A timer can be used to execute an expression repeatedly at regular intervals, for example to poll a service. Recall the definition of metronome from the previous chapter:

def metronome(t) = signal | Rwait(t) >> metronome()

The following example publishes "tick" once per second and "tock" once per second after an initial half-second delay. The publications alternate: "tick tock tick tock ...". Note that this program is not defined recursively; the recursion is entirely contained within metronome.

  metronome(1000) >> "tick"
| Rwait(500) >> metronome(1000) >> "tock"