We think this generated file is just a stopgap until Swift gains the full power of variadic generics, but either way, it got us wondering whether generated source code would be a good fit for our parser library, and then we could just rip them out once variadic generics arrive in Swift. We were most worried about compile times, but we do know that the Swift compiler team has greatly improved the performance of result builders. Our early experiments were extremely promising, so we went all in and we think it is the future of the library.

Before diving in, let’s first discuss result builders from first principles so that we all know how to leverage them to their full potential, and then we will see how these ideas apply to parsers. We will show that result builders go far beyond just simple ergonomics for our parsing library. They unlock all new forms of APIs that can be really, really powerful.

So, let’s get started.