GNU bug report logs - #33998
27.0.50; cl-delete does not delete the first list element

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From: João Távora <joaotavora <at> gmail.com>
To: Drew Adams <drew.adams <at> oracle.com>
Cc: 33998 <at> debbugs.gnu.org, Deus Max <deusmax <at> gmx.com>
Subject: bug#33998: 27.0.50; cl-delete does not delete the first list element
Date: Tue, 8 Jan 2019 18:57:12 +0000

On Tue, Jan 8, 2019 at 6:46 PM Drew Adams <drew.adams <at> oracle.com> wrote:
>
> > > Again, you should set the variable to the value
> > > returned by `cl-delete' only if that's what you
> > > want
> >
> > OK. Say you don't. Say you don't SETQ the variable
> > to the return value. What do you expect to be left
> > with in the value bound to the SEQ symbol?
> >
> > > Pretty simple, really.
> >
> > Then answer the pretty simple question above.
>
> Without setting variable `seq' (i.e., without
> the `setq'), that variable is still bound to
> whatever it was bound to prior to your invoking
> `(cl-delete thing seq)'.  Presumably it was
> bound to a cons.

It could have been bound to a vector. cl-delete (as delq, delete)
accepts sequences. And contrary to those two, it makes
no guarantees as to how it potentially destroys the original
sequence. So unless you're relying on a particular
implementation, relying on SEQ after calling cl-delete on it
is a bad, bad idea.

So yes, always (setq SEQ (cl-delete THING SEQ)).  With
the extraordinary exception that you're leaving the scope
where SEQ is visible.  Then you can bypass it.

João

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