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Subject: bug#71120: 29.3; buglet in cl-loop
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When I need a list of 100 random dice throws I write

	(cl-loop for i from 1 to 100 collect (random 6))

It compiles just fine.

If instead I use

	(cl-loop for _i from 1 to 100 collect (random 6))

then I get a compilation warning:

	foo.el:1:18: Warning: variable ‘_i’ not left unused

It should be the other way around.

The variable 'i' is unused in the first form and that deserves a warning 
at compile time. Otherwise the compiler will not help me catch the typo in

	(cl-loop for i from 0 to 10 do
	   (cl-loop for j from 0 to 10 do
	      (foo j j))) ;; <<<=== typo, I meant (foo i j)


--ph.schnoebelen, happy and thankful GNU Emacs user since 1983 (Thanks 
to all involved!!)

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From: help-debbugs@HIDDEN (GNU bug Tracking System)
To: Philippe Schnoebelen <phs@HIDDEN>
Subject: bug#71120: Acknowledgement (29.3; buglet in cl-loop)
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Date: Wed, 22 May 2024 14:49:03 +0000

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--=20
71120: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=3D71120
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Message sent to bug-gnu-emacs@HIDDEN:

Subject: bug#71120: 29.3; buglet in cl-loop
Resent-From: Philip Kaludercic <philipk@HIDDEN>
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To: Philippe Schnoebelen <phs@HIDDEN>
Cc: 71120 <at> debbugs.gnu.org, Mattias =?UTF-8?Q?Engdeg=C3=A5rd?= <mattiase@HIDDEN>, Stefan Monnier <monnier@HIDDEN>
From: Philip Kaludercic <philipk@HIDDEN>
In-Reply-To: <23fd70f8-6e01-45d7-b9f9-d81d53c95ffd@HIDDEN> (Philippe
 Schnoebelen's message of "Wed, 22 May 2024 10:43:36 +0200")
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Date: Wed, 29 May 2024 20:47:56 +0000
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Philippe Schnoebelen <phs@HIDDEN> writes:

> When I need a list of 100 random dice throws I write
>
> 	(cl-loop for i from 1 to 100 collect (random 6))
>
> It compiles just fine.
>
> If instead I use
>
> 	(cl-loop for _i from 1 to 100 collect (random 6))
>
> then I get a compilation warning:
>
> 	foo.el:1:18: Warning: variable =E2=80=98_i=E2=80=99 not left unused
>
> It should be the other way around.

The issue here is that the warning describes an issue in the output, in
the latter case

  (let* ((_i 1) (--cl-var-- nil))
    (while (<=3D _i 100)
      (setq --cl-var-- (cons (random 6) --cl-var--))
      (setq _i (+ _i 1)))
    (nreverse --cl-var--))

As you see, _i is both evaluated in (+ _i 1) and updated.

Here you have a minimal working example of the warning:

  (byte-compile (lambda () (let ((_x 3)) _x)))

  ;; Warning: variable =E2=80=98_x=E2=80=99 not left unused

My guess is that fixing this would require cl-loop to notice that the
counter is prefixed with a "_" and then use some other variable, but
that might lead to issues with existing code.  Perhaps this
transformation might be safe in that case:

  (let* ((<fresh-variable> 1) (--cl-var-- nil))
    (while (<=3D <fresh-variable> 100)
      (let ((_i <fresh-variable>))
	(setq --cl-var-- (cons (random 6) --cl-var--))
	(setq <fresh-variable> (+ <fresh-variable> 1))))
    (nreverse --cl-var--))

I have added Mattias and Stefan to the CCs, as they'll probably have
more qualified comments to add.

>
> The variable 'i' is unused in the first form and that deserves a
> warning at compile time. Otherwise the compiler will not help me catch
> the typo in
>
> 	(cl-loop for i from 0 to 10 do
> 	   (cl-loop for j from 0 to 10 do
> 	      (foo j j))) ;; <<<=3D=3D=3D typo, I meant (foo i j)
>
>
> --ph.schnoebelen, happy and thankful GNU Emacs user since 1983 (Thanks
>   to all involved!!)
>
>
>
>
>

--=20
	Philip Kaludercic on peregrine

Message sent to bug-gnu-emacs@HIDDEN:

