diff --git a/Cabal/src/Distribution/Simple/Glob/Internal.hs b/Cabal/src/Distribution/Simple/Glob/Internal.hs
index 4f0b91eca39..c987cbab082 100644
--- a/Cabal/src/Distribution/Simple/Glob/Internal.hs
+++ b/Cabal/src/Distribution/Simple/Glob/Internal.hs
@@ -49,7 +49,7 @@ data Glob
     GlobFile !GlobPieces
   | -- | Trailing dir; a glob ending in @/@.
     GlobDirTrailing
-  deriving (Eq, Show, Generic)
+  deriving (Eq, Ord, Show, Generic)
 
 instance Binary Glob
 instance Structured Glob
@@ -65,7 +65,7 @@ data GlobPiece
     Literal String
   | -- | A union of patterns, e.g. @dir/{a,*.txt,c}/...@
     Union [GlobPieces]
-  deriving (Eq, Show, Generic)
+  deriving (Eq, Ord, Show, Generic)
 
 instance Binary GlobPiece
 instance Structured GlobPiece
@@ -457,8 +457,8 @@ checkNameMatches spec glob candidate
       -- if multidot is supported, then this is a clean match
       if enableMultidot spec
         then pure (GlobMatch ())
-        else -- if not, issue a warning saying multidot is needed for the match
 
+        else -- if not, issue a warning saying multidot is needed for the match
           let (_, candidateExts) = splitExtensions $ takeFileName candidate
               extractExts :: GlobPieces -> Maybe String
               extractExts [] = Nothing
diff --git a/cabal-install/cabal-install.cabal b/cabal-install/cabal-install.cabal
index 95b7ce725f3..ba2be24f509 100644
--- a/cabal-install/cabal-install.cabal
+++ b/cabal-install/cabal-install.cabal
@@ -247,7 +247,8 @@ library
         regex-base  >= 0.94.0.0 && <0.95,
         regex-posix >= 0.96.0.0 && <0.97,
         safe-exceptions >= 0.1.7.0 && < 0.2,
-        semaphore-compat >= 1.0.0 && < 1.1
+        semaphore-compat >= 1.0.0 && < 1.1,
+        transformers >= 0.5.6.2 && < 0.7
 
     if flag(native-dns)
       if os(windows)
diff --git a/cabal-install/src/Distribution/Client/FileMonitor.hs b/cabal-install/src/Distribution/Client/FileMonitor.hs
index 084545d5e7e..4a53f30b6c7 100644
--- a/cabal-install/src/Distribution/Client/FileMonitor.hs
+++ b/cabal-install/src/Distribution/Client/FileMonitor.hs
@@ -52,6 +52,7 @@ import Data.Binary.Get (runGetOrFail)
 import qualified Data.ByteString.Lazy as BS
 import qualified Data.Hashable as Hashable
 import qualified Data.Map.Strict as Map
+import qualified Data.Set as Set
 
 import Control.Exception
 import Control.Monad
@@ -93,20 +94,20 @@ data MonitorFilePath
       , monitorKindDir :: !MonitorKindDir
       , monitorPathGlob :: !RootedGlob
       }
-  deriving (Eq, Show, Generic)
+  deriving (Eq, Ord, Show, Generic)
 
 data MonitorKindFile
   = FileExists
   | FileModTime
   | FileHashed
   | FileNotExists
-  deriving (Eq, Show, Generic)
+  deriving (Eq, Ord, Show, Generic)
 
 data MonitorKindDir
   = DirExists
   | DirModTime
   | DirNotExists
-  deriving (Eq, Show, Generic)
+  deriving (Eq, Ord, Show, Generic)
 
 instance Binary MonitorFilePath
 instance Binary MonitorKindFile
@@ -202,8 +203,8 @@ monitorFileHashedSearchPath notFoundAtPaths foundAtPath =
 -- globs, which monitor may files at once.
 data MonitorStateFileSet
   = MonitorStateFileSet
-      ![MonitorStateFile]
-      ![MonitorStateGlob]
+      !(Set MonitorStateFile)
+      !(Set MonitorStateGlob)
   -- Morally this is not actually a set but a bag (represented by lists).
   -- There is no principled reason to use a bag here rather than a set, but
   -- there is also no particular gain either. That said, we do preserve the
@@ -231,7 +232,7 @@ data MonitorStateFile
       !MonitorKindDir
       !FilePath
       !MonitorStateFileStatus
-  deriving (Show, Generic)
+  deriving (Eq, Ord, Show, Generic)
 
 data MonitorStateFileStatus
   = MonitorStateFileExists
@@ -244,7 +245,7 @@ data MonitorStateFileStatus
     MonitorStateDirModTime !ModTime
   | MonitorStateNonExistent
   | MonitorStateAlreadyChanged
-  deriving (Show, Generic)
+  deriving (Eq, Ord, Show, Generic)
 
 instance Binary MonitorStateFile
 instance Binary MonitorStateFileStatus
@@ -259,7 +260,7 @@ data MonitorStateGlob
       !MonitorKindDir
       !FilePathRoot
       !MonitorStateGlobRel
-  deriving (Show, Generic)
+  deriving (Eq, Ord, Show, Generic)
 
 data MonitorStateGlobRel
   = MonitorStateGlobDirs
@@ -272,7 +273,7 @@ data MonitorStateGlobRel
       !ModTime
       ![(FilePath, MonitorStateFileStatus)] -- invariant: sorted
   | MonitorStateGlobDirTrailing
-  deriving (Show, Generic)
+  deriving (Eq, Ord, Show, Generic)
 
