Add support for Error Types

[seveibar]

I like this module a lot! I think it would be even better if we were able to define possible resulting errors. I've adding backwards-compatible types for errors here, people might use this when they have known errors being returned by a function.

At seam.co we have really error-heavy code, so we usually have types for all the errors that could be returned by a function. This might be one step towards getting all those errors nicely typed.

Example code BEFORE ErrorType introduced:

  const result = await pReflect(
    getFromGithub("/users/thispersondoesnotexist99829384", {})
  )
  if (isRejected(result)) {
    result.reason // type: unknown
  }

Example code after:

const result = await pReflect<APIResponse, NotFoundError>(
  getFromGithub("/users/thispersondoesnotexist99829384", {})
)

if (isRejected(result)) {
  result.reason // type: NotFoundError
}

I'm interested in comments but I think due to Typescript's all-or-nothing generics this feature might not be very helpful. (see how I have to include APIResponse in the AFTER type above?)

[seveibar]

Not entirely satisfied with this, converting back to draft.

I've edited the original post to include some reasoning why the interface sort of has to be ugly and require redundant specification of the value type.

[sindresorhus]

I cannot think of a better way either. I think it's fine though. If you really want strongly-typed errors, you need some more boilerplate for now, if not, you don't specify any type parameter. We can add a TODO comment about using type placeholder when it eventually becomes a reality.

[sindresorhus]

Bump :)