Problem with cl-letf in winner.el

Problem with cl-letf in winner.el

[Alan Mackenzie]

Hello, Emacs.

In Emacs master branch:

In connection with bug #48249, I'm looking at winner.el.  In function
winner-set-conf, there is the following macro call:

    (cl-letf (((window-buffer miniwin))
              ((window-point  miniwin)))
      (set-window-configuration winconf))

..  I'm having trouble understanding it.  When I expand and pretty print
it, I get this:

    (let*
        ((v miniwin)
         (v miniwin)
         (old
          (window-point v))
         (old
          (window-buffer v)))
      (unwind-protect
          (set-window-configuration winconf)
        (set-window-point v old)
        (let*
            ((v old))
          (progn
            (set-window-buffer v v)
            v))))
    
This looks very strange indeed: under the let* there are two variables
called v and two variables called old.  This looks like a bug to me, but
alternatively, is there something I'm not understanding here?  (Just to
be specific, I expanded the macro with lexical-binding both nil and t,
and got the same results.)

Help, please!

-- 
Alan Mackenzie (Nuremberg, Germany).

Re: Problem with cl-letf in winner.el

[Stefan Monnier]

> This looks very strange indeed: under the let* there are two variables
> called v and two variables called old.

Try against with `print-gensym` and `print-circle` set to
a non-nil value: they're uninterned identifiers.


        Stefan

Re: Problem with cl-letf in winner.el

[Basil L. Contovounesios]

Alan Mackenzie <acm@muc.de> writes:

> In connection with bug #48249, I'm looking at winner.el.  In function
> winner-set-conf, there is the following macro call:
>
>     (cl-letf (((window-buffer miniwin))
>               ((window-point  miniwin)))
>       (set-window-configuration winconf))
>
> ..  I'm having trouble understanding it.  When I expand and pretty print
> it, I get this:
>
>     (let*
>         ((v miniwin)
>          (v miniwin)
>          (old
>           (window-point v))
>          (old
>           (window-buffer v)))
>       (unwind-protect
>           (set-window-configuration winconf)
>         (set-window-point v old)
>         (let*
>             ((v old))
>           (progn
>             (set-window-buffer v v)
>             v))))
>     
> This looks very strange indeed: under the let* there are two variables
> called v and two variables called old.  This looks like a bug to me, but
> alternatively, is there something I'm not understanding here?  (Just to
> be specific, I expanded the macro with lexical-binding both nil and t,
> and got the same results.)
>
> Help, please!

Enabling print-gensym and print-circle should make it clearer that the
seemingly duplicate vars are actually different uninterned symbols.

So my reading of that macro call is that it restores miniwin's buffer
and point after changing window configuration.

HTH,

-- 
Basil