diff --git a/CHANGES.md b/CHANGES.md
index 1d626dc8..863bc00f 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -65,6 +65,13 @@
     it easier for us to clean up the codebase. These can still be suppressed
     manually using an `OPTIONS_GHC` pragma with `-Wno-deprecations`.
 
+  * Extensible state type names no longer need to be unique, because the
+    `extensibleState` map in `XState` is now primarily keyed by the
+    machine-readable type representation rather than the human-readable type
+    name. Human-readable type names are still used for serialization of state
+    between restarts, and this representation now encodes module names as
+    well to avoid conflicts between types with equal names.
+
 ## 0.15 (September 30, 2018)
 
   * Reimplement `sendMessage` to deal properly with windowset changes made
diff --git a/src/XMonad/Core.hs b/src/XMonad/Core.hs
index 46a09397..e83a0336 100644
--- a/src/XMonad/Core.hs
+++ b/src/XMonad/Core.hs
@@ -1,6 +1,13 @@
-{-# LANGUAGE ExistentialQuantification, FlexibleInstances, GeneralizedNewtypeDeriving,
-             MultiParamTypeClasses, TypeSynonymInstances, DeriveDataTypeable,
-             LambdaCase, NamedFieldPuns, DeriveTraversable #-}
+{-# LANGUAGE DeriveDataTypeable #-}
+{-# LANGUAGE DeriveTraversable #-}
+{-# LANGUAGE ExistentialQuantification #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE LambdaCase #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE NamedFieldPuns #-}
+{-# LANGUAGE TypeSynonymInstances #-}
+{-# LANGUAGE ViewPatterns #-}
 
 -----------------------------------------------------------------------------
 -- |
@@ -23,7 +30,7 @@ module XMonad.Core (
     XConf(..), XConfig(..), LayoutClass(..),
     Layout(..), readsLayout, Typeable, Message,
     SomeMessage(..), fromMessage, LayoutMessages(..),
-    StateExtension(..), ExtensionClass(..), ConfExtension(..),
+    StateExtension(..), ExtensionClass(..), ConfExtension(..), showExtType,
     runX, catchX, userCode, userCodeDef, io, catchIO, installSignalHandlers, uninstallSignalHandlers,
     withDisplay, withWindowSet, isRoot, runOnWorkspaces,
     getAtom, spawn, spawnPID, xfork, xmessage, recompile, trace, whenJust, whenX,
@@ -73,7 +80,7 @@ data XState = XState
     , waitingUnmap     :: !(M.Map Window Int)            -- ^ the number of expected UnmapEvents
     , dragging         :: !(Maybe (Position -> Position -> X (), X ()))
     , numberlockMask   :: !KeyMask                       -- ^ The numlock modifier
-    , extensibleState  :: !(M.Map String (Either String StateExtension))
+    , extensibleState  :: !(M.Map (Either String TypeRep) (Either String StateExtension))
     -- ^ stores custom state information.
     --
     -- The module "XMonad.Util.ExtensibleState" in xmonad-contrib
@@ -420,6 +427,20 @@ data StateExtension =
 -- | Existential type to store a config extension.
 data ConfExtension = forall a. Typeable a => ConfExtension a
 
+-- | Serialize extension type name.
+-- Produces a (more) unique representation than the Show instance of TypeRep
+-- which only includes type names but not module/package names. 'showExtType'
+-- adds modules names as well. Package names are omitted to support migration
+-- of extensible state during xmonad version upgrades.
+showExtType :: TypeRep -> String
+showExtType = ($ "") . showTypeRep
+  where
+    showTypeRep (splitTyConApp -> (tc, tas)) =
+        showParen (not (null tas)) $
+        showTyCon tc . foldr (\ta -> ((showChar ' ' . showTypeRep ta) .)) id tas
+    showTyCon tc =
+        showString (tyConModule tc) . showChar '.' . showString (tyConName tc)
+
 -- ---------------------------------------------------------------------
 -- | General utilities
 --
diff --git a/src/XMonad/Operations.hs b/src/XMonad/Operations.hs
index f87330a6..9ef8cb56 100644
--- a/src/XMonad/Operations.hs
+++ b/src/XMonad/Operations.hs
@@ -23,13 +23,13 @@ import qualified XMonad.StackSet as W
 import Data.Maybe
 import Data.Monoid          (Endo(..),Any(..))
 import Data.List            (nub, (\\), find)
+import Data.Bifunctor       (bimap)
 import Data.Bits            ((.|.), (.&.), complement, testBit)
 import Data.Function        (on)
 import Data.Ratio
 import qualified Data.Map as M
 import qualified Data.Set as S
 
-import Control.Arrow (second)
 import Control.Monad.Reader
 import Control.Monad.State
 import qualified Control.Exception as C
@@ -473,12 +473,12 @@ data StateFile = StateFile
 -- so that xmonad can resume with that state intact.
 writeStateToFile :: X ()
 writeStateToFile = do
-    let maybeShow (t, Right (PersistentExtension ext)) = Just (t, show ext)
-        maybeShow (t, Left str) = Just (t, str)
-        maybeShow _ = Nothing
+    let showExt (Right t, Right (PersistentExtension ext)) = Just (showExtType t, show ext)
+        showExt (Left t, Left str) = Just (t, str)
+        showExt _ = Nothing
 
         wsData   = W.mapLayout show . windowset
-        extState = catMaybes . map maybeShow . M.toList . extensibleState
+        extState = catMaybes . map showExt . M.toList . extensibleState
 
     path <- asks $ stateFileName . directories
     stateData <- gets (\s -> StateFile (wsData s) (extState s))
@@ -502,7 +502,7 @@ readStateFile xmc = do
       sf <- join sf'
 
       let winset = W.ensureTags layout (workspaces xmc) $ W.mapLayout (fromMaybe layout . maybeRead lreads) (sfWins sf)
-          extState = M.fromList . map (second Left) $ sfExt sf
+          extState = M.fromList . map (bimap Left Left) $ sfExt sf
 
       return XState { windowset       = winset
                     , numberlockMask  = 0
diff --git a/xmonad.cabal b/xmonad.cabal
index c7778ee1..049ee4b8 100644
--- a/xmonad.cabal
+++ b/xmonad.cabal
@@ -1,5 +1,5 @@
 name:               xmonad
-version:            0.16.99999
+version:            0.16.999999
 synopsis:           A tiling window manager
 description:        xmonad is a tiling window manager for X. Windows are arranged
                     automatically to tile the screen without gaps or overlap, maximising