Improve development time application reload (#239)

* Consider SDL.TextEditingEvent as an action event * When a frame requires rendering, make sure the next one is also rendered to avoid artifacts. It still does not render every frame * Fix lint * Fix typo * Add foreign-store dependency * Reuse window and avoid reinitializing SDL when running on ghci and code is reloaded * Restore previous model and widget root on code changes * Attempt to check if model's type changed (failed, since fingerprinting is not reliable) * Add Show instances to various widgets * Add configuration for fingerprint functions * Merge new root with stored root, to account for code changes in new root and data in old * Store the complete MonomerCtx type for app reload * Use recover-rtti for model change detection * Process WidgetResult when merging reloaded widget root * Validate model changes in Composite * Fix lint * Fix unit tests * Force hpack version in monomer.cabal * Add appDisableModelReuse configuration option to always ignore previous model version * Minor setup docs update * Revert to AppConfig only being parameterized by e * Restore recover-rtti dependency * Revert "Revert to AppConfig only being parameterized by e" This reverts commit 0a66513807. * Revert to previous version without restore-rtti dependency * Try to overcome build issue by enforcing ubuntu-20.04 * Do not commit .dylib files * For hpack version * Fix docs * Fix missing imports * Fix typo
2024-09-11 12:05:35 +03:00 · 2023-02-24 18:14:08 +01:00 · 2023-02-24 18:14:08 +01:00 · 5b90aebae9
commit 5b90aebae9
parent e88d5d916e
19 changed files with 341 additions and 95 deletions
--- a/dev-test-app/Main.hs
+++ b/dev-test-app/Main.hs
@ -56,6 +56,7 @@ main = do
      appWindowIcon "./assets/images/icon.png",
      appTheme darkTheme,
      appFontDef "Regular" "./assets/fonts/Roboto-Regular.ttf",
-      appInitEvent AppInit
+      appInitEvent AppInit,
+      appModelFingerprint show
      ]
    model = AppModel 0
--- a/docs/tutorials/00-setup.md
+++ b/docs/tutorials/00-setup.md
@ -195,7 +195,7 @@ non-threaded modes.

 ## Development mode

-Since compilation times can be annoying, I personally prefer to rely on
+Since compilation times can be annoying, I prefer to rely on
 [ghcid](https://github.com/ndmitchell/ghcid) for a nicer development experience.
 First, you need to install it:

@ -209,19 +209,30 @@ Then, inside your project's directory:
 ghcid
 ```

+With this you will be running your application in interpreted mode (`ghcid`
+under the hood uses `ghci`), allowing you to make changes and test them almost
+immediately.
+
+If the `appModelFingerprint` setting is provided, Monomer will attempt to reuse
+the active model when the application is reloaded. This allows for faster
+iteration since the application will return to the previous state but with all
+the code and style changes that triggered the reload. Closing the window causes
+the old model to be discarded, and the application will start from scratch when
+reloaded.
+
+In general, this will work fine, but in some cases modifying data types can
+cause ghci to crash. Restarting ghcid/ghci will solve the issue. You can read
+more details
+[here](https://hackage.haskell.org/package/monomer/docs/Monomer-Main-Types.html#v:appModelFingerprint).
+
+### VS Code
+
 If you use Visual Studio Code, you can also use this [very nice
 extension](https://marketplace.visualstudio.com/items?itemName=ndmitchell.haskell-ghcid).

 Once installed, pressing `Ctrl-Shift-P` will allow you to invoke the
 `Start Ghcid` command. You can also run `ghcid` on the command line directly.

-With this you will be running your application in interpreted mode (`ghcid`
-under the hood uses `ghci`), allowing you to make changes and test them almost
-immediately.
-
-Note: when a file is saved, a new instance of the application will be in a new
-window. The previous window needs to be closed manually.
-
 ## Notes for Intel Mac users

 If you have a discrete GPU, and you'd rather have your application use the
@ -257,7 +268,7 @@ Reference: http://supermegaultragroovy.com/2016/12/10/auto-graphics-switching
 ### ghcid

 The sample project includes custom .ghci and .ghcid files. The most important
-change in .ghci is for Mac users, since macOS does not allow graphics setup to
+change in .ghci is for Mac users since macOS does not allow graphics setup to
 happen outside the main thread (by default `ghci` spawns a thread for user
 code). If you create your custom GHCi config and run into issues, check if you
 are providing the necessary flags.
--- a/monomer.cabal
+++ b/monomer.cabal
@ -175,6 +175,7 @@ library
    , data-default >=0.5 && <0.8
    , exceptions ==0.10.*
    , extra >=1.6 && <1.9
+    , foreign-store >=0.2 && <1.0
    , formatting >=6.0 && <8.0
    , http-client >=0.6 && <0.9
    , lens >=4.16 && <6
@ -220,6 +221,7 @@ executable books
    , data-default >=0.5 && <0.8
    , exceptions ==0.10.*
    , extra >=1.6 && <1.9
+    , foreign-store >=0.2 && <1.0
    , formatting >=6.0 && <8.0
    , http-client >=0.6 && <0.9
    , lens >=4.16 && <6
@ -262,6 +264,7 @@ executable dev-test-app
    , data-default >=0.5 && <0.8
    , exceptions ==0.10.*
    , extra >=1.6 && <1.9
+    , foreign-store >=0.2 && <1.0
    , formatting >=6.0 && <8.0
    , http-client >=0.6 && <0.9
    , lens >=4.16 && <6
@ -303,6 +306,7 @@ executable generative
    , data-default >=0.5 && <0.8
    , exceptions ==0.10.*
    , extra >=1.6 && <1.9
+    , foreign-store >=0.2 && <1.0
    , formatting >=6.0 && <8.0
    , http-client >=0.6 && <0.9
    , lens >=4.16 && <6
@ -347,6 +351,7 @@ executable opengl
    , data-default >=0.5 && <0.8
    , exceptions ==0.10.*
    , extra >=1.6 && <1.9
+    , foreign-store >=0.2 && <1.0
    , formatting >=6.0 && <8.0
    , http-client >=0.6 && <0.9
    , lens >=4.16 && <6
@ -393,6 +398,7 @@ executable ticker
    , data-default >=0.5 && <0.8
    , exceptions ==0.10.*
    , extra >=1.6 && <1.9
+    , foreign-store >=0.2 && <1.0
    , formatting >=6.0 && <8.0
    , http-client >=0.6 && <0.9
    , lens >=4.16 && <6
@ -438,6 +444,7 @@ executable todo
    , data-default >=0.5 && <0.8
    , exceptions ==0.10.*
    , extra >=1.6 && <1.9
+    , foreign-store >=0.2 && <1.0
    , formatting >=6.0 && <8.0
    , http-client >=0.6 && <0.9
    , lens >=4.16 && <6
@ -488,6 +495,7 @@ executable tutorial
    , data-default >=0.5 && <0.8
    , exceptions ==0.10.*
    , extra >=1.6 && <1.9
+    , foreign-store >=0.2 && <1.0
    , formatting >=6.0 && <8.0
    , http-client >=0.6 && <0.9
    , lens >=4.16 && <6
@ -581,6 +589,7 @@ test-suite monomer-test
    , data-default >=0.5 && <0.8
    , exceptions ==0.10.*
    , extra >=1.6 && <1.9
+    , foreign-store >=0.2 && <1.0
    , formatting >=6.0 && <8.0
    , hspec >=2.4 && <3.0
    , http-client >=0.6 && <0.9
--- a/package.yaml
+++ b/package.yaml
@ -35,6 +35,7 @@ dependencies:
  - data-default >= 0.5 && < 0.8
  - exceptions >= 0.10 && < 0.11
  - extra >= 1.6 && < 1.9
+  - foreign-store >= 0.2 && < 1.0
  - formatting >= 6.0 && < 8.0
  - http-client >= 0.6 && < 0.9
  - JuicyPixels >= 3.2.9 && < 3.5
--- a/src/Monomer/Core/Lens.hs
+++ b/src/Monomer/Core/Lens.hs
@ -16,7 +16,6 @@ module Monomer.Core.Lens where

 import Control.Lens.TH (abbreviatedFields, makeLensesWith, makePrisms)

-import Monomer.Common.Lens
 import Monomer.Core.StyleTypes
 import Monomer.Core.ThemeTypes
 import Monomer.Core.WidgetTypes
--- a/src/Monomer/Core/WidgetTypes.hs
+++ b/src/Monomer/Core/WidgetTypes.hs
@ -311,6 +311,8 @@ data WidgetEnv s e = WidgetEnv {
  _weOs :: Text,
  -- | Device pixel rate.
