promise

type WriteOnce interface {
	// Get reads the current value of this variable.  If this
	// variable is not set yet then this call blocks until this
	// variable gets a value.
	Get() interface{}

	// Set normally writes a value into this variable, unblocks every
	// goroutine waiting for this variable to have a value, and
	// returns true.  In the unhappy case that this variable is
	// already set, this method returns false without modifying the
	// variable's value.
	Set(interface{}) bool
}