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

[John ff]

-------- Original Message --------
From: Eli Zaretskii <eliz@gnu.org>
Sent: Sat Nov 09 16:44:28 GMT 2024
To: "Alfred M. Szmidt" <ams@gnu.org>
Cc: yuri.v.khan@gmail.com, arthur.miller@live.com, emacs-devel@gnu.org
Subject: Re: Is this a bug in while-let or do I missunderstand it?

> 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.

>       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.

The above description actually supports what Yuri was saying, not what
Arthur and you expect.