Try a separate holdKnown primitive

src/AFRP.hs

@@ -8,6 +8,7 @@ module AFRP
   , withEntities
   , Event(..)
   , hold
+  , holdKnown
   , events
   -- , switch
   , preMapAccum
@@ -326,6 +327,11 @@ hold def = mapAccum step def id
       Tick -> prev
       Event new -> new
 
+
+-- A variant of 'hold' which makes it explicit whether we have a previous value
+holdKnown :: (Serialize a, Eq a) => Mealy m (Event a) (Maybe a)
+holdKnown = arr (fmap Just) >>> hold Nothing
+
 snapshot :: (Serialize a, Eq a) => a -> Mealy m (Event (), a) a
 snapshot def = mapAccum step def id
   where

src/HomeAssistant/Controller/Children.hs

@@ -19,6 +19,7 @@ import Data.Aeson.Types (ToJSON (..))
 import Data.Aeson.Lens (key, _String)
 import Control.Lens ((^?), to)
 import Data.Text (Text)
+import Control.Monad (join)
 
 -- Let's see building some reasonable interface for utctime
 dow :: Day -> (WeekOfYear, Int)
@@ -131,8 +132,8 @@ data HumidifierAction
 instance Serialize HumidifierAction
 
 -- what is the humidifer doing right now?
-humidifierEntityAction :: HASS (Event Value) (Event (Maybe HumidifierAction))
-humidifierEntityAction = entityChangeEvent "humidifier.lapset_ilmankostutin"
+humidifierOperatingState :: HASS (Event Value) (Event (Maybe HumidifierAction))
+humidifierOperatingState = entityChangeEvent "humidifier.lapset_ilmankostutin"
   >>> arr (fmap entityAction)
   where
     entityAction v = v ^? key "event" . key "variables" . key "trigger" . key "to_state" . key "attributes" . key "action" . _String . to humidifierAction
@@ -149,7 +150,7 @@ instance ToJSON Percentage where
 childrenAirFilterController :: HASS (Event Value) ()
 childrenAirFilterController = proc ev -> do
   level <- pm25 >>> pm25Level >>> AFRP.changes >>> traceEvent -< ev
-  humidifier <- humidifierEntityAction >>> AFRP.hold Nothing -< ev
+  humidifier <- humidifierOperatingState >>> AFRP.holdKnown >>> arr join -< ev
   case level of
        Event PM25High ->
          AFRP.whenA (== Just Idle) (callService (fanOnTo (Percentage 80))) -< humidifier