 instance Binary MonitorStateGlob
 instance Binary MonitorStateGlobRel
@@ -283,9 +284,11 @@ instance Structured MonitorStateGlobRel
 -- | We can build a 'MonitorStateFileSet' from a set of 'MonitorFilePath' by
 -- inspecting the state of the file system, and we can go in the reverse
 -- direction by just forgetting the extra info.
-reconstructMonitorFilePaths :: MonitorStateFileSet -> [MonitorFilePath]
+reconstructMonitorFilePaths :: MonitorStateFileSet -> Set MonitorFilePath
 reconstructMonitorFilePaths (MonitorStateFileSet singlePaths globPaths) =
-  map getSinglePath singlePaths ++ map getGlobPath globPaths
+  Set.fromList $
+    map getSinglePath (Set.toList singlePaths)
+      <> map getGlobPath (Set.toList globPaths)
   where
     getSinglePath :: MonitorStateFile -> MonitorFilePath
     getSinglePath (MonitorStateFile kindfile kinddir filepath _) =
@@ -374,7 +377,7 @@ data MonitorChanged a b
     --
     -- The set of monitored files is also returned. This is useful
     -- for composing or nesting 'FileMonitor's.
-    MonitorUnchanged b [MonitorFilePath]
+    MonitorUnchanged b (Set MonitorFilePath)
   | -- | The monitor found that something changed. The reason is given.
     MonitorChanged (MonitorChangedReason a)
   deriving (Show)
@@ -569,14 +572,15 @@ probeFileSystem root (MonitorStateFileSet singlePaths globPaths) =
   runChangedM $ do
     sequence_
       [ probeMonitorStateFileStatus root file status
-      | MonitorStateFile _ _ file status <- singlePaths
+      | MonitorStateFile _ _ file status <- Set.toList singlePaths
       ]
     -- The glob monitors can require state changes
     globPaths' <-
-      sequence
-        [ probeMonitorStateGlob root globPath
-        | globPath <- globPaths
-        ]
+      Set.fromList
+        <$> sequence
+          [ probeMonitorStateGlob root globPath
+          | globPath <- Set.toList globPaths
+          ]
     return (MonitorStateFileSet singlePaths globPaths')
 
 -----------------------------------------------
@@ -849,7 +853,7 @@ updateFileMonitor
   -- ^ root directory
   -> Maybe MonitorTimestamp
   -- ^ timestamp when the update action started
-  -> [MonitorFilePath]
+  -> Set MonitorFilePath
   -- ^ files of interest relative to root
   -> a
   -- ^ the current key value
@@ -889,20 +893,20 @@ buildMonitorStateFileSet
   -- ^ existing file hashes
   -> FilePath
   -- ^ root directory
-  -> [MonitorFilePath]
+  -> Set MonitorFilePath
   -- ^ patterns of interest
   --   relative to root
   -> IO MonitorStateFileSet
 buildMonitorStateFileSet mstartTime hashcache root =
-  go [] []
+  go mempty mempty . Set.toList
   where
     go
-      :: [MonitorStateFile]
-      -> [MonitorStateGlob]
+      :: Set MonitorStateFile
+      -> Set MonitorStateGlob
       -> [MonitorFilePath]
       -> IO MonitorStateFileSet
     go !singlePaths !globPaths [] =
-      return (MonitorStateFileSet (reverse singlePaths) (reverse globPaths))
+      return (MonitorStateFileSet singlePaths globPaths)
     go
       !singlePaths
       !globPaths
@@ -916,7 +920,7 @@ buildMonitorStateFileSet mstartTime hashcache root =
               kinddir
               root
               path
-        go (monitorState : singlePaths) globPaths monitors
+        go (Set.insert monitorState singlePaths) globPaths monitors
     go
       !singlePaths
       !globPaths
@@ -929,7 +933,7 @@ buildMonitorStateFileSet mstartTime hashcache root =
             kinddir
             root
             globPath
-        go singlePaths (monitorState : globPaths) monitors
+        go singlePaths (Set.insert monitorState globPaths) monitors
 
 buildMonitorStateFile
   :: Maybe MonitorTimestamp
@@ -1129,7 +1133,7 @@ readCacheFileHashes monitor =
       collectAllFileHashes singlePaths
         `Map.union` collectAllGlobHashes globPaths
 
-    collectAllFileHashes :: [MonitorStateFile] -> Map FilePath (ModTime, Hash)
+    collectAllFileHashes :: Set MonitorStateFile -> Map FilePath (ModTime, Hash)
     collectAllFileHashes singlePaths =
       Map.fromList
         [ (fpath, (mtime, hash))
@@ -1138,14 +1142,14 @@ readCacheFileHashes monitor =
             _
             fpath
             (MonitorStateFileHashed mtime hash) <-
-            singlePaths
+            Set.toList singlePaths
         ]
 
