplist`, `cl-mapl`, 
and `cl-loop`'s `for VAR on LIST`, and think that these would be useful 
additions.

To get the sub-sequences, the code uses `seq-rest` and stops when the 
returned sub-sequence is empty according to `seq-empty-p`.  This is 
similar to how I would do it for a list, using `cdr` and `null`, but is 
that a good way to do it for arrays and other sequences?

Thank you.

[0001-Create-seq-functions-for-mapping-over-subsequences.patch (text/x-patch, attachment)]

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From: Stefan Kangas <stefankangas <at> gmail.com>
To: Okamsn <okamsn <at> protonmail.com>
Cc: nicolas <at> petton.fr, arstoffel <at> gmail.com, 67456-done <at> debbugs.gnu.org
Subject: Re: bug#67456: [PATCH] seq.el: Add functions for mapping over
 subsequences
Date: Mon, 24 Feb 2025 20:39:31 -0600

Okamsn <okamsn <at> protonmail.com> writes:

> Stefan Kangas wrote:
>> Okamsn <okamsn <at> protonmail.com> writes:
>>
>>> Augusto Stoffel wrote:
>>>> This operation has quadratic complexity for anything other than regular
>>>> linked lists.  I'm not sure it's a good idea to add it to a generic
>>>> sequence library...
>>>
>>>
>>> Thank you for the feedback.
>>>
>>> Do you know whether there is a better way to implement the idea for
>>> arrays? Is there a way to apply a function to a portion of the array
>>> without copying the sub-sequences, if that is what you mean?
>>
>> Hmm, it seems like this issue got stuck due to problems with algorithmic
>> complexity.  Performance is a real issue in seq.el, so I think we should
>> solve that before we consider installing anything.
>>
>> Are you still working on this?
>
> I am no longer working on this.  For some things, I opted to use the
> Stream package from ELPA, which now better supports arrays.  Although it
> has the same problem when first traversing a seq.el sequence, it avoids
> recreating things for the sub-sequence.
>
> I am fine with this bug being closed.

Thanks, I guess that means that it's unlikely that we'll make more
progress here.  I'm therefore closing this bug report.

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