RFC: revset relation operator

[Matt Mackall]

On Mon, 2015-10-12 at 10:07 -0400, Augie Fackler wrote:
> On Sun, Oct 11, 2015 at 05:45:24PM -0500, Matt Mackall wrote:
> > It would be nice if we had a convenience syntax for finding things
> > related to a given changeset. Currently, we have a couple of these for
> > parents, ancestors, and descendants.
> >
> >  foo^  # first parent
> >  foo^2 # second parent
> >  foo^^ # first parent of first parent
> >  foo~2 # same
> >  ::foo # ancestors, inclusive
> >  foo:: # descendants, inclusize
> >
> > But we have nothing directly equivalent to parents(), nor any shorthand
> > for children(), successors(), origin(), destination(), or any other type
> > of relation we may add. We'd also like a way to specify next, previous,
> > all, all exclusive, last, etc.
> >
> > So my proposal[1] is to add a bracketed postfix (one of {}, [], or <>,
> > but perhaps {} is the least bad) that looks like this:
> >
> >  foo{1}     # children(foo)
> >  foo{2}     # children(children(foo))
> >  foo{}      # defaults to 1, so children(foo)
> >  foo{0}     # synonym for foo
> >  foo{-1}    # parents(foo) (both of 'em)
> >  foo{-}     # same
> >  foo{*}     # foo:: - foo (exclusive)
> >  foo{**}    # foo:: (inclusive)
> >  foo{-*}    # ::foo - foo
> >  foo{$}     # heads(foo::) aka "the last children"
> >  foo{-4}::foo # last five changesets in a branch
> >
> > This makes a lot of our most common expressions shorter. To talk about
> > other types of relations, we use a suffix character:
> >
> >  foo{1g}    # immediate grafts of foo
> >  foo{-g}    # origin for grafts
> >  foo{o}     # successor(foo)
> >  foo{$o}    # final successor of foo
> 
> Neat. The one-character suffix thing gives me a little pause as
> possibly-not-entirely-self-documenting. Maybe it could be whole [a-z]
> words, and we could allow unambiguous prefixes?

s/prefixes/suffixes/, sure. Definitely makes more sense for the long
form.

I think the specific feedback I'm looking for here is how do folks feel
about:

- using {} for the operator
- allowing empty {} to be children()
- allowing bare "-" with no numeral
- using * and ** (and -* and -**) for all
- using $ (and -$) for end/last
- suffix characters/words for relation dimension
- any common operations I might be forgetting

This is kind of a lot of new weird syntax all at once and I haven't
completely convinced myself that it's not too horrible.

-- 
Mathematics is the supreme nostalgia of our time.