Instead of the previous refcount base garbage collection, we're now using
a basic tri-color mark&sweep collector. This is done to support cyclical
value relationships in the future (functions can form cycles, all values
implemented up to this point can not).
The collector maintains a set of roots and a set of objects (grouped into
blocks). The GC enabled objects are no longer allocated manually, but will
be allocated by the GC. The GC also wraps an allocator, this way the GC
knows, if we ran out of memory and will try to get out of this situation by
performing a full collection cycle.
The tri-color abstraction was chosen for two reasons:
- We don't have to maintain a list of objects that need to be marked, we
can simply grab the next grey one.
- It should allow us to later implement incremental collection (right now
we only do a stop-the-world collection).
This also switches to a bytecode based evaluation of the code: We no longer
directly evaluate the AST, but first compile it into a series of
instructions, that are evaluated in a separate step. This was done in
preparation for inplementing functions: We only need to turn a function
body into instructions instead of evaluating the node again with each call
of the function. Also, since an instruction list is implemented as a GC
object, this then removes manual memory management of the function body and
it's child nodes. Since the GC and the bytecode go hand in hand, this was
done in one (giant) commit.
As a downside, we've now lost the ability do do list matching on
assignments. I've already started to work on implementing this in the new
architecture, but left it out of this commit, as it's already quite a large
commit :)
If you assign into a member access (`foo.bar = baz` or `foo@bar = baz`), it
is no longer permitted that the LHS of the at/dot is an arbitrary
assignable. It now must be a variable, at or dot. This disallows some silly
constructs (e.g. `[foo]@bar = baz`), increases the similarity to function
parameters and should make writing the evaluation code for these more easy.
The previous representation didn't properly model the fact that an
assignable / parameter can only be expanded, if it's a list element. This
now better models this. Other than being more correct, this should also
make evaluating these a bit easier.
While I was at it, I also improved the error message for multiple
expansions on the same level and added tests for these.
