map.go

func Map(done <-chan struct{}, inputStream <-chan int, operator func(int)int) <-chan int {
    // 1
    mappedStream := make(chan int)
    go func() {
      defer close(mappedStream)
      // 2
      for {
        select {
        case <-done:
          return
        // 3
        case i, ok := <-inputStream:
          if !ok { return }

          //4
          select {
            case <-done:
                return
            case mappedStream <- operator(i):
          }
        }
      }
    }()
    return mappedStream