Add AST and en/pl examples

[zbraniecki]

Here's an initial proposal based on Option 2 from #6, using Placeholder from #4.

[zbraniecki]

@mihnita @stasm - how does it look to you?

[mihnita]

I think it implements (almost) the use case we wanted to implement.
It does not show the use of a numeric placeholder, but shows more in other areas (plural, select).

I find the examples really hard to read (just because there is a lot of "duck tape" building the objects, not because it is rust syntax). See below.

I find it a lot easier to just read the ast.rs
Which is where we want to focus anyway :-)

I think it "propagates" some of the existing problems
(like the lack of support for combinations of selectors, what is now nesting in the old MF and in Fluent)
But we don't want to do a "one big commit that resolves everything" anyway :-)

In general I wanted to stay away from your experiment here, so that what I came up with is independent
(if we independently come up with similar structures that is a good sign :-)
And we might be able to "merge" things in ways that would solve more of the requests.

So I apologize if I didn't get involved (until I've seen you pinging me directly :-)

---

I find the building of the message really hard to read.
Not a complaint, I have the same problems with my prototyping using protobuffers.
My json prototyping is a bit more readable than the proto one.
But still not readable enough.

The ast is pretty readable though.

I wonder if using serde to dump the message in a different syntax (json, yaml?) would be more readable.
Or maybe even some "macros" that generates the real ast?

For example:

PatternElement::Placeholder(InlineExpression::VariableReference(Identifier(
            "userName".into(),
        ))),

replaced by:

ph("userName")

The whole English message might be:

    let _en = Message_Single([
        ph("userName"),
        text(" published a post in "),
        ph("groupName"),
    ]);

I am not sure what exactly can be done with the rust macros.

[mihnita]

Suggestion: add the generic form, as represented by the code:
{userName} published a post in {groupName}
(or, if we don't want it ICU, {$userName} published a post in {$groupName})

[echeran]

LGTM.

I'm similar to Mihai in that I'm having to go off and experiment on my own to see if my ideas actually work as intended, and to learn more Rust in the process (including getting the compiler to do what I want). My work is in the fork of this repo, very much a WIP, I can already see some similarities and differences. But I think it'll help my ability to have an informed opinion to continue exploring & learning.

I think this covers all [https://github.com/unicode-org/message-format-wg/pull/94/files?short_path=50e1d2b#diff-50e1d2bd4aa7bab895a42ce05dfcc5c4](the examples) up to and including gender+plural. I think there was discussion in a diff thread on what would happen if both gender and plurals were needed as selectors in the same mutli-message group, and if not, that would be interesting to explore.