I don't think this is worth unit testing, at least right now. We'd need to mock the PTP, the TunnelHandle, the validator, and eventually the XPC speaker, all for a relatively simple abstraction that's mostly error handling.

This is a little frustrating - theTask type itself is generic on two type parameters,Success andFailure, but the only APIs you can await on a task are eithervalue with an untypedthrows orresult which returns a type-erasedany Error error variant. It's very similar for continuations, the type itself is generic but there's no way to return that typed error.

As an aside, I think I'm becoming more of a fan, for binaries at least, of just having a single opaque error type, propagating it with context, and then just displaying it, ala Go or Rust'sanyhow - it's so rare that one or more of the error variants are recoverable, and that the caller needs to know which one.