Re: [PATCH] branch: allow "-" as a short-hand for "previous branch"

On Mon, Aug 8, 2022 at 6:06 PM Junio C Hamano <gitster@pobox.com 
<mailto:gitster@pobox.com>> wrote:
 >
 > Johannes Schindelin <Johannes.Schindelin@gmx.de 
<mailto:Johannes.Schindelin@gmx.de>> writes:
 >
 > > @@ -1420,6 +1420,12 @@ static int 
interpret_nth_prior_checkout(struct repository *r,
 > >       const char *brace;
 > >       char *num_end;
 > >
 > > +     if (namelen == 1 && *name == '-') {
 > > +             brace = name;
 > > +             nth = 1;
 > > +             goto find_nth_checkout;
 > > +     }
 > > +
 > >       if (namelen < 4)
 > >               return -1;
 > >       if (name[0] != '@' || name[1] != '{' || name[2] != '-')
 >
 > If a solution along this line works, it would be far cleaner design
 > than the various hacks we have done in the past, noticing "-" and
 > replacing with "@{-1}".  For one thing, we wouldn't be receiving a
 > "-" from the end user on the command line and in response say @{-1}
 > does not make sense in the context in an error message. That alone
 > makes the above approach to deal with it at the lowest level quite
 > attractive.
 >
 > In the list archive, however, you may be able to find a few past
 > discussions on why this is not a good idea (some of which I may no
 > longer agree with).  One thing that still worries me a bit is that
 > we often disambiguate the command line arguments by seeing "is this
 > (still) a rev, or is this a file, or can it be interpreted as both?"
 > and "-" is not judged to be a "rev", IIRC.
 >
 > Luckily, not many commands we have take "-" as if it were a file and
 > make it read from the standard input stream, but if there were (or
 > if we were to add a command to behave like so), treating "-" to mean
 > the same thing as "@{-1}" everywhere may require the "does this look
 > like a rev?"  heuristics (which is used by the "earlier ones must be
 > rev and not file, later ones must be file and cannot be interpreted
 > as rev, for you to omit '--' from the command line" logic) to be
 > taught that a lone "-" can be a rev.
 >
 > So it is quite a lot of thing that the new code needs to get right
 > before getting there.

Agree. To make a substitution in the command line and to consider "-" in interpret_nth_prior_checkout, I see them as two very different games.

Previous to this, I thought about making also a "git diff -", https://github.com/gitgitgadget/git/pull/1314 <https://github.com/gitgitgadget/git/pull/1314>. Suddenly there was 5 commands with this substitution (checkout, merge, rebase, branch, diff) so I follow a little the path now Johannes suggests, making the substitution "- ~ @{-1}" deep in the system. For me, the implications, error cases, test cases... to consider was not worth the change to get what I was looking for: align the workflow "checkout/merge/branch -d".

Also discarded the "git diff -" change, because of so many flags and conditions "diff" has. So I only sent the "branch -" patch.