  _weDpr :: Double,
+  -- | Indicates whether the application is running on ghci.
+  _weIsGhci :: Bool,
  -- | The timestamp in milliseconds when the application started.
  _weAppStartTs :: Millisecond,
  -- | Provides helper functions for calculating text size.
--- a/src/Monomer/Helper.hs
+++ b/src/Monomer/Helper.hs
@ -15,10 +15,12 @@ module Monomer.Helper where

 import Control.Exception (SomeException, catch)
 import Control.Monad.IO.Class (MonadIO)
+import Data.Functor ((<&>))
 import Data.Sequence (Seq(..))
 import System.IO (hPutStrLn, stderr)

 import qualified Data.Sequence as Seq
+import qualified System.Environment as SE

 -- | Concats a list of Monoids or returns Nothing if empty.
 maybeConcat :: Monoid a => [a] -> Maybe a
@ -94,7 +96,12 @@ headMay :: [a] -> Maybe a
 headMay [] = Nothing
 headMay (x : _) = Just x

+-- | Attempts to print on stderr, with fallback to stdout on failure.
 putStrLnErr :: String -> IO ()
 putStrLnErr msg = catchAny
  (hPutStrLn stderr msg)
  (const $ putStrLn msg)
+
+-- | Checks if the application is running in ghci.
+isGhciRunning :: IO Bool
+isGhciRunning = SE.getProgName <&> (== "<interactive>")
--- a/src/Monomer/Main/Core.hs
+++ b/src/Monomer/Main/Core.hs
@ -9,6 +9,7 @@ Portability : non-portable
 Core glue for running an application.
 -}
 {-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE RecordWildCards #-}
 {-# LANGUAGE Strict #-}

@ -22,25 +23,27 @@ module Monomer.Main.Core (
 import Control.Concurrent
 import Control.Concurrent.STM.TChan (TChan, newTChanIO, readTChan, writeTChan)
 import Control.Exception
-import Control.Lens ((&), (^.), (.=), (.~), use)
+import Control.Lens ((&), (^.), (.=), (.~), _2, use)
 import Control.Monad (unless, void, when)
 import Control.Monad.Extra
 import Control.Monad.State
 import Control.Monad.STM (atomically)
 import Data.Default
 import Data.Either (isLeft)
-import Data.Maybe (fromMaybe)
+import Data.Maybe (fromMaybe, fromJust, isJust)
 import Data.Map (Map)
 import Data.List (foldl')
 import Data.Text (Text)
 import Data.Time
 import Data.Time.Clock.POSIX (utcTimeToPOSIXSeconds)
+import Data.Word (Word32)
 import Graphics.GL

 import qualified Data.Map as Map
-import qualified Data.Text as T
-import qualified SDL
 import qualified Data.Sequence as Seq
+import qualified Data.Text as T
+import qualified Foreign.Store as FS
+import qualified SDL

 import Monomer.Core
 import Monomer.Core.Combinators
@ -51,7 +54,7 @@ import Monomer.Main.Types
 import Monomer.Main.Util
 import Monomer.Main.WidgetTask
 import Monomer.Graphics
-import Monomer.Helper (catchAny, putStrLnErr)
+import Monomer.Helper (catchAny, putStrLnErr, isGhciRunning)
 import Monomer.Widgets.Composite

 import qualified Monomer.Lens as L
@ -60,7 +63,7 @@ import qualified Monomer.Lens as L
 Type of response an App event handler can return, with __s__ being the model and
 __e__ the user's event type.
 -}
-type AppEventResponse s e = EventResponse s e s ()
+type AppEventResponse s e = EventResponse s e s e

 -- | Type of an App event handler.
 type AppEventHandler s e
@ -73,7 +76,9 @@ type AppEventHandler s e
 -- | Type of the function responsible of creating the App UI.
 type AppUIBuilder s e = UIBuilder s e

+-- | Updated information for the current step of the event loop.
 data MainLoopArgs sp e ep = MainLoopArgs {
+  _mlIsGhci :: Bool,
  _mlOS :: Text,
  _mlTheme :: Theme,
  _mlAppStartTs :: Millisecond,
@ -87,12 +92,32 @@ data MainLoopArgs sp e ep = MainLoopArgs {
  _mlWidgetShared :: MVar (Map Text WidgetShared)
 }

+-- | State information for the rendering thread.
 data RenderState s e = RenderState {
  _rstDpr :: Double,
  _rstWidgetEnv :: WidgetEnv s e,
  _rstRootNode :: WidgetNode s e
 }

+{-|
+Information for hot reload of an application running on ghci.
+
+When running in interpreted mode, SDL initialization and window creation data
+will be reused on further code updates.
+-}
+data MonomerReloadData s e = MonomerReloadData {
+  -- | The active window.
+  _mrdWindow :: !SDL.Window,
+  -- | The OpenGL context associated to the window.
+  _mrdGlContext :: !SDL.GLContext,
+  -- | The latest context.
+  _mrdMonomerCtx :: !(MonomerCtx s e),
+  -- | The fingerprint of the model. Used to detect changes in the data type.
