Fix issues associated with multiple IHaskells installed.

This fix is only a halfway-fix. If there are multiple IHaskell libraries
installed, you can still end up loading an interpreted version of the
wrong one; however, now Display values are serialized to bytestring, so
this will not cause any issues with transferring those between the two
versions. Widget messages are not serializable and cannot be
serializable, so those remain a problem, but instead of crashing it will
instead emit a loud warning on stderr.

src/IHaskell/Display.hs

@@ -43,7 +43,7 @@ module IHaskell.Display (
     switchToTmpDir,
 
     -- * Internal only use
-    displayFromChan,
+    displayFromChanEncoded,
     serializeDisplay,
     Widget(..),
     ) where
@@ -151,9 +151,9 @@ displayChan = unsafePerformIO newTChanIO
 
 -- | Take everything that was put into the 'displayChan' at that point out, and make a 'Display' out
 -- of it.
-displayFromChan :: IO (Maybe Display)
-displayFromChan =
-  Just . many <$> unfoldM (atomically $ tryReadTChan displayChan)
+displayFromChanEncoded :: IO ByteString
+displayFromChanEncoded =
+  Serialize.encode <$> Just . many <$> unfoldM (atomically $ tryReadTChan displayChan)
 
 -- | Write to the display channel. The contents will be displayed in the notebook once the current
 -- execution call ends.

src/IHaskell/Eval/Evaluate.hs

@@ -333,7 +333,15 @@ evaluate kernelState code output widgetHandler = do
 
       -- Get displayed channel outputs. Merge them with normal display outputs.
       dispsMay <- if supportLibrariesAvailable state
-                    then extractValue "IHaskell.Display.displayFromChan" >>= liftIO
+                    then do
+                      getEncodedDisplays <- extractValue "IHaskell.Display.displayFromChanEncoded"
+                      case getEncodedDisplays of
+                        Left err -> error $ "Deserialization error (Evaluate.hs): " ++ err
+                        Right displaysIO -> do
+                          result <- liftIO displaysIO
+                          case Serialize.decode result of
+                            Left err  -> error $ "Deserialization error (Evaluate.hs): " ++ err
+                            Right res -> return res
                     else return Nothing
       let result =
             case dispsMay of
@@ -362,12 +370,27 @@ evaluate kernelState code output widgetHandler = do
 
 -- | Compile a string and extract a value from it. Effectively extract the result of an expression
 -- from inside the notebook environment.
-extractValue :: Typeable a => String -> Interpreter a
+extractValue :: Typeable a => String -> Interpreter (Either String a)
 extractValue expr = do
   compiled <- dynCompileExpr expr
   case fromDynamic compiled of
-    Nothing     -> error "Error casting types (Evaluate.hs): multiple IHaskell copies installed?"
-    Just result -> return result
+    Nothing     -> return (Left multipleIHaskells)
+    Just result -> return (Right result)
+
+  where
+    multipleIHaskells =
+      concat
+        [ "The installed IHaskell support libraries do not match"
+        , " the instance of IHaskell you are running.\n"
+        , "This *may* cause problems with functioning of widgets or rich media displays.\n"
+        , "This is most often caused by multiple copies of IHaskell"
+        , " being installed simultaneously in your environment.\n"
+        , "To resolve this issue, clear out your environment and reinstall IHaskell.\n"
+        , "If you are installing support libraries, make sure you only do so once:\n"
+        , "    # Run this without first running `stack install ihaskell`\n"
+        , "    stack install ihaskell-diagrams\n"
+        , "If you continue to have problems, please file an issue on Github."
+        ]
 
 flushWidgetMessages :: KernelState
                     -> [WidgetMsg]
@@ -375,12 +398,19 @@ flushWidgetMessages :: KernelState
                     -> Interpreter KernelState
 flushWidgetMessages state evalMsgs widgetHandler = do
   -- Capture all widget messages queued during code execution
-  messagesIO <- extractValue "IHaskell.Eval.Widgets.relayWidgetMessages"
-  messages <- liftIO messagesIO
-
-  -- Handle all the widget messages
-  let commMessages = evalMsgs ++ messages
-  liftIO $ widgetHandler state commMessages
+  extracted <- extractValue "IHaskell.Eval.Widgets.relayWidgetMessages"
+  liftIO $
+    case extracted of
+      Left err -> do
+        hPutStrLn stderr "Disabling IHaskell widget support due to an encountered error:"
+        hPutStrLn stderr err
+        return state
+      Right messagesIO -> do
+        messages <- messagesIO
+
+        -- Handle all the widget messages
+        let commMessages = evalMsgs ++ messages
+        widgetHandler state commMessages
 
 safely :: KernelState -> Interpreter EvalOut -> Interpreter EvalOut
 safely state = ghandle handler . ghandle sourceErrorHandler