eventual

type Getter func(time.Duration) (interface{}, bool)

type Value interface {
	// Set sets this Value to the given val.
	Set(val interface{})

	// Get waits up to timeout for the value to be set and returns it, or returns
	// nil if it times out or Cancel() is called. valid will be false in latter
	// case. If timeout is 0, Get won't wait. If timeout is -1, Get will wait
	// forever.
	Get(timeout time.Duration) (ret interface{}, valid bool)

	// Cancel cancels this value, signaling any waiting calls to Get() that no
	// value is coming. If no value was set before Cancel() was called, all future
	// calls to Get() will return nil, false. Subsequent calls to Set after Cancel
	// have no effect.
	Cancel()
}