Task concept

Task.js

type Result<L,R> =
  | {T: 'Right', value: R}
  | {T: 'Left', value: L}
  | {T: 'Thrown', error: any}
  | {T: 'Cancel'}

type Cancel = () => void

type RunHandler<L,R> =
  | (result: Result<L,R>) => void
  | {
    Right?: (value: R) => void,
    Left?: (value: L) => void,
    Thrown?: (error: any) => void,
    Cancel?: () => void,
  }

type RunOptions = {
  catch?: boolean
}

type Task<L,R> = {
  run(handler: RunHandler<L,R>, options?: RunOptions): Cancel
}

Alternative approach 1

• Instead of making it Task<Result<L,R,E>> internally make it just Task<A>
• Users can wrap an Either<L,R> into Task<A> by themselves
• Additionally provide a runAndCatch(onSuccess, onError) that uses different from run strategy and wraps everything to try..catch stopping on first exception and calling onError with it, or calling onSuccess as normal run(onSuccess) if no exceptions appeared
• Note: with runAndCatch we will not have an ability to recover, there no chainError(). Or we could actually have chainError() but this is probably bad idea...

In browser you would use run() and on the server you would use runAndCatch() all the rest of the code should be universal and dealing with Task<A> type, using Task<Either<L,R>> if necessary.

Alt 2

Like #1 but with Task<L,R>. L and R only for user's explicit "lefts" / "rights", catched exceptions never go there. Exceptions still go only into onError callback in runAndCatch and can't be recovered from etc.

And yeah, we don't need onCancel callback probably...

Alt 3

Like #2 but catched exceptions go into L.