Is there a netwire equivalent of stepper?

Question

Reactive-banana has a function named stepper (of type MonadMoment m => a -> Event a -> m (Behavior a) ), which converts an event into a behaviour where the behaviour's value is the value of the last event, or the initial value if no event has occurred yet.

In a talk by Conal Elliott, the name of which I cannot remember, he presents this as being one of the fundamental operations on events and behaviours. However, I cannot find a similar function in netwire. With my limited understanding of netwire, I would expect it to have type:

a -> Wire sem (Event a) a

• Does netwire have an equivalent function?
• If not, is there any reason why this is not possible?
• Is anything similar possible, ie ways of converting events into behaviours?

Answer 1

The function I was looking for is called hold , in Control.Wire.Interval .

This does not need an initial value, because the wire inhibits until the first event is received. If this behaviour is needed, it can be implemented like so

stepper init = hold <|> pure init

Answer 2

I can only guess why netwire doesn't provide that. Everything in Control.Wire.Event keeps the results in Event s preserving the knowledge of when they are happening.

You can get out of Event s by using one of the methods of switching in Control.Wire.Switch . You're looking for rSwitch .

-- Beware: untested, untype-checked code
stepper :: (Monad m) => a -> Wire s e m (Event a) a
stepper init = switcher . source
  where
     -- source :: Wire s e m (Event a) ((), Event (Wire s e m () a))
     source = arr (\e -> ((), pure <$> e))
     -- switcher :: Wire s e m ((), Event (Wire s e m () a)) a
     switcher = rSwitch (pure init)

In the code above pure is used as a -> Wire sem () a to create trivial wires.