Add responseIf and responseMaybe helpers (#250)

* Add responseIf and responseMaybe helpers

* Move helper functions into UserUtil, where other similar functions reside

docs/tutorials/01-basics.md

@@ -298,6 +298,10 @@ application (you can check the [lens](external/01-optics.md) tutorial to better
 understand those operators). If the model changed, this will trigger a call to
 the build UI function.
 
+Similar to `widgetIf` and `widgetMaybe`, a couple of helper functions exist for
+event responses: `responseIf` and `responseMaybe`. Their usage pattern is the
+same as in the widget helpers.
+
 ## Configuration
 
 By default, the starter app sets a few configuration options, including window

src/Monomer/Main/UserUtil.hs

@@ -133,3 +133,31 @@ Useful for conditionally setting a configuration value.
 configMaybe :: Monoid a => Maybe b -> (b -> a) -> a
 configMaybe Nothing _ = mempty
 configMaybe (Just val) fn = fn val
+
+{-|
+Returns the provided 'EventResponse' when True, otherwise returns a no-op.
+
+Useful for conditionally returning a response.
+
+@
+...
+_ -> [Model newModel, responseIf isValid (SetFocusOnKey "widgetKey")]
+@
+-}
+responseIf :: Bool -> EventResponse s e sp ep -> EventResponse s e sp ep
+responseIf True resp = resp
+responseIf False _ = NoOpResponse
+
+{-|
+Returns the provided 'EventResponse' when 'Just', otherwise returns a no-op.
+
+Useful for conditionally returning a response.
+
+@
+...
+_ -> [Model newModel, responseMaybe maybeResp]
+@
+-}
+responseMaybe :: Maybe (EventResponse s e sp ep) -> EventResponse s e sp ep
+responseMaybe (Just resp) = resp
+responseMaybe Nothing = NoOpResponse

src/Monomer/Widgets/Composite.hs

@@ -207,6 +207,8 @@ data EventResponse s e sp ep
   producer crashes without sending a value, composite will not know about it.
   -}
   | Producer (ProducerHandler e)
+  -- | Does nothing. Useful with 'responseIf' and 'responseMaybe' helpers.
+  | NoOpResponse
 
 {-|
 Configuration options for composite:
@@ -942,6 +944,7 @@ evtResponseToRequest widgetComp widgetKeys response = case response of
   Message key msg -> Just $ sendMsgTo widgetComp (CompMsgMessage key msg)
   Task task -> Just $ RunTask widgetId path task
   Producer producer -> Just $ RunProducer widgetId path producer
+  NoOpResponse -> Nothing
   where
     widgetId = widgetComp ^. L.info . L.widgetId
     path = widgetComp ^. L.info . L.path