Re: Function value changes after running function?

[Okam <okamsn@protonmail.com>]

On 2/5/21 10:48 AM, Stefan Monnier wrote:
> 
>> No, I wrote a macro like `cl-loop` here:
>>
>> https://github.com/okamsn/loopy
>>
>> I received a bug report that said that the above function would raise an
>> error after running a second time here:
> 
> In the current situation, the problem is:
> 
>      (setq loopy--main-body (nreverse loopy--main-body)
>            loopy--with-vars (nreverse loopy--with-vars))
> 
> which reverses the "with vars" received as arguments.
> If you replace `nreverse` with `reverse` here, this particular problem
> should disappear.
> 
> This said, this `nreverse` reverses the order of the bindings in (with ....)
> which maybe isn't what you intended (I suspect that the `nreverse` here
> is meant to reverse the bindings added to `loopy--with-vars` via
> `push`).
> So maybe a better solution is to replace
> 
>          (setq loopy--with-vars (cdr arg)))
> 
> with
> 
>          (setq loopy--with-vars (reverse (cdr arg))))
> 
> so that the subsequent `nreverse` puts them back in the original order.
> 
> 
>          Stefan
> 
> 

Thank you.  That does prevent the error, but would you please explain 
(or point me to where I could learn) how `nreverse` causes this error?

Why would invoking the function a second time change what the function 
does? The return value of `symbol-function` makes it look like the 
function passes different arguments to the macro after the first 
invocation. Is that a misleading return value, or is that actually 
happening? If that is what happens, why does that happen?