Subject: bug#71120: 29.3; buglet in cl-loop
Resent-From: Andrea Corallo <acorallo@HIDDEN>
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From: Andrea Corallo <acorallo@HIDDEN>
In-Reply-To: <87cyp4wdj7.fsf@HIDDEN> (Philip Kaludercic's message of "Wed, 
 29 May 2024 20:47:56 +0000")
References: <23fd70f8-6e01-45d7-b9f9-d81d53c95ffd@HIDDEN>
 <87cyp4wdj7.fsf@HIDDEN>
Date: Wed, 29 May 2024 17:33:49 -0400
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Philip Kaludercic <philipk@HIDDEN> writes:

> Philippe Schnoebelen <phs@HIDDEN> writes:
>
>> When I need a list of 100 random dice throws I write
>>
>> 	(cl-loop for i from 1 to 100 collect (random 6))
>>
>> It compiles just fine.
>>
>> If instead I use
>>
>> 	(cl-loop for _i from 1 to 100 collect (random 6))
>>
>> then I get a compilation warning:
>>
>> 	foo.el:1:18: Warning: variable =E2=80=98_i=E2=80=99 not left unused
>>
>> It should be the other way around.
>
> The issue here is that the warning describes an issue in the output, in
> the latter case
>
>   (let* ((_i 1) (--cl-var-- nil))
>     (while (<=3D _i 100)
>       (setq --cl-var-- (cons (random 6) --cl-var--))
>       (setq _i (+ _i 1)))
>     (nreverse --cl-var--))
>
> As you see, _i is both evaluated in (+ _i 1) and updated.
>
> Here you have a minimal working example of the warning:
>
>   (byte-compile (lambda () (let ((_x 3)) _x)))
>
>   ;; Warning: variable =E2=80=98_x=E2=80=99 not left unused
>
> My guess is that fixing this would require cl-loop to notice that the
> counter is prefixed with a "_" and then use some other variable, but
> that might lead to issues with existing code.  Perhaps this
> transformation might be safe in that case:
>
>   (let* ((<fresh-variable> 1) (--cl-var-- nil))
>     (while (<=3D <fresh-variable> 100)
>       (let ((_i <fresh-variable>))
> 	(setq --cl-var-- (cons (random 6) --cl-var--))
> 	(setq <fresh-variable> (+ <fresh-variable> 1))))
>     (nreverse --cl-var--))
>
> I have added Mattias and Stefan to the CCs, as they'll probably have
> more qualified comments to add.

This is the same transformation that came to my mind reading the orginal
report, I think it should be safe.

BTW Philippe, you can workaround the bug with:

(cl-loop repeat 100 collect (random 6))

Bests

  Andrea

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Andrea Corallo <acorallo@HIDDEN> writes:

> Philip Kaludercic <philipk@HIDDEN> writes:
>
>> Philippe Schnoebelen <phs@HIDDEN> writes:
>>
>>> When I need a list of 100 random dice throws I write
>>>
>>> 	(cl-loop for i from 1 to 100 collect (random 6))
>>>
>>> It compiles just fine.
>>>
>>> If instead I use
>>>
>>> 	(cl-loop for _i from 1 to 100 collect (random 6))
>>>
>>> then I get a compilation warning:
>>>
>>> 	foo.el:1:18: Warning: variable =E2=80=98_i=E2=80=99 not left unused
>>>
>>> It should be the other way around.
>>
>> The issue here is that the warning describes an issue in the output, in
>> the latter case
>>
>>   (let* ((_i 1) (--cl-var-- nil))
>>     (while (<=3D _i 100)
>>       (setq --cl-var-- (cons (random 6) --cl-var--))
>>       (setq _i (+ _i 1)))
>>     (nreverse --cl-var--))
>>
>> As you see, _i is both evaluated in (+ _i 1) and updated.
>>
>> Here you have a minimal working example of the warning:
>>
>>   (byte-compile (lambda () (let ((_x 3)) _x)))
>>
>>   ;; Warning: variable =E2=80=98_x=E2=80=99 not left unused
>>
>> My guess is that fixing this would require cl-loop to notice that the
>> counter is prefixed with a "_" and then use some other variable, but
>> that might lead to issues with existing code.  Perhaps this
>> transformation might be safe in that case:
>>
>>   (let* ((<fresh-variable> 1) (--cl-var-- nil))
>>     (while (<=3D <fresh-variable> 100)
>>       (let ((_i <fresh-variable>))
>> 	(setq --cl-var-- (cons (random 6) --cl-var--))
>> 	(setq <fresh-variable> (+ <fresh-variable> 1))))
>>     (nreverse --cl-var--))
>>
>> I have added Mattias and Stefan to the CCs, as they'll probably have
>> more qualified comments to add.
>
> This is the same transformation that came to my mind reading the orginal
> report, I think it should be safe.