-    collectAllGlobHashes :: [MonitorStateGlob] -> Map FilePath (ModTime, Hash)
+    collectAllGlobHashes :: Set MonitorStateGlob -> Map FilePath (ModTime, Hash)
     collectAllGlobHashes globPaths =
       Map.fromList
         [ (fpath, (mtime, hash))
-        | MonitorStateGlob _ _ _ gstate <- globPaths
+        | MonitorStateGlob _ _ _ gstate <- Set.toList globPaths
         , (fpath, (mtime, hash)) <- collectGlobHashes "" gstate
         ]
 
diff --git a/cabal-install/src/Distribution/Client/Glob.hs b/cabal-install/src/Distribution/Client/Glob.hs
index 90054a8f64f..943268096f5 100644
--- a/cabal-install/src/Distribution/Client/Glob.hs
+++ b/cabal-install/src/Distribution/Client/Glob.hs
@@ -39,7 +39,7 @@ data RootedGlob
       -- ^ what the glob is relative to
       Glob
       -- ^ the glob
-  deriving (Eq, Show, Generic)
+  deriving (Eq, Ord, Show, Generic)
 
 instance Binary RootedGlob
 instance Structured RootedGlob
@@ -49,7 +49,7 @@ data FilePathRoot
   | -- | e.g. @"/"@, @"c:\"@ or result of 'takeDrive'
     FilePathRoot FilePath
   | FilePathHomeDir
-  deriving (Eq, Show, Generic)
+  deriving (Eq, Ord, Show, Generic)
 
 instance Binary FilePathRoot
 instance Structured FilePathRoot
diff --git a/cabal-install/src/Distribution/Client/ProjectBuilding/PackageFileMonitor.hs b/cabal-install/src/Distribution/Client/ProjectBuilding/PackageFileMonitor.hs
index b93064ea7be..5fa19b63bb3 100644
--- a/cabal-install/src/Distribution/Client/ProjectBuilding/PackageFileMonitor.hs
+++ b/cabal-install/src/Distribution/Client/ProjectBuilding/PackageFileMonitor.hs
@@ -226,7 +226,7 @@ updatePackageConfigFileMonitor
       pkgFileMonitorConfig
       srcdir
       Nothing
-      []
+      mempty
       pkgconfig
       ()
     where
@@ -238,7 +238,7 @@ updatePackageBuildFileMonitor
   -> MonitorTimestamp
   -> ElaboratedConfiguredPackage
   -> BuildStatusRebuild
-  -> [MonitorFilePath]
+  -> Set MonitorFilePath
   -> BuildResultMisc
   -> IO ()
 updatePackageBuildFileMonitor
@@ -285,7 +285,7 @@ updatePackageRegFileMonitor
       pkgFileMonitorReg
       srcdir
       Nothing
-      []
+      mempty
       ()
       mipkg
 
diff --git a/cabal-install/src/Distribution/Client/ProjectBuilding/UnpackedPackage.hs b/cabal-install/src/Distribution/Client/ProjectBuilding/UnpackedPackage.hs
index 5b651746dc1..a78ebaff142 100644
--- a/cabal-install/src/Distribution/Client/ProjectBuilding/UnpackedPackage.hs
+++ b/cabal-install/src/Distribution/Client/ProjectBuilding/UnpackedPackage.hs
@@ -91,6 +91,7 @@ import qualified Data.ByteString as BS
 import qualified Data.ByteString.Lazy as LBS
 import qualified Data.ByteString.Lazy.Char8 as LBS.Char8
 import qualified Data.List.NonEmpty as NE
+import qualified Data.Set as Set
 
 import Control.Exception (ErrorCall, Handler (..), SomeAsyncException, assert, catches)
 import System.Directory (canonicalizePath, createDirectoryIfMissing, doesDirectoryExist, doesFileExist, removeFile)
@@ -473,7 +474,7 @@ buildInplaceUnpackedPackage
             let listSimple =
                   execRebuild srcdir (needElaboratedConfiguredPackage pkg)
                 listSdist =
-                  fmap (map monitorFileHashed) $
+                  fmap (Set.fromList . map monitorFileHashed) $
                     allPackageSourceFiles verbosity srcdir
                 ifNullThen m m' = do
                   xs <- m
@@ -499,19 +500,20 @@ buildInplaceUnpackedPackage
                     listSimple
 
             let dep_monitors =
-                  map monitorFileHashed $
-                    elabInplaceDependencyBuildCacheFiles
-                      distDirLayout
-                      pkgshared
-                      plan
-                      pkg
+                  Set.fromList $
+                    map monitorFileHashed $
+                      elabInplaceDependencyBuildCacheFiles
+                        distDirLayout
+                        pkgshared
+                        plan
+                        pkg
             updatePackageBuildFileMonitor
               packageFileMonitor
               srcdir
               timestamp
               pkg
               buildStatus
-              (monitors ++ dep_monitors)
+              (monitors <> dep_monitors)
               buildResult
         PBHaddockPhase{runHaddock} -> do
           runHaddock
@@ -933,4 +935,3 @@ withTempInstalledPackageInfoFile verbosity tempdir action =
           unlines warns
 
       return ipkg
-
diff --git a/cabal-install/src/Distribution/Client/ProjectConfig.hs b/cabal-install/src/Distribution/Client/ProjectConfig.hs
index 23d8cbab932..97428671936 100644
--- a/cabal-install/src/Distribution/Client/ProjectConfig.hs
+++ b/cabal-install/src/Distribution/Client/ProjectConfig.hs
@@ -685,7 +685,7 @@ readProjectLocalConfigOrDefault verbosity httpTransport distDirLayout = do
     then do
       readProjectFileSkeleton verbosity httpTransport distDirLayout "" "project file"
     else do
-      monitorFiles [monitorNonExistentFile projectFile]
+      monitorFiles (Set.singleton (monitorNonExistentFile projectFile))
       return (singletonProjectConfigSkeleton defaultImplicitProjectConfig)
 
