bug#18837: 25.0.50; `sort` -- modify function to preserve original list.

bug#18837: 25.0.50; `sort` -- modify function to preserve original list.

[Keith David Bershatsky]

The doc-string for the function `sort` states that `SEQ is modified by side effects`; however, it does not state in what way the original list gets modified or why such a feature might somehow be useful.  I attempted to find out the answer to this question on `superuser.com`, however, the community has expressed an interest in closing the question without offering an explanation on this issue:

http://superuser.com/questions/831608/emacs-sort-usefulness-of-side-effect-modifying-initial-list

In my use case, I want to preserve an initial list that is set to a let-bound variable so that I can use it again elsewhere in the function.  To achieve this objection, I used `copy-list`.

Unless the modified `SEQ` is somehow useful, perhaps it would be appropriate to modify the function `sort` so that it preserves the initial list.

Thanks,

Keith

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In GNU Emacs 25.0.50.1 (x86_64-apple-darwin10.8.0, NS appkit-1038.36 Version 10.6.8 (Build 10K549))
 of 2014-10-01 on MP.local
Repository revision: 117996 dmantipov@yandex.ru-20141001132108-zdsxru2390mqyjlu
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bug#18837: 25.0.50; `sort` -- modify function to preserve original list.

[Stefan Monnier]

> The doc-string for the function `sort` states that `SEQ is modified by
> side effects`; however, it does not state in what way the original
> list gets modified or why such a feature might somehow be useful.

The "usefulness" is to avoid allocating another list in the case where
the unsorted list won't be used later on anyway.


        Stefan

bug#18837: 25.0.50; `sort` -- modify function to preserve original list.

[Keith David Bershatsky]

Thank you, Stefan, for taking a look at this Emacs report (#18837).

Here is my workaround:

    (defun lawlist-sort (seq predicate)
      (let ((disposable-list (copy-list seq)))
        (sort disposable-list predicate)))

There may be situations where `copy-list` might not work, but that is beyond my present level of understanding / knowledge.  I suppose that there might be some time consumption issues involved with the function `copy-list` for extremely complex lists, but none of my lists would ever get so large/complex that this would be an issue.

Thanks,

Keith

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At Sat, 25 Oct 2014 22:44:45 -0400,
Stefan Monnier wrote:
> 
> > The doc-string for the function `sort` states that `SEQ is modified by
> > side effects`; however, it does not state in what way the original
> > list gets modified or why such a feature might somehow be useful.
> 
> The "usefulness" is to avoid allocating another list in the case where
> the unsorted list won't be used later on anyway.
> 
> 
>         Stefan

bug#18837: 25.0.50; `sort` -- modify function to preserve original list.

[Stefan Kangas]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

>> The doc-string for the function `sort` states that `SEQ is modified by
>> side effects`; however, it does not state in what way the original
>> list gets modified or why such a feature might somehow be useful.
>
> The "usefulness" is to avoid allocating another list in the case where
> the unsorted list won't be used later on anyway.

It doesn't seem like there is anything more to do here, so I'm closing
this bug.

Best regards,
Stefan Kangas