This represents a significant departure from how the XSLT-based TAME handles
the `@values@` param, but it will end up having the same effect. It builds
upon prior work, utilizing the fact that referencing a template in TAMER
will expand it.
The problem is this: allowing trees in `Meta` would add yet another
container; we have `Pkg` and `Tpl` already. This was the same problem with
template application---I didn't want to add support for binding arguments
separately, and so re-used templates themselves, reaching the generalization
I just mentioned above.
`Meta` is intended to be a lexical metasyntatic variable. That keeps its
implementation quite simple. But if we start allowing trees, that gets
rather complicated really quickly, and starts to require much more complex
AIR parser state.
But we can accomplish the same behavior by desugaring into an existing
container---a template---and placing the body within it. Then, in the
future, we'll parse `param-copy` into a simple `Air::RefIdent`, which will
expand the closed template and produce the same result as it does today in
the XSLT-based system.
This leaves open issues of closure (variable binding) in complex scenarios,
such as in templates that introduce metavariables to be utilized by the
body. That's never a practice I liked, but we'll see how things evolve.
Further, this does not yet handle nested template applications.
But this saved me a ton of work. Desugaring is much simpler.
The question is going to be how the XSLT-based compiler responds to this for
large packages with thousands of template applications. I'll have to see
if it's worth the hit at that time, or if we should inline it when
generating the `xmli` file, producing the same `@values@` as
before. But as it stands at this moment, the output is _not_ compatible
with the current compiler, as it expects `@values@` to be a tree, so a
modification would have to be made there.
DEV-13708
This applies to template application only; there's still some work to do for
template parameters in definitions (well, for deriving them in `xmli` at
least). And, as you can see, there's still a lot of TODO items here.
I ended up backtracking on tree edges to Meta, and even on cross edges to
Meta, because it complicated xmli derivation with no benefit right now;
maybe a cross edge will be re-added in the future, but I need to move on and
see where this takes me.
But, it works.
DEV-13708
This this a big change that's difficult to break up, and I don't have the
energy after it.
This introduces nullary template application, short- and long-form. Note
that a body of the short form is a `@values@` argument, so that's not
supported yet.
This continues to formalize the idea of what "template application" and
"template expansion" mean in TAMER. It makes a separate `TplApply`
unnecessary, because now application is simply a reference to a
template. Expansion and application are one and the same: when a template
expands, it'll re-bind metavariables to the parent context. So in a
template context, this amounts to application.
But applying a closed template will have nothing to bind, and so is
equivalent to expansion. And since `Meta` objects are not valid outside of
a `Tpl` context, applying a non-closed template outside of another template
will be invalid.
So we get all of this with a single primitive (getting the "value" of a
template).
The expansion is conceptually like `,@` in Lisp, where we're splicing trees.
It's a mess in some spots, but I want to get this committed before I do a
little bit of cleanup.
Mike Gerwitz