Do not include unreachable branches in generated Erlang

[giacomocavalieri]

When compiling to Erlang we could be using the exhaustiveness checking information to trim away unreachable branches from the generated Erlang code

[jackprogramsjp]

Hi. I would like to work on this issue.
I have some prior experience with compilers.
I'll start looking at the unreachable case clauses and the exhaustiveness checking information.

[lpil]

Go for it! Thanks Jack!

[jackprogramsjp]

@lpil

I read over the codebase and please correct me if I misinterpreted the codebase. I have some questions to how I should implement this because there could be more than one way to.

So, in the type checker, I see the case exhaustiveness is checked and then if unreachable then a warning is set. However, the typed clauses themselves aren't mutated/changed if there are any unreachable.

Then in the Erlang code generation, it walks over the clauses, but there's still those unreachable clauses.

I also checked out the JS code generation, however it seems to use the Decision tree instead, the compiled case tree, the unreachable cases are not there anymore.

Would it be better to go through the compiled_case.tree in the Erlang code generation? Or, instead in the check_case_exhaustiveness function, store Diagnostic information or a HashSet or something that describes which clauses are unreachable in CompiledCase, or store information in the TypedClause?

Hopefully, I'm not missing something or there's a much simpler solution!

Thanks!

[lpil]

store Diagnostic information or a HashSet or something that describes which clauses are unreachable in CompiledCase

Sounds good! The indexes of the unreachable ones could be stored and then referenced during code generation.