[PATCH RFC] revset: lookup descendents for negative arguments to ancestor operator

[David Soria Parra]

On Wed, May 31, 2017 at 10:50:15PM +0900, Yuya Nishihara wrote:
> On Mon, 29 May 2017 23:28:20 +0900, Yuya Nishihara wrote:
> > On Mon, 29 May 2017 14:45:39 +0200, Denis Laxalde wrote:
> > 
> > I agree it's a bit puzzling, but I also think it's an acceptable extension to
> > the `~` operator as long as descendants history is linear. However, from
> > implementation POV, it would be unnecessarily expensive to make sure there's
> > no more than one child.
> 
> So I'm against the current proposal. FWIW, if we can define the first
> descendant as the nearest child in revision number space, the computation
> will be cheaper.

I considered this and decided to not introduce this notion of nearest
child. I feel that the notion of revnumber as an ordering is unintuitive for
users of a decentralized version control system as it adds an concept of
ordering on the existing dag, and rev numbers are rarely used by users for
ordering (hence people use :: far more than : in revsets). I feel while the
computation is faster it might often lead to unexpected results. The user must
know the revision numbers of children in order to know what 'hg update .~-2'
would do. That's why I felt restricting it the way i did in the patch was more
sensibile for userbility.