+  _mrdModelFp :: !String,
+  -- | The latest widget tree.
+  _mrdRoot :: !(WidgetNode s e)
+}
+
 {-|
 Runs an application, creating the UI with the provided function and initial
 model, handling future events with the event handler.
@ -105,34 +130,45 @@ startApp
  => s                    -- ^ The initial model.
  -> AppEventHandler s e  -- ^ The event handler.
  -> AppUIBuilder s e     -- ^ The UI builder.
-  -> [AppConfig e]        -- ^ The application config.
+  -> [AppConfig s e]      -- ^ The application config.
  -> IO ()                -- ^ The application action.
-startApp model eventHandler uiBuilder configs = do
-  (window, dpr, epr, glCtx) <- initSDLWindow config
-  vpSize <- getViewportSize window dpr
+startApp newModel eventHandler uiBuilder configs = do
+  isGhci <- isGhciRunning
  channel <- newTChanIO

-  let monomerCtx = initMonomerCtx window channel vpSize dpr epr model
+  (model, oldRoot) <- retrieveModelAndRoot config newModel newRoot
+  (window, glCtx, ctx) <- retrieveSDLWindow config channel model

-  runStateT (runAppLoop window glCtx channel appWidget config) monomerCtx
-  destroySDLWindow window
+  when isGhci $
+    setReloadData (MonomerReloadData window glCtx ctx modelFp newRoot)
+
+  resp <- runStateT (runAppLoop window glCtx channel oldRoot newRoot config) ctx
+
+  -- Even when running on ghci, if exitApplication == True it means the user
+  -- closed the window and it will need to be created again on reload.
+  when (not isGhci || resp ^. _2 . L.exitApplication) $ do
+    destroySDLWindow window
+    resetReloadData
  where
    config = mconcat configs
    compCfgs
      = (onInit <$> _apcInitEvent config)
      ++ (onDispose <$> _apcDisposeEvent config)
      ++ (onResize <$> _apcResizeEvent config)
-    appWidget = composite_ "app" id uiBuilder eventHandler compCfgs
+    ~modelFp = maybe "" ($ newModel) (_apcModelFingerprintFn config)
+    newRoot = composite_ "app" id uiBuilder eventHandler compCfgs

 runAppLoop
  :: (MonomerM sp ep m, Eq sp, WidgetEvent e, WidgetEvent ep)
  => SDL.Window
  -> SDL.GLContext
  -> TChan (RenderMsg sp ep)
+  -> Maybe (WidgetNode sp ep)
  -> WidgetNode sp ep
-  -> AppConfig e
+  -> AppConfig s e
  -> m ()
-runAppLoop window glCtx channel widgetRoot config = do
+runAppLoop window glCtx channel mRootOld newRoot config = do
+  isGhci <- liftIO isGhciRunning
  dpr <- use L.dpr
  winSize <- use L.windowSize

@ -150,10 +186,16 @@ runAppLoop window glCtx channel widgetRoot config = do
  os <- liftIO getPlatform
  widgetSharedMVar <- liftIO $ newMVar Map.empty
  fontManager <- liftIO $ makeFontManager fonts dpr
+  -- Restore previous state
+  hovered <- getHoveredPath
+  focused <- getFocusedPath
+  overlay <- getOverlayPath
+  dragged <- getDraggedMsgInfo

  let wenv = WidgetEnv {
    _weOs = os,
    _weDpr = dpr,
+    _weIsGhci = isGhci,
    _weAppStartTs = appStartTs,
    _weFontManager = fontManager,
    _weFindBranchByPath = const Seq.empty,
@ -164,10 +206,10 @@ runAppLoop window glCtx channel widgetRoot config = do
    _weWidgetShared = widgetSharedMVar,
    _weWidgetKeyMap = Map.empty,
    _weCursor = Nothing,
-    _weHoveredPath = Nothing,
-    _weFocusedPath = emptyPath,
-    _weOverlayPath = Nothing,
-    _weDragStatus = Nothing,
+    _weHoveredPath = hovered,
+    _weFocusedPath = focused,
+    _weOverlayPath = overlay,
+    _weDragStatus = dragged,
    _weMainBtnPress = Nothing,
    _weModel = model,
    _weInputStatus = def,
@ -178,9 +220,13 @@ runAppLoop window glCtx channel widgetRoot config = do
    _weViewport = Rect 0 0 (winSize ^. L.w) (winSize ^. L.h),
    _weOffset = def
  }
-  let pathReadyRoot = widgetRoot
+  let tmpRoot = newRoot
        & L.info . L.path .~ rootPath
        & L.info . L.widgetId .~ WidgetId (wenv ^. L.timestamp) rootPath
+  let mergeNewRoot newRoot oldRoot = result where
+        result = widgetMerge (newRoot ^. L.widget) wenv newRoot oldRoot
+  let result = maybe (resultNode tmpRoot) (mergeNewRoot tmpRoot) mRootOld
+  let appRoot = result ^. L.node
  let makeMainThreadRenderer = do
        renderer <- liftIO $ makeRenderer fonts dpr
        L.renderMethod .= Left renderer
@ -192,10 +238,10 @@ runAppLoop window glCtx channel widgetRoot config = do

      liftIO . void . forkOS $
        {-
-        The wenv and widgetRoot values are not used, since they are replaced
+        The wenv and appRoot values are not used, since they are replaced
        during MsgInit. Kept to avoid issues with the Strict pragma.
