I love Go for writing servers. And in fact, I do it professionally. But I totally agree that for parsers, it’s not the right tool for the job.

First off, the only way to express union types is with runtime reflection. You might as well be coding in Python (but without the convenient syntax sugar).

Second off, “if err != nil” is really terrible in parsers. I’m actually somewhat of a defender of Go’s error handling approach in servers. Sure, it could have used a more convenient syntax. But in servers, I almost never return an error without handling it or adding additional context. The same isn’t true in parser’s though. Almost half of my parser code was error checks that simply wouldn’t exist in other languages.

For Rust, I think the value proposition is if you are also writing a virtual machine or an interpreter, your compiler front end can be written in the same language as your backend. Your other alternatives are C and C++, but then you don’t have sum types. You could write the front end in Ocaml, but then you would have to write the backend and runtime in some other language anyways.