Re: pcase-memoize: equal first branch, yet different

[Stefan Monnier <monnier@iro.umontreal.ca>]

> I get a not so useful message using `pcase' in the following scenario
> using trunk:
[...]
>                          `(lambda (_event)
>                             (interactive "e")
>                             (pcase ,flag
>                               (`"-" (dired-sort-menu-set-switches ""))
>                               ((or `"t" `"S")
>                                (dired-sort-menu-set-switches ,flag))
>                               (`"r"
>                               (dired-sort-menu-toggle-reverse)))))))
[...]
> This works well, but every time I click on a flag, I get this message:
> | pcase-memoize: equal first branch, yet different
> What does that mean?  It's annoying.

The pcase macro's expansion can take a significant amount of time to
generate.  When done during byte-compilation, it's a non-issue, but in
the above example it will be done every time you click.

For this reason, pcase uses a memoization mechanism, to try and reuse
previous expansions.  Sadly, this is unreliable because it is difficult
to figure out when two chunks of code are actually identical and to do
so without incurring undue overheads.

The message is just a warning, so you can ignore it.  But the better
option is to try and fix the underlying problem and make sure pcase is
not expanded repeatedly.

E.g. using lexical-binding:

                   keymap ,(make-mode-line-mouse-map
                            'mouse-2
                            (lambda (_event)
                              (interactive "e")
                              (pcase flag
                                (`"-" (dired-sort-menu-set-switches ""))
                                ((or `"t" `"S")
                                 (dired-sort-menu-set-switches ,flag))
                                (`"r"
                                 (dired-sort-menu-toggle-reverse)))))))

or using CL's lexical-let:

                   keymap ,(make-mode-line-mouse-map
                            'mouse-2
                            (lexical-let ((flag flag))
                              (lambda (_event)
                                (interactive "e")
                                (pcase flag
                                  (`"-" (dired-sort-menu-set-switches ""))
                                  ((or `"t" `"S")
                                   (dired-sort-menu-set-switches ,flag))
                                  (`"r"
                                   (dired-sort-menu-toggle-reverse))))))))

-- Stefan


PS: Why do you use (interactive "e") if you then don't use the
`event' argument.