        -}
-        startRenderThread stpChan channel window glCtx fonts dpr wenv widgetRoot
+        startRenderThread stpChan channel window glCtx fonts dpr wenv appRoot

      setupRes <- liftIO . atomically $ readTChan stpChan

@ -212,7 +258,10 @@ runAppLoop window glCtx channel widgetRoot config = do
      makeMainThreadRenderer

  handleResourcesInit
-  (newWenv, newRoot, _) <- handleWidgetInit wenv pathReadyRoot
+
+  (newWenv, newAppRoot, _) <- if isJust mRootOld
+    then handleWidgetResult wenv True result
+    else handleWidgetInit wenv appRoot

  {-
  Deferred initialization step to account for Widgets that rely on OpenGL. They
@ -221,13 +270,14 @@ runAppLoop window glCtx channel widgetRoot config = do
  -}
  case setupRes of
    RenderSetupMulti -> do
-      liftIO . atomically $ writeTChan channel (MsgInit newWenv newRoot)
+      liftIO . atomically $ writeTChan channel (MsgInit newWenv newAppRoot)

      unless (isLinux newWenv) $
        liftIO $ watchWindowResize channel
    _ -> return ()

  let loopArgs = MainLoopArgs {
+    _mlIsGhci = isGhci,
    _mlOS = os,
    _mlTheme = theme,
    _mlMaxFps = maxFps,
@ -237,7 +287,7 @@ runAppLoop window glCtx channel widgetRoot config = do
    _mlFrameAccumTs = 0,
    _mlFrameCount = 0,
    _mlExitEvents = exitEvents,
-    _mlWidgetRoot = newRoot,
+    _mlWidgetRoot = newAppRoot,
    _mlWidgetShared = widgetSharedMVar
  }

@ -249,7 +299,7 @@ mainLoop
  :: (MonomerM sp ep m, WidgetEvent e)
  => SDL.Window
  -> FontManager
-  -> AppConfig e
+  -> AppConfig s e
  -> MainLoopArgs sp e ep
  -> m ()
 mainLoop window fontManager config loopArgs = do
@ -301,6 +351,7 @@ mainLoop window fontManager config loopArgs = do
  let wenv = WidgetEnv {
    _weOs = _mlOS,
    _weDpr = dpr,
+    _weIsGhci = _mlIsGhci,
    _weAppStartTs = _mlAppStartTs,
    _weFontManager = fontManager,
    _weFindBranchByPath = findChildBranchByPath wenv _mlWidgetRoot,
@ -379,6 +430,7 @@ mainLoop window fontManager config loopArgs = do
  let tempDelay = abs (frameLength - fromIntegral remainingMs * 1000)
  let nextFrameDelay = min frameLength tempDelay
  let latestRenderTs = if renderNeeded then startTs else _mlLatestRenderTs
+  let newModel = newWenv ^. L.model
  let newLoopArgs = loopArgs {
    _mlLatestRenderTs = latestRenderTs,
    _mlFrameStartTs = startTs,
@ -387,6 +439,10 @@ mainLoop window fontManager config loopArgs = do
    _mlWidgetRoot = newRoot
  }

+  when _mlIsGhci $ do
+    ctx <- get
+    liftIO $ updateReloadData ctx newRoot
+
  liftIO $ threadDelay nextFrameDelay

  shouldQuit <- use L.exitApplication
@ -595,3 +651,66 @@ getElapsedTimestampSince :: MonadIO m => Millisecond -> m Millisecond
 getElapsedTimestampSince start = do
  ts <- getCurrentTimestamp
  return (ts - start)
+
+-- Hot reload support
+
+reloadStoreId :: Word32
+reloadStoreId = 0
+
+getReloadData :: IO (Maybe (MonomerReloadData s e))
+getReloadData = FS.lookupStore reloadStoreId >>= \case
+  Just{} -> Just <$> FS.readStore (FS.Store reloadStoreId)
+  _ -> return Nothing
+
+setReloadData :: MonomerReloadData s e -> IO ()
+setReloadData = FS.writeStore (FS.Store reloadStoreId)
+
+resetReloadData :: IO ()
+resetReloadData = FS.deleteStore (FS.Store reloadStoreId)
+
+updateReloadData :: MonomerCtx s e -> WidgetNode s e -> IO ()
+updateReloadData context widgetRoot = do
+  whenJustM getReloadData $ \rd ->
+    setReloadData rd {
+      _mrdMonomerCtx = context,
+      _mrdRoot = widgetRoot
+    }
+
+{-|
+When running in GHCi, avoids reinitializing SDL, reuses the existing window and
+restores the model and (merged) widget tree when code is reloaded.
+-}
+retrieveSDLWindow
+  :: AppConfig s e
+  -> TChan (RenderMsg s e)
+  -> s
+  -> IO (SDL.Window, SDL.GLContext, MonomerCtx s e)
+retrieveSDLWindow config channel model = do
+  getReloadData >>= \case
+    Just rd -> return (_mrdWindow rd, _mrdGlContext rd, newCtx) where
+      ctx = _mrdMonomerCtx rd
+      newCtx = ctx {
+        _mcMainModel = model,
+        _mcRenderMethod = Right channel
+      }
+    Nothing -> do
+      (window, dpr, epr, ctxRender) <- initSDLWindow config
+      vpSize <- getViewportSize window dpr
+      let newCtx = initMonomerCtx window channel vpSize dpr epr model
+      return (window, ctxRender, newCtx)
+
+retrieveModelAndRoot
+  :: WidgetModel s
+  => AppConfig s e
+  -> s
+  -> WidgetNode s e
+  -> IO (s, Maybe (WidgetNode s e))
+retrieveModelAndRoot config newModel newRoot = getReloadData >>= \case
+  Just rd
+    | attemptModelReuse && fingerprint == _mrdModelFp rd ->
+        return (_mrdMonomerCtx rd ^. L.mainModel, Just (_mrdRoot rd))
+  _ -> do
+    return (newModel, Nothing)
+  where
+    attemptModelReuse = isJust (_apcModelFingerprintFn config)
+    ~fingerprint = fromJust (_apcModelFingerprintFn config) newModel
--- a/src/Monomer/Main/Platform.hs
+++ b/src/Monomer/Main/Platform.hs
@ -57,7 +57,7 @@ defaultWindowSize :: (Int, Int)
 defaultWindowSize = (800, 600)

 -- | Creates and initializes a window using the provided configuration.
-initSDLWindow :: AppConfig e -> IO (SDL.Window, Double, Double, SDL.GLContext)
+initSDLWindow :: AppConfig s e -> IO (SDL.Window, Double, Double, SDL.GLContext)
 initSDLWindow config = do
  SDL.initialize [SDL.InitVideo]

@ -156,7 +156,7 @@ initSDLWindow config = do
      Just MainWindowMaximized -> True
      _ -> False

-setWindowIcon :: SDL.Window -> AppConfig e -> IO ()
+setWindowIcon :: SDL.Window -> AppConfig s e -> IO ()
 setWindowIcon (SIT.Window winPtr) config =
  forM_ (_apcWindowIcon config) $ \iconPath ->
    flip catchAny handleException $ do
--- a/src/Monomer/Main/Types.hs
+++ b/src/Monomer/Main/Types.hs
@ -35,7 +35,6 @@ import qualified SDL
 import qualified SDL.Raw.Types as SDLR

 import Monomer.Common
-import Monomer.Core.Combinators
 import Monomer.Core.StyleTypes
 import Monomer.Core.ThemeTypes
 import Monomer.Core.WidgetTypes
@ -153,7 +152,7 @@ data MainWindowState
  deriving (Eq, Show)

 -- | Main application config.
-data AppConfig e = AppConfig {
+data AppConfig s e = AppConfig {
  -- | Initial size of the main window.
  _apcWindowState :: Maybe MainWindowState,
  -- | Title of the main window.
@ -208,10 +207,13 @@ data AppConfig e = AppConfig {
  -- | Whether compositing should be disabled. Defaults to False.
  _apcDisableCompositing :: Maybe Bool,
  -- | Whether the screensaver should be disabled. Defaults to False.
