I think that, while the lack of type annotations on every variable is great since hiding boilerplate is great, it could be confusing to the uninitiated.

To elaborate, lowpower etc. on this line are Behaviors, which emit values by time:

    react <$> lowpower <*> fridgepowerchange <*> wifiusers

while lowpower, fridgepowerchange, wifiusers in the react function bind to the values that are emitted by the Behaviors. (Just by looking at the code we can infer that their types are Bool, (Num a, Ord a) => a, and PowerChange respectively.)

    react lowpower fridgepowerchange wifiusers
            | lowpower = Just PowerOff
            | wifiusers > 0 = Just PowerOn
            | otherwise = fridgepowerchange

None of these are simply state variables!

Although this might have confused GP, I think this is nice since we can associate the name 'lowpower' for its semantics (The low power signal) and not have to worry about "this lowpower is a Behavior while that lowpower is a Bool".