Halogen.Hooks.Extra.Hooks.UseGet

useGet :: forall m a. MonadEffect m => a -> Hook m (UseGet a) (HookM m a)

Use this hook when you wish to ensure that your reference to a state value or the component's input is not "stale" or outdated. Usually, this happens in when you define a computation in one "Hook evaluation cycle," but you do not run the computation until a future "Hook evaluation cycle." This typically occurs when running a useLifecycleEffect/useTickEffect's cleanup/finalizer/unsubscribe computation.

Let's see an example of an effect's finalizer referring to a stale value in code. If you don't use useGet in this situation, you will refer to what the value used to be (a stale value), not what the value is now:

myComponent :: forall q i o m. MonadAff m => H.Component HH.HTML q i o m
myComponent = Hooks.component \_ _ -> Hooks.do
  thisIsFive_NotSix /\ stateId <- Hooks.useState 5
  let modifyState = Hooks.modify_ stateId

  Hooks.captures {} Hooks.useTickEffect do
    -- The `thisIsFive_NotSix` state reference is currently `5` and
    -- is up-to-date because this effect body runs immediately
    -- after the Hook evaluation in which it is defined.
    -- Thus, this will print "5" to the console.
    logShow thisIsFive_NotSix

    -- Now we change the value to 6
    modifyState (_ + 1)

    pure $ Just $ do
      -- The effect cleanup, however, will not run after the Hook
      -- evaluation in which it is defined. Thus, the `thisIsFive_NotSix`
      -- state reference is still `5` even though we previously
      -- updated the real value to 6.
      -- Thus, this will print "5" to the console when it should print "6".
      logShow thisIsFive_NotSix

To ensure we refer to the latest value and not a stale one, we use this hook to do so.

myComponent :: forall q i o m. MonadAff m => H.Component HH.HTML q i o m
myComponent = Hooks.component \_ _ -> Hooks.do
  thisIsFive_NotSix /\ stateId <- Hooks.useState 5
  let modifyState = Hooks.modify_ stateId

  -- This returns a function to get the latest state/input value.
  getState <- useGet thisIsFive_NotSix

  Hooks.captures {} Hooks.useTickEffect do
    logShow thisIsFive_NotSix

    modifyState (_ + 1)

    pure $ Just $ do
      -- Now we get the latest value rather than using the stale value.
      -- This correctly prints "6".
      thisIsSix_NotFive <- getState
      logShow thisIsSix_NotFive

data UseGet :: Type -> HookType