-  _apcDisableScreensaver :: Maybe Bool
+  _apcDisableScreensaver :: Maybe Bool,
+  -- | Extracts a String based fingerprint from the application's model.
+  --   See 'appFingerprint' for more details.
+  _apcModelFingerprintFn :: Maybe (s -> String)
 }

-instance Default (AppConfig e) where
+instance Default (AppConfig s e) where
  def = AppConfig {
    _apcWindowState = Nothing,
    _apcWindowTitle = Nothing,
@ -233,10 +235,11 @@ instance Default (AppConfig e) where
    _apcInvertWheelX = Nothing,
    _apcInvertWheelY = Nothing,
    _apcDisableCompositing = Nothing,
-    _apcDisableScreensaver = Nothing
+    _apcDisableScreensaver = Nothing,
+    _apcModelFingerprintFn = Nothing
  }

-instance Semigroup (AppConfig e) where
+instance Semigroup (AppConfig s e) where
  (<>) a1 a2 = AppConfig {
    _apcWindowState = _apcWindowState a2 <|> _apcWindowState a1,
    _apcWindowTitle = _apcWindowTitle a2 <|> _apcWindowTitle a1,
@ -258,38 +261,39 @@ instance Semigroup (AppConfig e) where
    _apcInvertWheelX = _apcInvertWheelX a2 <|> _apcInvertWheelX a1,
    _apcInvertWheelY = _apcInvertWheelY a2 <|> _apcInvertWheelY a1,
    _apcDisableCompositing = _apcDisableCompositing a2 <|> _apcDisableCompositing a1,
-    _apcDisableScreensaver = _apcDisableScreensaver a2 <|> _apcDisableScreensaver a1
+    _apcDisableScreensaver = _apcDisableScreensaver a2 <|> _apcDisableScreensaver a1,
+    _apcModelFingerprintFn = _apcModelFingerprintFn a2 <|> _apcModelFingerprintFn a1
  }

-instance Monoid (AppConfig e) where
+instance Monoid (AppConfig s e) where
  mempty = def

 -- | Initial size of the main window.
-appWindowState :: MainWindowState -> AppConfig e
+appWindowState :: MainWindowState -> AppConfig s e
 appWindowState title = def {
  _apcWindowState = Just title
 }

 -- | Title of the main window.
-appWindowTitle :: Text -> AppConfig e
+appWindowTitle :: Text -> AppConfig s e
 appWindowTitle title = def {
  _apcWindowTitle = Just title
 }

 -- | Whether the main window is resizable.
-appWindowResizable :: Bool -> AppConfig e
+appWindowResizable :: Bool -> AppConfig s e
 appWindowResizable resizable = def {
  _apcWindowResizable = Just resizable
 }

 -- | Whether the main window has a border.
-appWindowBorder :: Bool -> AppConfig e
+appWindowBorder :: Bool -> AppConfig s e
 appWindowBorder border = def {
  _apcWindowBorder = Just border
 }

 -- | Path to an icon file in BMP format.
-appWindowIcon :: Text -> AppConfig e
+appWindowIcon :: Text -> AppConfig s e
 appWindowIcon path = def {
  _apcWindowIcon = Just path
 }
@ -314,7 +318,7 @@ This flag is no longer necessary for those cases, since the library will:
 -}
 {-# DEPRECATED appRenderOnMainThread
  "Should no longer be needed. Check appRenderOnMainThread's Haddock page." #-}
-appRenderOnMainThread :: AppConfig e
+appRenderOnMainThread :: AppConfig s e
 appRenderOnMainThread = def {
  _apcUseRenderThread = Just False
 }
@ -324,7 +328,7 @@ Max number of FPS the application will run on. It does not necessarily mean
 rendering will happen every frame, but events and schedules will be checked at
 this rate and may cause it.
 -}
-appMaxFps :: Int -> AppConfig e
+appMaxFps :: Int -> AppConfig s e
 appMaxFps fps = def {
  _apcMaxFps = Just fps
 }
@ -334,7 +338,7 @@ Scale factor to apply to the viewport. This factor only affects the content, not
 the size of the window. It is applied in addition to the detected display scale
 factor, and can be useful if the detected value is not the desired.
 -}
-appScaleFactor :: Double -> AppConfig e
+appScaleFactor :: Double -> AppConfig s e
 appScaleFactor factor = def {
  _apcScaleFactor = Just factor
 }
@ -377,22 +381,22 @@ Considering the above, when SDL_GetDisplayDPI fails, the library assumes that a
 screen width larger than 1920 belongs to an HiDPI display and uses a scale
 factor of 2. This factor is used to scale the window size and the content.
 -}
-appDisableAutoScale :: Bool -> AppConfig e
+appDisableAutoScale :: Bool -> AppConfig s e
 appDisableAutoScale disable = def {
  _apcDisableAutoScale = Just disable
 }

 {-|
-Available fonts to the application, loaded from the specified path. 
+Available fonts to the application, loaded from the specified path.
 Specifying no fonts will make it impossible to render text.
 -}
-appFontDef :: Text -> Text -> AppConfig e
+appFontDef :: Text -> Text -> AppConfig s e
 appFontDef name path = def {
  _apcFonts = [ FontDefFile name path ]
 }

 {-|
-Available fonts to the application, loaded from the bytes in memory. 
+Available fonts to the application, loaded from the bytes in memory.
 Specifying no fonts will make it impossible to render text.

 One use case for this function is to embed fonts in the application, without the need to distribute the font files.
@ -401,49 +405,49 @@ The [file-embed](https://hackage.haskell.org/package/file-embed-0.0.15.0/docs/Da
 appFontDefMemory "memoryFont" $(embedFile "dirName/fileName")
@
 -}
-appFontDefMem :: Text -> ByteString -> AppConfig e
+appFontDefMem :: Text -> ByteString -> AppConfig s e
 appFontDefMem name bytes = def {
  _apcFonts = [ FontDefMem name bytes ]
 }

 -- | Initial theme.
-appTheme :: Theme -> AppConfig e
+appTheme :: Theme -> AppConfig s e
 appTheme t = def {
  _apcTheme = Just t
 }

 -- | Initial event, useful for loading resources.
-appInitEvent :: e -> AppConfig e
+appInitEvent :: e -> AppConfig s e
 appInitEvent evt = def {
  _apcInitEvent = [evt]
 }

 -- | Dispose event, useful for closing resources.
-appDisposeEvent :: e -> AppConfig e
+appDisposeEvent :: e -> AppConfig s e
 appDisposeEvent evt = def {
  _apcDisposeEvent = [evt]
 }

 -- | Exit event, useful for cancelling an application close event.
-appExitEvent :: e -> AppConfig e
+appExitEvent :: e -> AppConfig s e
 appExitEvent evt = def {
  _apcExitEvent = [evt]
 }

 -- | Resize event handler.