 defaultImplicitProjectConfig :: ProjectConfig
@@ -739,12 +739,12 @@ readProjectFileSkeleton
     exists <- liftIO $ doesFileExist extensionFile
     if exists
       then do
-        monitorFiles [monitorFileHashed extensionFile]
+        monitorFiles (Set.singleton (monitorFileHashed extensionFile))
         pcs <- liftIO readExtensionFile
-        monitorFiles $ map monitorFileHashed (projectSkeletonImports pcs)
+        monitorFiles (Set.fromList (map monitorFileHashed (projectSkeletonImports pcs)))
         pure pcs
       else do
-        monitorFiles [monitorNonExistentFile extensionFile]
+        monitorFiles (Set.singleton (monitorNonExistentFile extensionFile))
         return mempty
     where
       extensionFile = distProjectFile extensionName
@@ -782,7 +782,7 @@ readGlobalConfig :: Verbosity -> Flag FilePath -> Rebuild ProjectConfig
 readGlobalConfig verbosity configFileFlag = do
   config <- liftIO (loadConfig verbosity configFileFlag)
   configFile <- liftIO (getConfigFilePath configFileFlag)
-  monitorFiles [monitorFileHashed configFile]
+  monitorFiles (Set.singleton (monitorFileHashed configFile))
   return (convertLegacyGlobalConfig config)
 
 reportParseResult :: Verbosity -> String -> FilePath -> OldParser.ParseResult ProjectConfigSkeleton -> IO ProjectConfigSkeleton
@@ -1230,7 +1230,7 @@ readSourcePackageLocalDirectory
   -- ^ The package @.cabal@ file
   -> Rebuild (PackageSpecifier (SourcePackage UnresolvedPkgLoc))
 readSourcePackageLocalDirectory verbosity dir cabalFile = do
-  monitorFiles [monitorFileHashed cabalFile]
+  monitorFiles (Set.singleton (monitorFileHashed cabalFile))
   root <- askRoot
   let location = LocalUnpackedPackage (root </> dir)
   liftIO $
@@ -1246,7 +1246,7 @@ readSourcePackageLocalTarball
   -> FilePath
   -> Rebuild (PackageSpecifier (SourcePackage UnresolvedPkgLoc))
 readSourcePackageLocalTarball verbosity tarballFile = do
-  monitorFiles [monitorFile tarballFile]
+  monitorFiles (Set.singleton (monitorFile tarballFile))
   root <- askRoot
   let location = LocalTarballPackage (root </> tarballFile)
   liftIO $
@@ -1286,7 +1286,7 @@ fetchAndReadSourcePackageRemoteTarball
         return ()
 
       -- Read
-      monitorFiles [monitorFile tarballFile]
+      monitorFiles (Set.singleton (monitorFile tarballFile))
       let location = RemoteTarballPackage tarballUri tarballFile
       liftIO $
         fmap (mkSpecificSourcePackage location)
@@ -1437,7 +1437,7 @@ syncAndReadSourcePackagesRemoteRepos
           (_ : _ : _) -> liftIO $ throwIO $ MultipleCabalFilesFound packageDir
           [cabalFileName] -> do
             let cabalFilePath = packageDir </> cabalFileName
-            monitorFiles [monitorFileHashed cabalFilePath]
+            monitorFiles (Set.singleton (monitorFileHashed cabalFilePath))
             gpd <- liftIO $ readSourcePackageCabalFile verbosity cabalFilePath =<< BS.readFile cabalFilePath
 
             -- write sdist tarball, to repoPath-pgkid
diff --git a/cabal-install/src/Distribution/Client/ProjectPlanning.hs b/cabal-install/src/Distribution/Client/ProjectPlanning.hs
index 6344249a8a6..8db6fbe7329 100644
--- a/cabal-install/src/Distribution/Client/ProjectPlanning.hs
+++ b/cabal-install/src/Distribution/Client/ProjectPlanning.hs
@@ -488,7 +488,7 @@ configureCompiler
         -- the compiler will configure (and it does vary between compilers).
         -- We do know however that the compiler will only configure the
         -- programs it cares about, and those are the ones we monitor here.
-        monitorFiles (programsMonitorFiles progdb'')
+        monitorFiles (Set.fromList (programsMonitorFiles progdb''))
 
         return result
     where
@@ -941,7 +941,7 @@ getInstalledPackages
   -> PackageDBStack
   -> Rebuild InstalledPackageIndex
 getInstalledPackages verbosity compiler progdb platform packagedbs = do
-  monitorFiles . map monitorFileOrDirectory
+  monitorFiles . Set.fromList . map monitorFileOrDirectory
     =<< liftIO
       ( IndexUtils.getInstalledPackagesMonitorFiles
           verbosity
@@ -1165,10 +1165,11 @@ getPackageSourceHashes verbosity withRepoCtx solverPlan = do
             return (pkgid, srchash)
           | (pkgid, tarball) <- allTarballFilePkgs
           ]
-  monitorFiles
-    [ monitorFile tarball
-    | (_pkgid, tarball) <- allTarballFilePkgs
-    ]
+  monitorFiles $
+    Set.fromList
+      [ monitorFile tarball
+      | (_pkgid, tarball) <- allTarballFilePkgs
+      ]
 
   -- Return the combination
   return $!
diff --git a/cabal-install/src/Distribution/Client/RebuildMonad.hs b/cabal-install/src/Distribution/Client/RebuildMonad.hs
index 33303ea3243..cc8f559cf01 100644
--- a/cabal-install/src/Distribution/Client/RebuildMonad.hs
+++ b/cabal-install/src/Distribution/Client/RebuildMonad.hs
@@ -69,15 +69,16 @@ import Distribution.Simple.Utils (debug)
 
 import Control.Concurrent.MVar (MVar, modifyMVar, newMVar)
 import Control.Monad.Reader as Reader
-import Control.Monad.State as State
+import Control.Monad.Writer.Strict as Writer
 import qualified Data.Map.Strict as Map
+import qualified Data.Set as Set
 import System.Directory
 import System.FilePath
 
