Halogen-debounce.purs

type Debouncer =
  { var :: AVar Unit
  , timer :: Fiber Unit
  }

type State =
  { debouncer :: Maybe Debouncer
  }

debounceTimeout :: Milliseconds -> AVar Unit -> Aff (avar :: AVAR | eff) (Fiber Unit)
debounceTimeout ms var = forkAff do
  delay ms
  putVar unit var

eval (SomeInput next) = do
  st <- H.get
  case st.debouncer of
    -- No debouncing yet
    Nothing -> do
      var   <- H.liftAff makeEmptyVar
      timer <- H.liftAff (debounceTimeout (Milliseconds 500.0) var)
      _ <- H.fork do
        -- Wait for the var to resolve
        H.liftAff (takeVar var)
        -- Reset state and run debounced effect
        H.modify _ { debouncer = Nothing }  
        runMyCode
        
      let
        debouncer = { var, timer }
      H.modify _ { debouncer = Just debouncer }

    -- We are already debouncing
    Just { var, timer } -> do
      -- Kill the old timer and allocate a new one
      H.liftAff $ killFiber timer
      nextTimer <- H.liftAff (debounceTimeout (Milliseconds 500.0) var)
      
      let
        debouncer = { var, timer: nextTimer }
      H.modify _ { debouncer = Just debouncer }