future-blog-post-14

futureImpl2.go

type futureImpl struct {
    done chan struct{}
    cancel chan struct{}
    val interface{}
    err error
    o *sync.Once
}

func (f *futureImpl) Cancel() {
    select {
    case <-f.done:
        //already finished
    case <-f.cancel:
        //already cancelled
    default:
        f.o.Do(func() {
            close(f.cancel) //should only be called once, since the closed cancel channel will always return
        })
    }
}

func (f *futureImpl) IsCancelled() bool {
    select {
    case <-f.cancel:
        return true
    default:
        return false
    }
}