 -- | A monad layered on top of 'IO' to help with re-running actions when the
 -- input files and values they depend on change. The crucial operations are
 -- 'rerunIfChanged' and 'monitorFiles'.
-newtype Rebuild a = Rebuild (ReaderT FilePath (StateT [MonitorFilePath] IO) a)
+newtype Rebuild a = Rebuild (ReaderT FilePath (WriterT (Set MonitorFilePath) IO) a)
   deriving (Functor, Applicative, Monad, MonadIO)
 
 -- | Use this within the body action of 'rerunIfChanged' to declare that the
@@ -87,20 +88,20 @@ newtype Rebuild a = Rebuild (ReaderT FilePath (StateT [MonitorFilePath] IO) a)
 --
 -- Relative paths are interpreted as relative to an implicit root, ultimately
 -- passed in to 'runRebuild'.
-monitorFiles :: [MonitorFilePath] -> Rebuild ()
-monitorFiles filespecs = Rebuild (State.modify (filespecs ++))
+monitorFiles :: Set MonitorFilePath -> Rebuild ()
+monitorFiles filespecs = Rebuild (Writer.tell filespecs)
 
 -- | Run a 'Rebuild' IO action.
-unRebuild :: FilePath -> Rebuild a -> IO (a, [MonitorFilePath])
-unRebuild rootDir (Rebuild action) = runStateT (runReaderT action rootDir) []
+unRebuild :: FilePath -> Rebuild a -> IO (a, Set MonitorFilePath)
+unRebuild rootDir (Rebuild action) = runWriterT (runReaderT action rootDir)
 
 -- | Run a 'Rebuild' IO action.
 runRebuild :: FilePath -> Rebuild a -> IO a
-runRebuild rootDir (Rebuild action) = evalStateT (runReaderT action rootDir) []
+runRebuild rootDir (Rebuild action) = fst <$> runWriterT (runReaderT action rootDir)
 
 -- | Run a 'Rebuild' IO action.
-execRebuild :: FilePath -> Rebuild a -> IO [MonitorFilePath]
-execRebuild rootDir (Rebuild action) = execStateT (runReaderT action rootDir) []
+execRebuild :: FilePath -> Rebuild a -> IO (Set MonitorFilePath)
+execRebuild rootDir (Rebuild action) = execWriterT (runReaderT action rootDir)
 
 -- | The root that relative paths are interpreted as being relative to.
 askRoot :: Rebuild FilePath
@@ -236,7 +237,7 @@ delayInitSharedResources action = do
 matchFileGlob :: RootedGlob -> Rebuild [FilePath]
 matchFileGlob glob = do
   root <- askRoot
-  monitorFiles [monitorFileGlobExistence glob]
+  monitorFiles (Set.singleton (monitorFileGlobExistence glob))
   liftIO $ Glob.matchFileGlob root glob
 
 getDirectoryContentsMonitored :: FilePath -> Rebuild [FilePath]
@@ -248,7 +249,7 @@ getDirectoryContentsMonitored dir = do
 
 createDirectoryMonitored :: Bool -> FilePath -> Rebuild ()
 createDirectoryMonitored createParents dir = do
-  monitorFiles [monitorDirectoryExistence dir]
+  monitorFiles (Set.singleton (monitorDirectoryExistence dir))
   liftIO $ createDirectoryIfMissing createParents dir
 
 -- | Monitor a directory as in 'monitorDirectory' if it currently exists or
@@ -256,11 +257,12 @@ createDirectoryMonitored createParents dir = do
 monitorDirectoryStatus :: FilePath -> Rebuild Bool
 monitorDirectoryStatus dir = do
   exists <- liftIO $ doesDirectoryExist dir
-  monitorFiles
-    [ if exists
-        then monitorDirectory dir
-        else monitorNonExistentDirectory dir
-    ]
+  monitorFiles $
+    Set.singleton
+      ( if exists
+          then monitorDirectory dir
+          else monitorNonExistentDirectory dir
+      )
   return exists
 
 -- | Like 'doesFileExist', but in the 'Rebuild' monad.  This does
@@ -269,16 +271,17 @@ doesFileExistMonitored :: FilePath -> Rebuild Bool
 doesFileExistMonitored f = do
   root <- askRoot
   exists <- liftIO $ doesFileExist (root </> f)
-  monitorFiles
-    [ if exists
-        then monitorFileExistence f
-        else monitorNonExistentFile f
-    ]
+  monitorFiles $
+    Set.singleton
+      ( if exists
+          then monitorFileExistence f
+          else monitorNonExistentFile f
+      )
   return exists
 
 -- | Monitor a single file
 need :: FilePath -> Rebuild ()
-need f = monitorFiles [monitorFileHashed f]
+need f = monitorFiles (Set.singleton (monitorFileHashed f))
 
 -- | Monitor a file if it exists; otherwise check for when it
 -- gets created.  This is a bit better for recompilation avoidance
@@ -289,11 +292,12 @@ needIfExists :: FilePath -> Rebuild ()
 needIfExists f = do
   root <- askRoot
   exists <- liftIO $ doesFileExist (root </> f)
-  monitorFiles
-    [ if exists
-        then monitorFileHashed f
-        else monitorNonExistentFile f
-    ]
+  monitorFiles $
+    Set.singleton
+      ( if exists
+          then monitorFileHashed f
+          else monitorNonExistentFile f
+      )
 
