Refactor + Expand IHaskell.Eval.Widgets

- Send [method .= custom] messages via widgetSendCustom
- Send arbitrary JSON messages via widgetSendValue
- Refactor IHaskell.Eval.Widgets, tidy up handleMessage

src/IHaskell/Eval/Widgets.hs

 module IHaskell.Eval.Widgets (
     widgetSendOpen,
-    widgetSendUpdate,
     widgetSendView,
+    widgetSendUpdate,
+    widgetSendCustom,
     widgetSendClose,
+    widgetSendValue,
     relayWidgetMessages,
     widgetHandler,
     ) where
@@ -62,6 +64,14 @@ widgetSendView = queue . View . Widget
 widgetSendClose :: IHaskellWidget a => a -> Value -> IO ()
 widgetSendClose = widgetSend Close
 
+-- | Send a [method .= custom, content .= value] comm_msg
+widgetSendCustom :: IHaskellWidget a => a -> Value -> IO ()
+widgetSendCustom = widgetSend Custom
+
+-- | Send a custom Value
+widgetSendValue :: IHaskellWidget a => a -> Value -> IO ()
+widgetSendValue widget = queue . JSONValue (Widget widget)
+
 -- | Handle a single widget message. Takes necessary actions according to the message type, such as
 -- opening comms, storing and updating widget representation in the kernel state etc.
 handleMessage :: (Message -> IO ())
@@ -70,8 +80,6 @@ handleMessage :: (Message -> IO ())
               -> WidgetMsg
               -> IO KernelState
 handleMessage send replyHeader state msg = do
-  let oldComms = openComms state
-
   case msg of
     Open widget initVal stateVal -> do
       let target = targetName widget
@@ -118,24 +126,24 @@ handleMessage send replyHeader state msg = do
           return newState
         else return state
 
-    View widget -> do
-      let uuid = getCommUUID widget
-          present = isJust $ Map.lookup uuid oldComms
+    View widget -> sendMessage widget (toJSON DisplayWidget)
 
-      -- If the widget is present, we send a display message on it's comm.
-      when present $ do
-        header <- dupHeader replyHeader CommDataMessage
-        send . CommData header uuid $ toJSON DisplayWidget
-      return state
+    Update widget value -> sendMessage widget (toJSON $ UpdateState value)
+
+    Custom widget value -> sendMessage widget (toJSON $ CustomContent value)
+
+    JSONValue widget value -> sendMessage widget value
 
-    Update widget value -> do
+  where
+    oldComms = openComms state
+    sendMessage widget value = do
       let uuid = getCommUUID widget
           present = isJust $ Map.lookup uuid oldComms
 
       -- If the widget is present, we send an update message on its comm.
       when present $ do
         header <- dupHeader replyHeader CommDataMessage
-        send . CommData header uuid . toJSON $ UpdateState value
+        send $ CommData header uuid value
       return state
 
 -- Handle messages one-by-one, while updating state simultaneously

src/IHaskell/Types.hs

@@ -182,6 +182,7 @@ data LintStatus = LintOn
                 | LintOff
   deriving (Eq, Show)
 
+-- | Send JSON objects with specific formats
 data WidgetMsg = Open Widget Value Value
                |
                -- ^ Cause the interpreter to open a new comm, and register the associated widget in
@@ -197,16 +198,24 @@ data WidgetMsg = Open Widget Value Value
                -- ^ Cause the interpreter to send a comm_msg containing a display command for the
                -- frontend.
                 Close Widget Value
-  -- ^ Cause the interpreter to close the comm associated with the widget. Also sends data with
-  -- comm_close.
+               |
+               -- ^ Cause the interpreter to close the comm associated with the widget. Also sends
+               -- data with comm_close.
+                Custom Widget Value
+               |
+               -- ^ A [method .= custom, content = value] message
+                JSONValue Widget Value
+  -- ^ A json object that is sent to the widget without modifications.
   deriving (Show, Typeable)
 
 data WidgetMethod = UpdateState Value
+                  | CustomContent Value
                   | DisplayWidget
 
 instance ToJSON WidgetMethod where
   toJSON DisplayWidget = object ["method" .= "display"]
   toJSON (UpdateState v) = object ["method" .= "update", "state" .= v]
+  toJSON (CustomContent v) = object ["method" .= "custom", "content" .= v]
 
 -- | Output of evaluation.
 data EvaluationResult =