Sv: Is this a bug in while-let or do I missunderstand it?

[arthur miller <arthur.miller@live.com>]

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>> From: "Alfred M. Szmidt" <ams@gnu.org>
>> Cc: arthur.miller@live.com, emacs-devel@gnu.org
>> Date: Sat, 09 Nov 2024 11:33:45 -0500
>>
>>        (while-let ((run (some-condition)))
>>         (message "running"))
>>
>>    Do you expect that to evaluate (some-condition) once, then, if it’s
>>    initially true, run forever?
>>
>> That is how it is described in the manual, so yes (some-condition)
>> should only be done once, and not every iteration.  See (elisp)
>> Conditionals .
>
>Which could mean that the manual is wrong and needs to be fixed.
>The above description actually supports what Yuri was saying, not what
>Arthur and you expect.

Mnjah; if you consider this scatchy C:

{
  int foo = ..;
   ...
  for (int i=0, j=0; u < 10i++ )
  {
        do something with i, j
        .....
        do something with foo
  }

  i,j are not visible here
  ...
}

In other words, there might be variables live outisde of
the loop-scope we wish to access in the loop, and that is
what Yuri's example shows. However, i,j are not re-initiated
on each iteration, but remembers their value. The effecto of
while-let in current implementation is that i,j are re-initiated
in each iteration, not re-evaluated, if that makes it clear.

I am not sure how to illustrate in a better way. The net effect is
that lexical variables declared in while-let loop are "read-only".

They are not, but since they are re-iniated, it is pointless to
write to them.

Of course, all loop predicates should be evaled on each iteration,
but not re-iniated on each iteration. If that makes sense. Sorry,
I am not very good at writing.

When I see at my own KISS version, I see also it only initiates
variable, but does not re-evaluate function calls on each iteration;
I didn't really udnerstand it from the beginning, so this discussion
has cleared my mind a bit too.

However I am not sure exact how to fix it. But I believe a loop
where we can't update loop invariantes is a bit strange too.
________________________________
Från: Alfred M. Szmidt <ams@gnu.org>
Skickat: den 9 november 2024 17:33
Till: Yuri Khan <yuri.v.khan@gmail.com>
Kopia: arthur.miller@live.com <arthur.miller@live.com>; emacs-devel@gnu.org <emacs-devel@gnu.org>
Ämne: Re: Is this a bug in while-let or do I missunderstand it?

   > If it wasn't clear, the unintuitive part is that while-let was to
   > establish the local environment, so that we don't need to type:
   >
   > (let ((som-var (init-form)))
   >     (while some-var
   >         ... ))

   But if it did it that way, the condition (init-form) would only be
   evaluated once, and I’d find *that* counterintuitive. Consider the
   usual form of a while loop:

       (while-let ((run (some-condition)))
         (message "running"))

   Do you expect that to evaluate (some-condition) once, then, if it’s
   initially true, run forever?

That is how it is described in the manual, so yes (some-condition)
should only be done once, and not every iteration.  See (elisp)
Conditionals .

      It can be convenient to bind variables in conjunction with using a
   conditional.  It's often the case that you compute a value, and then
   want to do something with that value if it's non-‘nil’.  The
   straightforward way to do that is to just write, for instance:

        (let ((result1 (do-computation)))
          (when result1
            (let ((result2 (do-more result1)))
              (when result2
                (do-something result2)))))

      Since this is a very common pattern, Emacs provides a number of
   macros to make this easier and more readable.  The above can be written
   the following way instead:

... following the various with various FOO-let forms, ending with
while-let.

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