 -- | Like 'findFileWithExtension', but in the 'Rebuild' monad.
 findFileWithExtensionMonitored
diff --git a/cabal-install/src/Distribution/Client/VCS.hs b/cabal-install/src/Distribution/Client/VCS.hs
index 7c071de31eb..b3a77e49525 100644
--- a/cabal-install/src/Distribution/Client/VCS.hs
+++ b/cabal-install/src/Distribution/Client/VCS.hs
@@ -90,6 +90,7 @@ import Control.Monad.Trans
 import qualified Data.Char as Char
 import qualified Data.List as List
 import qualified Data.Map as Map
+import qualified Data.Set as Set
 import System.Directory
   ( doesDirectoryExist
   , removeDirectoryRecursive
@@ -274,7 +275,7 @@ syncSourceRepos
   -> Rebuild ()
 syncSourceRepos verbosity vcs repos = do
   files <- liftIO $ vcsSyncRepos vcs verbosity (vcsProgram vcs) repos
-  monitorFiles files
+  monitorFiles (Set.fromList files)
 
 -- ------------------------------------------------------------
 
diff --git a/cabal-install/tests/UnitTests/Distribution/Client/FileMonitor.hs b/cabal-install/tests/UnitTests/Distribution/Client/FileMonitor.hs
index f3c8145bc49..3d49feb7261 100644
--- a/cabal-install/tests/UnitTests/Distribution/Client/FileMonitor.hs
+++ b/cabal-install/tests/UnitTests/Distribution/Client/FileMonitor.hs
@@ -8,6 +8,7 @@ import Control.Concurrent (threadDelay)
 import Control.Exception
 import Control.Monad
 import Data.Proxy (Proxy (..))
+import Data.Set (Set)
 import qualified Data.Set as Set
 import qualified System.Directory as IO
 import System.FilePath
@@ -188,10 +189,10 @@ testCorruptMonitorCache step =
     reason @?= MonitorCorruptCache
 
     step "Updating file monitor"
-    updateMonitor root monitor [] () ()
+    updateMonitor root monitor mempty () ()
     (res, files) <- expectMonitorUnchanged root monitor ()
     res @?= ()
-    files @?= []
+    files @?= mempty
 
     step "Writing broken file again"
     IO.writeFile (fileMonitorCacheFile monitor) "broken"
@@ -203,11 +204,11 @@ testEmptyMonitor :: Assertion
 testEmptyMonitor =
   withFileMonitor $ \root monitor -> do
     touchFile root "a"
-    updateMonitor root monitor [] () ()
+    updateMonitor root monitor mempty () ()
     touchFile root "b"
     (res, files) <- expectMonitorUnchanged root monitor ()
     res @?= ()
-    files @?= []
+    files @?= mempty
 
 -- monitor a file that is expected to exist
 testMissingFile :: Assertion
@@ -228,13 +229,13 @@ testMissingFile = do
       withFileMonitor $ \root monitor -> do
         -- a file that doesn't exist at snapshot time is considered to have
         -- changed
-        updateMonitor root monitor [monitorKind file] () ()
+        updateMonitor root monitor (Set.singleton (monitorKind file)) () ()
         reason <- expectMonitorChanged root monitor ()
         reason @?= MonitoredFileChanged file
 
         -- a file doesn't exist at snapshot time, but gets added afterwards is
         -- also considered to have changed
-        updateMonitor root monitor [monitorKind file] () ()
+        updateMonitor root monitor (Set.singleton (monitorKind file)) () ()
         touch root file
         reason2 <- expectMonitorChanged root monitor ()
         reason2 @?= MonitoredFileChanged file
@@ -257,7 +258,7 @@ testChangedFile mtimeChange = do
     test monitorKind touch touch' file =
       withFileMonitor $ \root monitor -> do
         touch root file
-        updateMonitor root monitor [monitorKind file] () ()
+        updateMonitor root monitor (Set.singleton (monitorKind file)) () ()
         threadDelay mtimeChange
         touch' root file
         reason <- expectMonitorChanged root monitor ()
@@ -268,28 +269,28 @@ testChangedFileMtimeVsContent mtimeChange =
   withFileMonitor $ \root monitor -> do
     -- if we don't touch the file, it's unchanged
     touchFile root "a"
-    updateMonitor root monitor [monitorFile "a"] () ()
+    updateMonitor root monitor (Set.singleton (monitorFile "a")) () ()
     (res, files) <- expectMonitorUnchanged root monitor ()
     res @?= ()
-    files @?= [monitorFile "a"]
+    files @?= Set.fromList [monitorFile "a"]
 
     -- if we do touch the file, it's changed if we only consider mtime
-    updateMonitor root monitor [monitorFile "a"] () ()
+    updateMonitor root monitor (Set.singleton (monitorFile "a")) () ()
     threadDelay mtimeChange
     touchFile root "a"
     reason <- expectMonitorChanged root monitor ()
     reason @?= MonitoredFileChanged "a"
 
     -- but if we touch the file, it's unchanged if we consider content hash
-    updateMonitor root monitor [monitorFileHashed "a"] () ()
+    updateMonitor root monitor (Set.singleton (monitorFileHashed "a")) () ()
     threadDelay mtimeChange
     touchFile root "a"
     (res2, files2) <- expectMonitorUnchanged root monitor ()
     res2 @?= ()
-    files2 @?= [monitorFileHashed "a"]
+    files2 @?= Set.fromList [monitorFileHashed "a"]
 
