RFC: revset relation operator

[Sean Farley]

Durham Goode <durham at fb.com> writes:

> On 10/12/15 7:07 AM, 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?
> I thought about using a multidimensional array style syntax for allowing 
> full words instead of single letters:
>
> foo{1g} == foo{1, graft}
>
> which allow for more self-documenting uses.
>
> I also thought about using [] to have the nice side effect of implying a 
> array style access:
>
> foo[-1]
> foo[1, graft]
> foo[$][-1o]

Surprising to me: this actually works in bash.