Referencing a local in an action block before its definition generates invalid code

[krame505]

Currently locals in Copper action blocks share the same environment handling as production bodies, since action blocks are really production bodies in disguise. This means that we can reference them before they are defined (since production bodies are one big let-rec), but the translation works differently than for a production, generating non-compilable code that references uninitialized variables. I can think of a few possible solutions for this:

• Evaluate and initialize all locals before all other action code. Not a great option since assigning to a parser attribute and then using it to initialize a local would unexpectedly give the old value. Also this is still broken for mutually recursive locals.
• Change how the environment works so locals are only visible before being initialized. Probably the ideal option, but would require some annoying changes.
• Initialize all parser attributes to null, raise an error at runtime when referencing an uninitialized local. This is what I did to fix parser attributes in #301, and would be a fairly quick fix.

Thoughts?

[ericvanwyk]

Perhaps do the third option now and leave this issue open until we can do option 2?

[krame505]

Just tried implementing option 3, turns out this doesn't quite work. All variables captured by an anonymous class (as used in lambdas and pattern matching) must be effectively final, so that rules out setting it to null and mutating the variable later when it is initialized.

A slightly less invasive solution than 2, rather than modifying the existing environment, might be to add a secondary environment for action blocks that "flows down" in the expected way, purely for tracking what locals have been initialized so far.