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
}

Alt 3

Like #2 but catched exceptions go into L.