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

[Joost Kremers]

On Tue, Nov 12 2024, arthur miller wrote:
> The scope of the let-binding is the same in both.

I don't see how. With `while`, `result` is let-bound outside the loop, with
`while-let` it's bound inside the loop, even after macroexpanding it:

```
(macroexpand-all '(while-let ((result (foo)))
                    (bar result)))

==> 

(catch 'done15
  (while t
    (let*
        ((result
          (and t
               (foo))))
      (if result
          (progn
            (bar result))
        (throw 'done15 nil)))))
```

In fact, with `while`, you can use `result` even before you enter the loop:

```

(let ((result (do-something)))
  (do-something-with result)
  ...
  (while result
    (do-something-else-with result)
    (setq result (do-yet-another-thing))))
```

So clearly the scope of `result` is wider.

> The crucial
> difference is that in the first example, the user have control over
> when and how 'result' is evaluated. The user can for example do:
>
> (let ((result (do-computation)))
>   (while result
>     (do-stuff-with result)
>     (setq result (do-some-other-computation))))

Yes, and if that's what you need, you should use `while`.

> Whereas in the other example, the code is automatically generated
> to pass in the original condition calculation, and the user can not
> interfere with the computation of the condition.

Yes, but that's exactly the point of `while-let`. `while-let` is not there
to replace `while`, it's a different macro for a different (though related)
purpose.

Mind you, what you want to do can still be done with `while-let`, provided
you establish the relevant binding *outside* the loop:

```
(let ((continue t))
  (while-let ((a (foo))
              (b (bar))
              (  continue))
    (do-some-stuff)
    ...
    (when (stop-condition)
      (setq continue nil))))
```

I think I understand where your confusion is coming from, and I've tried to
address it in my proposed patch for the documentation. If you feel it's
still not clear enough, I'll happily take another look.

-- 
Joost Kremers
Life has its moments