-appResizeEvent :: (Rect -> e) -> AppConfig e
+appResizeEvent :: (Rect -> e) -> AppConfig s e
 appResizeEvent evt = def {
  _apcResizeEvent = [evt]
 }

 -- | Defines which mouse button is considered main.
-appMainButton :: Button -> AppConfig e
+appMainButton :: Button -> AppConfig s e
 appMainButton btn = def {
  _apcMainButton = Just btn
 }

 -- | Defines which mouse button is considered secondary or context button.
-appContextButton :: Button -> AppConfig e
+appContextButton :: Button -> AppConfig s e
 appContextButton btn = def {
  _apcContextButton = Just btn
 }
@ -452,7 +456,7 @@ appContextButton btn = def {
 Whether the horizontal wheel/trackpad movement should be inverted. In general
 platform detection should do the right thing.
 -}
-appInvertWheelX :: Bool -> AppConfig e
+appInvertWheelX :: Bool -> AppConfig s e
 appInvertWheelX invert = def {
  _apcInvertWheelX = Just invert
 }
@ -461,7 +465,7 @@ appInvertWheelX invert = def {
 Whether the vertical wheel/trackpad movement should be inverted. In general
 platform detection should do the right thing.
 -}
-appInvertWheelY :: Bool -> AppConfig e
+appInvertWheelY :: Bool -> AppConfig s e
 appInvertWheelY invert = def {
  _apcInvertWheelY = Just invert
 }
@ -471,10 +475,10 @@ Whether compositing should be disabled. Linux only, ignored in other platforms.
 Defaults to False.

 Desktop applications should leave compositing as is since disabling it may
-cause visual glitches in other programs. When creating games or fullscreen
+cause visual glitches in other programs. When creating games or full-screen
 applications, disabling compositing may improve performance.
 -}
-appDisableCompositing :: Bool -> AppConfig e
+appDisableCompositing :: Bool -> AppConfig s e
 appDisableCompositing disable = def {
  _apcDisableCompositing = Just disable
 }
@ -484,9 +488,56 @@ Whether the screensaver should be disabled. Defaults to False.

 Desktop applications should leave the screensaver as is since disabling it also
 affects power saving features, including turning off the screen. When creating
-games or fullscreen applications, disabling the screensaver may make sense.
+games or full-screen applications, disabling the screensaver may make sense.
 -}
-appDisableScreensaver :: Bool -> AppConfig e
+appDisableScreensaver :: Bool -> AppConfig s e
 appDisableScreensaver disable = def {
  _apcDisableScreensaver = Just disable
 }
+
+{-|
+Generates a fingerprint from the application's model. This is used to identify
+whether the application should attempt to reuse the model between reloads when
+running in interpreted mode. Since Monomer uses the model to build the UI,
+reusing the old model allows for quicker iteration as the application will be
+restored to its previous state before being reloaded. By default, unless a
+fingerprint function is provided, the model will not be reused and the
+application will start from scratch.
+
+The fingerprint function is applied to the user provided model on application's
+startup only, not on subsequent updates during the application's lifetime. When
+a reload occurs, the original fingerprint of the model is compared against the
+fingerprint of the newly provided version of the model; if they match, it is
+assumed that the latest version of the original model can be reused. If the
+fingerprint changed because its data or data type changed, the new model will be
+used. The rationale is that, since the model is provided by the developer at
+startup, if the fingerprints match then only changes to business logic/UI have
+been made.
+
+A fingerprint function is used because trying to compare two different versions
+of a data type using its 'Eq' instance will result in ghci crashing. Creating a
+string based fingerprint as soon as the instance is available is a workaround
+for this issue. Ideally, the fingerprint would incorporate enough information to
+detect type and data changes.
+
+A simple approach is using the 'show' function to generate the fingerprint,
+although in some cases it may not be possible, maybe because the type does not
+implement it or it does not include enough information.
+
+An alternative to 'show' is <https://hackage.haskell.org/package/recover-rtti
+recover-rtti>'s __anythingToString__. This function returns a string
+representation of the data, although it does not include the name of record
+fields. In general this is not an issue, but changing a field's type from 'Int'
+to 'Long' will go undetected and cause a crash.
+
+Ideally, we could use "GHC.Fingerprint.Fingerprint" to detect changes in the
+model's type, but unfortunately this is not reliable since this fingerprint is
+based on the type's name only.
+
+GHC issue with more details: https://gitlab.haskell.org/ghc/ghc/-/issues/7897.
+Related Hint issue: https://github.com/haskell-hint/hint/issues/31.
+-}
+appModelFingerprint :: (s -> String) -> AppConfig s e
+appModelFingerprint fn = def {
+  _apcModelFingerprintFn = Just fn
+}
--- a/src/Monomer/Widgets/Composite.hs
+++ b/src/Monomer/Widgets/Composite.hs
@ -245,6 +245,7 @@ data CompositeCfg s e sp ep = CompositeCfg {
  _cmcMergeRequired :: Maybe (MergeRequired s e),
  _cmcMergeReqs :: [MergeReqsHandler s e sp],
  _cmcMergeModel :: Maybe (MergeModelHandler s e sp),
+  _cmcModelFingerprintFn :: Maybe (s -> String),
  _cmcOnInitReq :: [WidgetRequest s e],
  _cmcOnDisposeReq :: [WidgetRequest s e],
  _cmcOnResize :: [Rect -> e],
@ -255,9 +256,10 @@ data CompositeCfg s e sp ep = CompositeCfg {

 instance Default (CompositeCfg s e sp ep) where
  def = CompositeCfg {
-    _cmcMergeModel = Nothing,
    _cmcMergeRequired = Nothing,
    _cmcMergeReqs = [],
+    _cmcMergeModel = Nothing,
+    _cmcModelFingerprintFn = Nothing,
    _cmcOnInitReq = [],
    _cmcOnDisposeReq = [],
    _cmcOnResize = [],
@ -268,9 +270,10 @@ instance Default (CompositeCfg s e sp ep) where

 instance Semigroup (CompositeCfg s e sp ep) where
  (<>) c1 c2 = CompositeCfg {
-    _cmcMergeModel = _cmcMergeModel c2 <|> _cmcMergeModel c1,
    _cmcMergeRequired = _cmcMergeRequired c2 <|> _cmcMergeRequired c1,
    _cmcMergeReqs = _cmcMergeReqs c1 <> _cmcMergeReqs c2,
+    _cmcMergeModel = _cmcMergeModel c2 <|> _cmcMergeModel c1,
+    _cmcModelFingerprintFn = _cmcModelFingerprintFn c2 <|> _cmcModelFingerprintFn c1,
    _cmcOnInitReq = _cmcOnInitReq c1 <> _cmcOnInitReq c2,
    _cmcOnDisposeReq = _cmcOnDisposeReq c1 <> _cmcOnDisposeReq c2,
    _cmcOnResize = _cmcOnResize c1 <> _cmcOnResize c2,
@ -342,6 +345,16 @@ compositeMergeReqs fn = def {
  _cmcMergeReqs = [fn]
 }

+{-|
+Generates a fingerprint to detect if the model can be reused.
