All Spec attributes are now represented as `attr(attribute_name, ... args ...)`, e.g.
`attr(node, "hdf5")` instead of `node("hdf5")`, as we *have* to maintain the `attr()`
form anyway, and it simplifies the encoding to just maintain one form of the Spec
information.
Background
----------
In #20644, we unified the way conditionals are done in the concretizer, but this
introduced a nasty aspect to the encoding: we have to maintain everything we want in
general conditions in two forms: `predicate(...)` and `attr("predicate", ...)`. For
example, here's the start of the table of spec attributes we had to maintain:
```prolog
node(Package) :- attr("node", Package).
virtual_node(Virtual) :- attr("virtual_node", Virtual).
hash(Package, Hash) :- attr("hash", Package, Hash).
version(Package, Version) :- attr("version", Package, Version).
...
```
```prolog
attr("node", Package) :- node(Package).
attr("virtual_node", Virtual) :- virtual_node(Virtual).
attr("hash", Package, Hash) :- hash(Package, Hash).
attr("version", Package, Version) :- version(Package, Version).
...
```
This adds cognitive load to understanding how the concretizer works, as you have to
understand the equivalence between the two forms of spec attributes. It also makes the
general condition logic in #20644 hard to explain, and it's easy to forget to add a new
equivalence to this list when adding new spec attributes (at least two people have been
bitten by this).
Solution
--------
- [x] remove the equivalence list from `concretize.lp`
- [x] simplify `spec_clauses()`, `condition()`, and other functions in `asp.py` that need
to deal with `Spec` attributes.
- [x] Convert all old-form spec attributes in `concretize.lp` to the `attr()` form
- [x] Simplify `display.lp`, where we also had to maintain a list of spec attributes. Now
we only need to show `attr/2`, `attr/3`, and `attr/4`.
- [x] Simplify model extraction logic in `asp.py`.
Performance
-----------
This seems to result in a smaller grounded problem (as there are no longer duplicated
`attr("foo", ...)` / `foo(...)` predicates in the program), but it also adds a slight
performance overhead vs. develop. Ultimately, simplifying the encoding will be a win,
particularly for improving error messages.
Notes
-----
This will simplify future node refactors in `concretize.lp` (e.g., not identifying nodes
by package name, which we need for separate build dependencies).
I'm still not entirely used to reading `attr()` notation, but I thnk it's ultimately
clearer than what we did before. We need more uniform naming, and it's now clear what is
part of a solution. We should probably continue making the encoding of `concretize.lp`
simpler and more self-explanatory. It may make sense to rename `attr` to something like
`node_attr` and to simplify the names of node attributes. It also might make sense to do
something similar for other types of predicates in `concretize.lp`.
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