WorkflowTest.hs

type WidgetFlow t m a = Workflow t (VtyWidget t m) a

testScreen
 :: (Reflex t, Show a, Show b, Monad m)
 => a
 -- ^ Input
 -> (a -> VtyWidget t m (Event t b))
 -- ^ Screen to test
 -> (b -> WidgetFlow t m ())
 -- ^ Next continuation 
 -> WidgetFlow t m ()
testScreen theInput screenToTest next = Workflow $ do
  inputScreenFinishedEvent <- makeInputScreen theInput
  pure ((), ffor inputScreenFinishedEvent $ \() -> Workflow $ do
    testScreenFinished <- screenToTest theInput
    pure ((), ffor testScreenFinished $ \output -> Workflow $ do
      outputScreenFinishedEvent <- makeOutputScreen output
      pure ((), ffor outputScreenFinishedEvent $ \() -> next output)))

testScreen theInput screenToTest next = initWf
 where
  initWf = Workflow $ do
    inputScreenFinishedEvent <- makeInputScreen theInput
    pure ((), screenToTestWf <$ inputScreenFinishedEvent)
  screenToTestWf = Workflow $ do
    testScreenFinished <- screenToTest theInput
    pure ((), nextWf <$> testScreenFinished)
  nextWf output = Workflow $ do
    outputScreenFinishedEvent <- makeOutputScreen output
    pure ((), next output <$ outputScreenFinishedEvent)

Embracing the <$ for events might make this feel a little less CPS-y

Just realized you were talking about passing in that next arg. Maybe return the output event like:

caller = do
  outputEvent <- switch . current <$> workflow (testScreenWf theInput screenToTest)
  do next something with outputEvent

testScreenWf :: a -> (a -> VtyWidget t m (Event t b)) -> WidgetFlow t m (Event t b)
testScreenWf theInput screenToTest = initWf
 where
  initWf = Workflow $ do
    inputScreenFinishedEvent <- makeInputScreen theInput
    pure (never, screenToTestWf <$ inputScreenFinishedEvent)
  screenToTestWf = Workflow $ do
    outputEvent <- screenToTest theInput
    pure (outputEvent, never)

Hard to say without seeing more code. This kinda sequencing stuff can be awkward. I think workflow probably is the way to go, but you do need them to be "in scope" to call them from each other, AFAIK. So the testScreen will have to know about what's "coming next".