+
+See 'Monomer.Main.Types.appModelFingerprint' for details.
+-}
+compositeModelFingerprint :: (s -> String) -> CompositeCfg s e sp ep
+compositeModelFingerprint fn = def {
+  _cmcModelFingerprintFn = Just fn
+}
+
 {-|
 Generate events during the merge process.

@ -361,7 +374,7 @@ Allows updating the composite model with information from the parent model.
 Useful when the composite needs a more complex model than what the user is
 binding.

-For example, a database record may be binded as the model from the parent, but
+For example, a database record may be bound as the model from the parent, but
 the composite needs its own boolean flags to toggle visibility on different
 sections.

@ -380,6 +393,7 @@ data Composite s e sp ep = Composite {
  _cmpMergeRequired :: MergeRequired s e,
  _cmpMergeReqs :: [MergeReqsHandler s e sp],
  _cmpMergeModel :: Maybe (MergeModelHandler s e sp),
+  _cmpModelFingerprintFn :: Maybe (s -> String),
  _cmpOnInitReq :: [WidgetRequest s e],
  _cmpOnDisposeReq :: [WidgetRequest s e],
  _cmpOnResize :: [Rect -> e],
@ -389,10 +403,11 @@ data Composite s e sp ep = Composite {
 }

 data CompositeState s e = CompositeState {
+  _cpsFingerprint :: !String,
  _cpsModel :: !(Maybe s),
  _cpsRoot :: !(WidgetNode s e),
  _cpsWidgetKeyMap :: WidgetKeyMap s e
-}
+} deriving Show

 data ReducedEvents s e sp ep = ReducedEvents {
  _reModel :: s,
@ -483,6 +498,7 @@ compositeD_ wType wData uiBuilder evtHandler configs = newNode where
    _cmpMergeRequired = mergeReq,
    _cmpMergeReqs = _cmcMergeReqs config,
    _cmpMergeModel = _cmcMergeModel config,
+    _cmpModelFingerprintFn = _cmcModelFingerprintFn config,
    _cmpOnInitReq = _cmcOnInitReq config,
    _cmpOnDisposeReq = _cmcOnDisposeReq config,
    _cmpOnResize = _cmcOnResize config,
@ -490,7 +506,7 @@ compositeD_ wType wData uiBuilder evtHandler configs = newNode where
    _cmpOnEnabledChange = _cmcOnEnabledChange config,
    _cmpOnVisibleChange = _cmcOnVisibleChange config
  }
-  state = CompositeState Nothing widgetRoot M.empty
+  state = CompositeState "" Nothing widgetRoot M.empty
  widget = createComposite composite state
  !newNode = defaultWidgetNode wType widget

@ -542,6 +558,8 @@ compositeInit comp state wenv widgetComp = newResult where

  WidgetResult root reqs = widgetInit (tempRoot ^. L.widget) cwenv tempRoot
  !newState = state {
+    -- Using model could cause issues in merge if the model type changes
+    _cpsFingerprint = modelFingerprint comp cwenv userModel,
    _cpsModel = Just model,
    _cpsRoot = root,
    _cpsWidgetKeyMap = collectWidgetKeys M.empty root
@ -567,14 +585,19 @@ compositeMerge comp state wenv newComp oldComp = newResult where
  widgetId = oldComp ^. L.info . L.widgetId
  oldState = widgetGetState (oldComp ^. L.widget) wenv oldComp
  validState = fromMaybe state (useState oldState)
-  CompositeState oldModel oldRoot oldWidgetKeys = validState
+  CompositeState oldFingerprint oldModel oldRoot oldWidgetKeys = validState

  !mergeModel = _cmpMergeModel comp
  !parentModel = wenv ^. L.model
  !userModel = getUserModel comp wenv
+  !newFingerprint = modelFingerprint comp cwenv userModel
+  !discardModelReload = isReload && oldFingerprint /= newFingerprint
+
  !model = case mergeModel of
-    Just merge -> merge cwenv parentModel (fromJust oldModel) userModel where
-      cwenv = convertWidgetEnv wenv oldWidgetKeys userModel
+    Just merge
+      | not discardModelReload -> mergedModel where
+        cwenv = convertWidgetEnv wenv oldWidgetKeys userModel
+        mergedModel = merge cwenv parentModel (fromJust oldModel) userModel
    _ -> userModel

  -- Creates new UI using provided function
@ -589,10 +612,11 @@ compositeMerge comp state wenv newComp oldComp = newResult where
  enabledChg = nodeEnabledChanged oldComp newComp
  flagsChanged = visibleChg || enabledChg
  themeChanged = wenv ^. L.themeChanged
+  isReload = isWidgetReload wenv
  mergeRequired
-    | isJust oldModel = modelChanged || flagsChanged || themeChanged
+    | isJust oldModel = modelChanged || flagsChanged || themeChanged || isReload
    | otherwise = True
-  initRequired = not (nodeMatches tempRoot oldRoot)
+  initRequired = not (nodeMatches tempRoot oldRoot) || discardModelReload
  useNewRoot = initRequired || mergeRequired

  WidgetResult !newRoot !tmpReqs
@ -600,6 +624,7 @@ compositeMerge comp state wenv newComp oldComp = newResult where
    | mergeRequired = widgetMerge tempWidget cwenv tempRoot oldRoot
    | otherwise = resultNode oldRoot
  !newState = validState {
+    _cpsFingerprint = newFingerprint,
    _cpsModel = Just model,
    _cpsRoot = newRoot,
    _cpsWidgetKeyMap = collectWidgetKeys M.empty newRoot
@ -619,9 +644,10 @@ compositeMerge comp state wenv newComp oldComp = newResult where
    | otherwise = []
  evts = RaiseEvent <$> Seq.fromList (visibleEvts ++ enabledEvts)

+  resizeReqs = [ResizeWidgets widgetId | initRequired]
  mergeReqsFns = _cmpMergeReqs comp
  mergeHelper f = f cwenv newRoot oldRoot parentModel (fromJust oldModel) model
-  mergeReqs = concatMap mergeHelper mergeReqsFns
+  mergeReqs = resizeReqs ++ concatMap mergeHelper mergeReqsFns
  extraReqs = seqCatMaybes (toParentReq widgetId <$> Seq.fromList mergeReqs)

  tmpResult = WidgetResult newRoot (RenderOnce <| tmpReqs <> extraReqs <> evts)
@ -1073,6 +1099,7 @@ convertWidgetEnv :: WidgetEnv sp ep -> WidgetKeyMap s e -> s -> WidgetEnv s e
