RE: [Yaml-core] getting started on ypath

[Oren Ben-K. <or...@ri...>]

Clark C . Evans [mailto:cc...@cl...] wrote:
> | - Why do you need the { key: ~, value: *ROOT } pair? It is 
> | the first entry
> | in *every* path, and as such it seems it can be omitted. 
> | Further, I want to
> | be able to "apply-path-to-graph(path, graph)". Placing a 
> | reference to the
> | root at the start of the path kills this useful ability.
> 
> You need to have the root node stored since inside of a YPATH you
> may need to jump up to the "top" of the tree within a predicate.

I don't see how it follows... I re-thought about it, however, and I see
there's not much point in applying to one graph the results of applying a
YAPTH to another graph. What would be interesting would be to convert the
results of a YPATH into a simpler YPATH, for example /a/../b/c into /a/c.

That said, there is still a question of how to represent a path returned by
the YPATH engine. Your answer was "a sequence of { key: ..., value: ... }
pairs". Which leaves open the questions of:

- How to represent going into a list (one possible answer - { key: ~, value:
<index> }).

- What happens when selecting a *key node* rather than a value node?

- And, of course - what happens with structured keys.

> | >   - item: []
> | >     role: >
> | >        This describes a predicate.  The predicate acts as
> | >        a "filter" on the current selected contexts, knocking
> | >        out those that don't return true.  Within a predicate
> | >        strings are treated as literals, unless prefixed with
> | >        ./ or / or some other way that identifies a subordinate
> | >        path expression.
> | 
> | Which implies some sort of different quoting/interpretation 
> | rules for
> | strings inside predicates. I'm not certain what you have in 
> | mind, but I
> | think the rules should be the same inside and outside predicates.
> 
> The quoting rules are the same.  The interpretation will be different
> as anything that must be a "path" inside a predicate must start with
> a './' or '/' or '../' or some other path-ish marker.

I see; as opposed to the start of the whole YPATH expression that may start
with a word (key value). Is this distinction really called for? Can foo[bar]
mean anything else other than /foo[./bar]?

BTW, this touches on David's "default is string" issue. Obviously in YPATH
strings values containing a '/' would have to be quoted.

Have fun,

	Oren Ben-Kiki