Yes, this is being used in LSP which vendors fiber. Thanks you for the review!

So, we have this for Deferred in Async and its usage is not unanimous. It's like marmite, some love it some hate it. @rgrinberg if this is only used in LSP, I'd rather that this code lives in LSP. It doesn't need to be part of the Fiber module.

Presumably, if we want to discourage the use of Result.t Fiber.t for error handling, we should also remove fold_errors and collect_errors, which are currently provided by the interface but appear to be unused in Dune.

I personally don't feel strongly one way or another, but if we want to stick to with_error_handler as the idiomatic way of handling errors, we can replace collect_errors with something like:

val with_all_errors_handler :
  (unit -> 'a t) -> on_error:(Exn_with_backtrace.t list -> unit) -> 'a t

P.S.: In a parallel discussion with @jeremiedimino, we also thought that it would be useful to make with_error_handler deterministic, i.e. return not the earliest error (like now), but the first error in the order of parallel branches.

In a parallel discussion with @jeremiedimino, we also thought that it would be useful to make with_error_handler deterministic, i.e. return not the earliest error (like now), but the first error in the order of parallel branches.

Sounds like a good idea. Shall we make an issue for this at least?

@rgrinberg Done: #3439.