 convertWidgetEnv wenv widgetKeyMap model = WidgetEnv {
  _weOs = _weOs wenv,
  _weDpr = _weDpr wenv,
+  _weIsGhci = _weIsGhci wenv,
  _weAppStartTs = _weAppStartTs wenv,
  _weFontManager = _weFontManager wenv,
  _weFindBranchByPath = _weFindBranchByPath wenv,
@ -1119,3 +1146,10 @@ lookupNode widgetKeys desc key = case M.lookup key widgetKeys of

 sendMsgTo :: Typeable i => WidgetNode s e -> i -> WidgetRequest sp ep
 sendMsgTo node msg = SendMessage (node ^. L.info . L.widgetId) msg
+
+modelFingerprint :: Composite s e sp ep -> WidgetEnv s e -> s -> String
+modelFingerprint comp wenv !model
+  | isWidgetReload wenv && isJust fingerprintFn = fromJust fingerprintFn model
+  | otherwise = "<ignored>"
+  where
+    fingerprintFn = _cmpModelFingerprintFn comp
--- a/src/Monomer/Widgets/Container.hs
+++ b/src/Monomer/Widgets/Container.hs
@ -652,8 +652,9 @@ mergeWrapper container wenv newNode oldNode = newResult where

  flagsChanged = nodeFlagsChanged oldNode pNode
  themeChanged = wenv ^. L.themeChanged
+  isReload = isWidgetReload wenv
  mResult
-    | mergeRequired || flagsChanged || themeChanged = vResult
+    | isReload || mergeRequired || flagsChanged || themeChanged = vResult
    | otherwise = pResult & L.node . L.children .~ oldNode ^. L.children

  mNode = mResult ^. L.node
--- a/src/Monomer/Widgets/Containers/Box.hs
+++ b/src/Monomer/Widgets/Containers/Box.hs
@ -323,7 +323,7 @@ boxFilterEvent handler = def {

 newtype BoxState s = BoxState {
  _bxsModel :: Maybe s
-}
+} deriving (Show)

 -- | Creates a box widget with a single node as child.
 box
@ -357,7 +357,7 @@ makeBox config state = widget where
    containerIgnoreEmptyArea = ignoreEmptyArea && emptyHandlersCount == 0,
    containerGetCurrentStyle = getCurrentStyle,
    containerInit = init,
-    containerMergeChildrenReq = mergeRequired,
+    containerMergeChildrenReq = mergeChildrenReq,
    containerMerge = merge,
    containerFilterEvent = filterEvent,
    containerHandleEvent = handleEvent,
@ -373,10 +373,12 @@ makeBox config state = widget where
    newNode = node
      & L.widget .~ makeBox config newState

-  mergeRequired wenv node oldNode oldState = required where
+  mergeChildrenReq wenv node oldNode oldState = required where
    newModel = wenv ^. L.model
+    isReload = isWidgetReload wenv
    required = case (_boxMergeRequired config, _bxsModel oldState) of
-      (Just mergeReqFn, Just oldModel) -> mergeReqFn wenv oldModel newModel
+      (Just mergeReqFn, Just oldModel)
+        -> isReload || mergeReqFn wenv oldModel newModel
      _ -> True

  merge wenv node oldNode oldState = resultNode newNode where
--- a/src/Monomer/Widgets/Containers/Keystroke.hs
+++ b/src/Monomer/Widgets/Containers/Keystroke.hs
@ -152,7 +152,10 @@ instance Default KeyStroke where

 newtype KeyStrokeState e = KeyStrokeState {
  _kssLatest :: [(KeyStroke, e)]
-} deriving (Eq, Show)
+} deriving (Eq)
+
+instance Show (KeyStrokeState e) where
+  show (KeyStrokeState keys) = show (map fst keys)

 data KeyEntry
  = KeyEntryCode KeyCode
--- a/src/Monomer/Widgets/Containers/SelectList.hs
+++ b/src/Monomer/Widgets/Containers/SelectList.hs
@ -315,8 +315,9 @@ makeSelectList widgetData items makeRow config state = widget where

  mergeChildrenReq wenv node oldNode oldState = result where
    oldItems = _prevItems oldState
+    isReload = isWidgetReload wenv
    mergeRequiredFn = fromMaybe (const (/=)) (_slcMergeRequired config)
-    result = mergeRequiredFn wenv oldItems items
+    result = isReload || mergeRequiredFn wenv oldItems items

  merge wenv node oldNode oldState = resultNode newNode where
    selected = currentValue wenv
--- a/src/Monomer/Widgets/Containers/ThemeSwitch.hs
+++ b/src/Monomer/Widgets/Containers/ThemeSwitch.hs
@ -88,7 +88,7 @@ themeClearBg_ clear = def {
 data ThemeSwitchState = ThemeSwitchState {
  _tssPrevTheme :: Maybe Theme,
  _tssChanged :: Bool
-}
+} deriving (Show)

 -- | Switches to a new theme starting from its child node.
 themeSwitch
--- a/src/Monomer/Widgets/Single.hs
+++ b/src/Monomer/Widgets/Single.hs
@ -499,7 +499,7 @@ handleFocusRequest wenv oldNode evt mResult = newResult where

  newReq = SetFocus (node ^. L.info . L.widgetId)
  newResult
-    | isFocusReq && isJust mResult = (& L.requests %~ (|> newReq)) <$> mResult
+    | isFocusReq && isJust mResult = (L.requests %~ (|> newReq)) <$> mResult
    | isFocusReq = Just $ resultReqs node [newReq]
    | otherwise = mResult

--- a/src/Monomer/Widgets/Util/Widget.hs
+++ b/src/Monomer/Widgets/Util/Widget.hs
@ -34,7 +34,8 @@ module Monomer.Widgets.Util.Widget (
  nodeMatches,
  handleWidgetIdChange,
  delayedMessage,
-  delayedMessage_
+  delayedMessage_,
+  isWidgetReload
 ) where

 import Control.Concurrent (threadDelay)
@ -213,3 +214,6 @@ delayedMessage_
 delayedMessage_ widgetId path msg delay = RunTask widgetId path $ do
  threadDelay (fromIntegral delay * 1000)
  return msg
+
+isWidgetReload :: WidgetEnv s e -> Bool
+isWidgetReload wenv = wenv ^. L.isGhci && wenv ^. L.timestamp == 0
--- a/test/unit/Monomer/TestUtil.hs
+++ b/test/unit/Monomer/TestUtil.hs
@ -177,6 +177,7 @@ mockWenv :: s -> WidgetEnv s e
 mockWenv model = WidgetEnv {
  _weOs = "Mac OS X",
  _weDpr = 2,
+  _weIsGhci = False,
  _weAppStartTs = 0,
  _weFontManager = mockFontManager,
  _weFindBranchByPath = const Seq.empty,