As a datapoint, SBCL does expands this with inside a

(declare (ignore i))

Unfortuantelly we don't support it :/ and our 'ignore' doesn't work here
as goes in the opposite direction :(

  Andrea

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>>   (let* ((<fresh-variable> 1) (--cl-var-- nil))
>>     (while (<= <fresh-variable> 100)
>>       (let ((_i <fresh-variable>))
>> 	(setq --cl-var-- (cons (random 6) --cl-var--))
>> 	(setq <fresh-variable> (+ <fresh-variable> 1))))
>>     (nreverse --cl-var--))
>>
>> I have added Mattias and Stefan to the CCs, as they'll probably have
>> more qualified comments to add.

Not really: There's a similar problem with `cl-destructuring-bind` and
similarly, when I started to look at the corresponding code I quickly
gave up.

At least for `cl-destructuring-bind` I think I understand the intended
behavior well enough that I might be able to implement code which
behaves as we want (tho I don't know how to *modify* the existing code
to reach that stage).

In the case of `cl-loop` the intended behavior is much too complex for
my poor head, so I don't think I'd be able to decide whether it does the
right thing for all cases.


        Stefan

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Philippe Schnoebelen <phs@HIDDEN> writes:

> When I need a list of 100 random dice throws I write
>=20
> 	(cl-loop for i from 1 to 100 collect (random 6))
>=20
> It compiles just fine.
>=20
> If instead I use
>=20
> 	(cl-loop for _i from 1 to 100 collect (random 6))
>=20
> then I get a compilation warning:
>=20
> 	foo.el:1:18: Warning: variable =E2=80=98_i=E2=80=99 not left =
unused

Quite unfair that you have no a priori way of knowing whether your =
variable name is actually the one that `cl-loop` uses for iteration or =
just one bound for each iteration (as in `dotimes`).

A sloppy reading of Common Lisp's `loop` spec, which we don't need to =
follow since this isn't CL but we'd be fools to deviate too far from =
without a good reason, didn't tell me anything.

In particular nothing about whether the user is allowed to alter the =
variable in order to change the iteration. For example, what should

  (cl-loop for i from 1 to 100
           when (=3D i 3) do (setq i 98)
           collect i)

return? Perhaps better not touch that.

Thus I don't think there's anything we really need to do here, do you?

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Mattias Engdeg=C3=A5rd <mattias.engdegard@HIDDEN> writes:

> Philippe Schnoebelen <phs@HIDDEN> writes:
>
>> When I need a list of 100 random dice throws I write
>>=20
>> 	(cl-loop for i from 1 to 100 collect (random 6))
>>=20
>> It compiles just fine.
>>=20
>> If instead I use
>>=20
>> 	(cl-loop for _i from 1 to 100 collect (random 6))
>>=20
>> then I get a compilation warning:
>>=20
>> 	foo.el:1:18: Warning: variable =E2=80=98_i=E2=80=99 not left unused
>
> Quite unfair that you have no a priori way of knowing whether your variab=
le name is actually the one that `cl-loop` uses for iteration or just one b=
ound for each iteration (as in `dotimes`).
>
> A sloppy reading of Common Lisp's `loop` spec, which we don't need to fol=
low since this isn't CL but we'd be fools to deviate too far from without a=
 good reason, didn't tell me anything.
>
> In particular nothing about whether the user is allowed to alter the vari=
able in order to change the iteration. For example, what should
>
>   (cl-loop for i from 1 to 100
>            when (=3D i 3) do (setq i 98)
>            collect i)
>
> return? Perhaps better not touch that.

If the spec doesn't say anything usually means it's left to the
implementors (IOW it's UB).

> Thus I don't think there's anything we really need to do here, do you?

I, for one, think the nicest option is the one SBCL (and I guess other
CL implementations) are using, that is to have 'i' 'ignorable', the
problem is that we don't support this at language level.

  Andrea

Last modified: Thu, 30 May 2024 13:30:03 UTC

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Copyright (C) 1999 Darren O. Benham, 1997 nCipher Corporation Ltd, 1994-97 Ian Jackson.