     -- finally if we change the content it's changed
-    updateMonitor root monitor [monitorFileHashed "a"] () ()
+    updateMonitor root monitor (Set.singleton (monitorFileHashed "a")) () ()
     threadDelay mtimeChange
     touchFileContent root "a"
     reason2 <- expectMonitorChanged root monitor ()
@@ -314,25 +315,25 @@ testUpdateDuringAction mtimeChange = do
     test monitorSpec touch file =
       withFileMonitor $ \root monitor -> do
         touch root file
-        updateMonitor root monitor [monitorSpec] () ()
+        updateMonitor root monitor (Set.singleton (monitorSpec)) () ()
 
         -- start doing an update action...
         threadDelay mtimeChange -- some time passes
         touch root file -- a file gets updates during the action
         threadDelay mtimeChange -- some time passes then we finish
-        updateMonitor root monitor [monitorSpec] () ()
+        updateMonitor root monitor (Set.singleton (monitorSpec)) () ()
         -- we don't notice this change since we took the timestamp after the
         -- action finished
         (res, files) <- expectMonitorUnchanged root monitor ()
         res @?= ()
-        files @?= [monitorSpec]
+        files @?= Set.fromList [monitorSpec]
 
         -- Let's try again, this time taking the timestamp before the action
         timestamp' <- beginUpdateFileMonitor
         threadDelay mtimeChange -- some time passes
         touch root file -- a file gets updates during the action
         threadDelay mtimeChange -- some time passes then we finish
-        updateMonitorWithTimestamp root monitor timestamp' [monitorSpec] () ()
+        updateMonitorWithTimestamp root monitor timestamp' (Set.singleton (monitorSpec)) () ()
         -- now we do notice the change since we took the snapshot before the
         -- action finished
         reason <- expectMonitorChanged root monitor ()
@@ -356,7 +357,7 @@ testRemoveFile = do
     test monitorKind touch remove file =
       withFileMonitor $ \root monitor -> do
         touch root file
-        updateMonitor root monitor [monitorKind file] () ()
+        updateMonitor root monitor (Set.singleton (monitorKind file)) () ()
         remove root file
         reason <- expectMonitorChanged root monitor ()
         reason @?= MonitoredFileChanged file
@@ -366,10 +367,10 @@ testNonExistentFile :: Assertion
 testNonExistentFile =
   withFileMonitor $ \root monitor -> do
     -- a file that doesn't exist at snapshot time or check time is unchanged
-    updateMonitor root monitor [monitorNonExistentFile "a"] () ()
+    updateMonitor root monitor (Set.singleton (monitorNonExistentFile "a")) () ()
     (res, files) <- expectMonitorUnchanged root monitor ()
     res @?= ()
-    files @?= [monitorNonExistentFile "a"]
+    files @?= Set.fromList [monitorNonExistentFile "a"]
 
     -- if the file then exists it has changed
     touchFile root "a"
@@ -377,7 +378,7 @@ testNonExistentFile =
     reason @?= MonitoredFileChanged "a"
 
     -- if the file then exists at snapshot and check time it has changed
-    updateMonitor root monitor [monitorNonExistentFile "a"] () ()
+    updateMonitor root monitor (Set.singleton (monitorNonExistentFile "a")) () ()
     reason2 <- expectMonitorChanged root monitor ()
     reason2 @?= MonitoredFileChanged "a"
 
@@ -387,11 +388,11 @@ testNonExistentFile =
     -- can depend on that content, whereas if the action expected a file not to
     -- exist and it now does not, it'll give the same result, irrespective of
     -- the fact that the file might have existed in the meantime.
-    updateMonitor root monitor [monitorNonExistentFile "a"] () ()
+    updateMonitor root monitor (Set.singleton (monitorNonExistentFile "a")) () ()
     removeFile root "a"
     (res2, files2) <- expectMonitorUnchanged root monitor ()
     res2 @?= ()
-    files2 @?= [monitorNonExistentFile "a"]
+    files2 @?= Set.fromList [monitorNonExistentFile "a"]
 
 testChangedFileType :: Int -> Assertion
 testChangedFileType mtimeChange = do
@@ -421,7 +422,7 @@ testChangedFileType mtimeChange = do
     test monitorKind touch remove touch' =
       withFileMonitor $ \root monitor -> do
         touch root "a"
-        updateMonitor root monitor [monitorKind] () ()
+        updateMonitor root monitor (Set.singleton (monitorKind)) () ()
         threadDelay mtimeChange
         remove root "a"
         touch' root "a"
@@ -436,10 +437,10 @@ testMultipleMonitorKinds :: Int -> Assertion
 testMultipleMonitorKinds mtimeChange =
   withFileMonitor $ \root monitor -> do
     touchFile root "a"
-    updateMonitor root monitor [monitorFile "a", monitorFileHashed "a"] () ()
+    updateMonitor root monitor (Set.fromList [monitorFile "a", monitorFileHashed "a"]) () ()
     (res, files) <- expectMonitorUnchanged root monitor ()
     res @?= ()
-    files @?= [monitorFile "a", monitorFileHashed "a"]
+    files @?= Set.fromList [monitorFile "a", monitorFileHashed "a"]
     threadDelay mtimeChange
     touchFile root "a" -- not changing content, just mtime
     reason <- expectMonitorChanged root monitor ()
@@ -449,14 +450,16 @@ testMultipleMonitorKinds mtimeChange =
     updateMonitor
       root
       monitor
-      [ monitorDirectory "dir"
-      , monitorDirectoryExistence "dir"
-      ]
+      ( Set.fromList
+          [ monitorDirectory "dir"
+          , monitorDirectoryExistence "dir"
+          ]
+      )
       ()
       ()
     (res2, files2) <- expectMonitorUnchanged root monitor ()
     res2 @?= ()
-    files2 @?= [monitorDirectory "dir", monitorDirectoryExistence "dir"]
+    files2 @?= Set.fromList [monitorDirectory "dir", monitorDirectoryExistence "dir"]
     threadDelay mtimeChange
     touchFile root ("dir" </> "a") -- changing dir mtime, not existence
     reason2 <- expectMonitorChanged root monitor ()
@@ -471,19 +474,19 @@ testGlobNoChange =
   withFileMonitor $ \root monitor -> do
     touchFile root ("dir" </> "good-a")
     touchFile root ("dir" </> "good-b")
-    updateMonitor root monitor [monitorFileGlobStr "dir/good-*"] () ()
+    updateMonitor root monitor (Set.singleton (monitorFileGlobStr "dir/good-*")) () ()
     (res, files) <- expectMonitorUnchanged root monitor ()
     res @?= ()
-    files @?= [monitorFileGlobStr "dir/good-*"]
+    files @?= Set.fromList [monitorFileGlobStr "dir/good-*"]
 
