RFC: revset relation operator

[Matt Mackall]

On Tue, 2015-10-13 at 13:28 -0700, Sean Farley wrote:
> 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.

The first one will suddenly but probably harmlessly fail if you have
files named "foo-" or "foo1".

Since we're on this topic, <> is interesting if you fail to quote it:

$ echo quux > bar

$ echo foo<bar>
bash: syntax error near unexpected token `newline'

$ echo foo<> bar
foo  <- oops, silently lost operator

$ cat bar
quux  <- but bar is undamaged

$ echo foo<nosuch> bar
bash: nosuch: no such file or directory

$ echo foo<exists> bar # oops!
$ cat bar
foo <- oops

It's pretty hard to hurt yourself with this syntax but not impossible.
And it always requires quoting. {} is probably the best choice since it
almost always works without quoting. 

The "$" thing can get us in trouble if we follow it with a letter since
that'll trigger env substitution. So maybe the relation type needs to go
first.

-- 
Mathematics is the supreme nostalgia of our time.