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

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]

[-- Attachment #1: Type: text/plain, Size: 55 bytes --]

Given a file foo.el that defines a pure function foo:


[-- Attachment #2: foo.el --]
[-- Type: application/emacs-lisp, Size: 122 bytes --]

[-- Attachment #3: Type: text/plain, Size: 58 bytes --]


and another file bar.el that mutates the result of foo:


[-- Attachment #4: bar.el --]
[-- Type: application/emacs-lisp, Size: 99 bytes --]

[-- Attachment #5: Type: text/plain, Size: 3633 bytes --]


byte-compiling bar.el emits a mutate-constant warning, even though the
result of foo is a fresh list:

$ for f in foo bar; do emacs -Q -batch -L . -f batch-byte-compile $f.el; done

In toplevel form:
bar.el:3:2: Warning: ‘setcar’ on constant list (arg 1)

Am I missing something?

Thanks,

-- 
Basil

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

[Mattias Engdegård]

17 juni 2023 kl. 13.32 skrev Basil Contovounesios <contovob@tcd.ie>:

> Given a file foo.el that defines a pure function foo:

> (defun foo (x) (declare (pure t)) (list x))

> and another file bar.el that mutates the result of foo:

> (setcar (foo nil) t)

> byte-compiling bar.el emits a mutate-constant warning, even though the
> result of foo is a fresh list:

No, the `pure` declaration means that the function can be evaluated at compile time which the compiler happily does, yielding a constant list, which your code then attempts to modify.

This is why the function `list` itself is not declared `pure` -- while it does look like a pure function when speaking informally, users relies on it returning a freshly allocated list that can be modified and that makes it non-pure. (If lists were immutable, then `list` would naturally be pure.)

Only the mutate-constant warning is new here; previously, the compiler would have let you make this mistake undisturbed.

Thus either you remove the pure-declaration from your function, or you don't mutate what it returns.

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]

Mattias Engdegård [2023-06-17 13:58 +0200] wrote:

> 17 juni 2023 kl. 13.32 skrev Basil Contovounesios <contovob@tcd.ie>:
>
>> Given a file foo.el that defines a pure function foo:
>
>> (defun foo (x) (declare (pure t)) (list x))
>
>> and another file bar.el that mutates the result of foo:
>
>> (setcar (foo nil) t)
>
>> byte-compiling bar.el emits a mutate-constant warning, even though the
>> result of foo is a fresh list:
>
> No, the `pure` declaration means that the function can be evaluated at
> compile time which the compiler happily does, yielding a constant
> list, which your code then attempts to modify.
>
> This is why the function `list` itself is not declared `pure` -- while it does
> look like a pure function when speaking informally, users relies on it returning
> a freshly allocated list that can be modified and that makes it non-pure. (If
> lists were immutable, then `list` would naturally be pure.)
>
> Only the mutate-constant warning is new here; previously, the compiler
> would have let you make this mistake undisturbed.
>
> Thus either you remove the pure-declaration from your function, or you
> don't mutate what it returns.

Right.

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).

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?

Thanks,

-- 
Basil

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

[Mattias Engdegård]

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.

> 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.

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]

tags 64127 notabug wontfix
close 64127
quit

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