Identifier lookups, as done using the graph methods today, look up from a
cache representing the global environment.
Templates must not contribute to this environment until expansion. Further,
metavariables will not be present in this environment. To avoid confusion
and help obviate accidental contributions to this environment, the methods
have been renamed. This will also allow for the creation of more general
methods down the line.
DEV-13708
Previous to this commit, ontological cross edges were declared
statically. But this doesn't fare well with the decided implementation for
template application.
The documentation details it, but we have Tpl->Ident which could mean "I
define this Ident once expanded", or it could mean "this is a reference to a
template I will be applying". The former is a tree edge, the latter is a
cross edge, and that determination can only be made by inspecting edge data
at runtime.
It could have been resolved by introducing new Object types, but that is a
lot of work for little benefit, especially given that only (right now) the
visitor uses this information.
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.
Also known as metavariables or template parameters.
This is a bit of a tortured excursion, trying to figure out how I want to
best represent this. I have a number of pages of hand-written notes that
I'd like to distill over time, but the rendered graph ontology (via
`asg-ontviz`) demonstrates the broad idea.
`AirTpl::TplApply` highlights some remaining questions. What I had _wanted_
to do is to separate the concepts of application and expansion, and support
partial application and such. But it's going to be too much work for now,
when it isn't needed---partial application can be worked around by simply
creating new templates and duplicating params, as we do today, although that
sucks and is a maintenance issue. But I'd rather address that head-on in
the future.
So it's looking like Option B is going to be the approach for now, with
templates being closed (as in, no free metavariables) and expanded at the
same time. This simplifies the parser and error conditions significantly
and makes it easier to utilize anonymous templates, since it'll still be the
active context.
My intent is to get at least the graph construction sorted out---not the
actual expansion and binding yet---enough that I can use templates to
represent parts of NIR that do not have proper graph representations or
desugaring yet, so that I can spit them back out again in the `xmli` file
and incrementally handle them. That was an option I had considered some
months ago, but didn't want to entertain it at the time because I wasn't
sure what doing so would look like; while it was an attractive approach
since it pushes existing primitives into the template system (something I've
wanted to do for years), I didn't want to potentially tank performance or
compromise the design for it after I had spent so much effort on all of this
so far.
But my efforts have yielded a system that significantly exceeds my initial
performance expectations, with a decent abstractions, and so this seems
viable.
DEV-13708
This sets us up to be able to determine how `Dangling` expressions will be
rooted into templates.
This new strategy isn't yet handling `Dangling`; I wanted to get this
committed first so that the `Dangling` refactoring is more clear.
DEV-13708
This sets the stage for template parsing, and finally decides how we're
going to represent templates on the ASG. This is going to start simple,
since my original plans for improving how templates are
handled (conceptually) is going to have to wait.
This is the last difficult object type to figure out, with respect to graph
representation and derivation, so I wanted to get it out of the way.
DEV-13708
The previous commit demonstrated the amount of boilerplate necessary for
introducing new `ObjectKind`s; this abstracts away a lot of that
boilerplate, and allows for declarative relationship definition for the
ASG's ontology.
DEV-13708
There's quite a bit of boilerplate here that'll eventually need factoring
out. But it's also clear that it is somewhat onerous to add new object
types.
Note that a good chunk of this burden is _intentional_, via exhaustiveness
checks---adding a new type of object is an exceptional occurrence (well, in
principle, but we haven't added them all yet, so it'll be more common
initially), and we'd rather be safe to ensure that everything is properly
considering how that new type of object interacts with it.
Let's not confuse coupling with safety---the latter causes a burden because
of the former, not because of itself; it provides a service to us.
But, nonetheless, we'll want to reduce this burden somewhat since there are
a number more to add.
DEV-13708
Mike Gerwitz