bug#64127: 30.0.50; mutate-constant warning with pure function

[Basil Contovounesios via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

tags 64127 notabug wontfix
close 64127
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Mattias Engdegård [2023-06-17 19:03 +0200] wrote:

> 17 juni 2023 kl. 18.28 skrev Basil Contovounesios <contovob@tcd.ie>:
>
>> Which approach do you think the dash.el library in GNU ELPA should
>> follow?  It generally defines nondestructive operations over lists, some
>> of which even claim in their docstring that they return a partial or
>> complete copy of their arguments (modulo the parts changed by the
>> operation).
>
> Its doc strings seem to talk a lot about how the functions returns a new this or
> a copy of that. Maybe that provides the licence to return a constant when those
> words are absent, or perhaps the users will just assume mutability in absence of
> stern warnings. I don't know how dash.el is used in practice, so perhaps it's
> prudent to stay off the `pure` declarations.

Done for some of the likelier candidates in
https://github.com/magnars/dash.el/commit/d5182da04c.

>> Is it okay for a pure function to say it returns a copy in its
>> docstring, with the onus lying on the caller to realise that a pure
>> function call may be byte-compiled to a runtime constant?  Or should all
>> such functions be impurified?
>
> A pure function cannot in general be guaranteed to return an eq-unique
> value. By definition it will, if all its arguments are constants, be
> called at compile-time to generate a constant used in the program.
>
> There is nothing wrong with returning a newly created object from a
> `pure`-declared function, as long as reasonable steps are taken to prevent the
> returned value from being mutated. Depending on the context this can be as
> simple as not saying that it returns a new object.

Makes sense, thanks for elaborating.

-- 
Basil