 testGlobAddMatch :: Int -> Assertion
 testGlobAddMatch mtimeChange =
   withFileMonitor $ \root monitor -> do
     touchFile root ("dir" </> "good-a")
-    updateMonitor root monitor [monitorFileGlobStr "dir/good-*"] () ()
+    updateMonitor root monitor (Set.singleton (monitorFileGlobStr "dir/good-*")) () ()
     (res, files) <- expectMonitorUnchanged root monitor ()
     res @?= ()
-    files @?= [monitorFileGlobStr "dir/good-*"]
+    files @?= Set.fromList [monitorFileGlobStr "dir/good-*"]
     threadDelay mtimeChange
     touchFile root ("dir" </> "good-b")
     reason <- expectMonitorChanged root monitor ()
@@ -494,7 +497,7 @@ testGlobRemoveMatch mtimeChange =
   withFileMonitor $ \root monitor -> do
     touchFile root ("dir" </> "good-a")
     touchFile root ("dir" </> "good-b")
-    updateMonitor root monitor [monitorFileGlobStr "dir/good-*"] () ()
+    updateMonitor root monitor (Set.singleton (monitorFileGlobStr "dir/good-*")) () ()
     threadDelay mtimeChange
     removeFile root "dir/good-a"
     reason <- expectMonitorChanged root monitor ()
@@ -505,12 +508,12 @@ testGlobChangeMatch mtimeChange =
   withFileMonitor $ \root monitor -> do
     touchFile root ("dir" </> "good-a")
     touchFile root ("dir" </> "good-b")
-    updateMonitor root monitor [monitorFileGlobStr "dir/good-*"] () ()
+    updateMonitor root monitor (Set.singleton (monitorFileGlobStr "dir/good-*")) () ()
     threadDelay mtimeChange
     touchFile root ("dir" </> "good-b")
     (res, files) <- expectMonitorUnchanged root monitor ()
     res @?= ()
-    files @?= [monitorFileGlobStr "dir/good-*"]
+    files @?= Set.fromList [monitorFileGlobStr "dir/good-*"]
 
     touchFileContent root ("dir" </> "good-b")
     reason <- expectMonitorChanged root monitor ()
@@ -520,7 +523,7 @@ testGlobAddMatchSubdir :: Int -> Assertion
 testGlobAddMatchSubdir mtimeChange =
   withFileMonitor $ \root monitor -> do
     touchFile root ("dir" </> "a" </> "good-a")
-    updateMonitor root monitor [monitorFileGlobStr "dir/*/good-*"] () ()
+    updateMonitor root monitor (Set.singleton (monitorFileGlobStr "dir/*/good-*")) () ()
     threadDelay mtimeChange
     touchFile root ("dir" </> "b" </> "good-b")
     reason <- expectMonitorChanged root monitor ()
@@ -531,7 +534,7 @@ testGlobRemoveMatchSubdir mtimeChange =
   withFileMonitor $ \root monitor -> do
     touchFile root ("dir" </> "a" </> "good-a")
     touchFile root ("dir" </> "b" </> "good-b")
-    updateMonitor root monitor [monitorFileGlobStr "dir/*/good-*"] () ()
+    updateMonitor root monitor (Set.singleton (monitorFileGlobStr "dir/*/good-*")) () ()
     threadDelay mtimeChange
     removeDir root ("dir" </> "a")
     reason <- expectMonitorChanged root monitor ()
@@ -542,12 +545,12 @@ testGlobChangeMatchSubdir mtimeChange =
   withFileMonitor $ \root monitor -> do
     touchFile root ("dir" </> "a" </> "good-a")
     touchFile root ("dir" </> "b" </> "good-b")
-    updateMonitor root monitor [monitorFileGlobStr "dir/*/good-*"] () ()
+    updateMonitor root monitor (Set.singleton (monitorFileGlobStr "dir/*/good-*")) () ()
     threadDelay mtimeChange
     touchFile root ("dir" </> "b" </> "good-b")
     (res, files) <- expectMonitorUnchanged root monitor ()
     res @?= ()
-    files @?= [monitorFileGlobStr "dir/*/good-*"]
+    files @?= Set.fromList [monitorFileGlobStr "dir/*/good-*"]
 
     touchFileContent root "dir/b/good-b"
     reason <- expectMonitorChanged root monitor ()
@@ -557,7 +560,7 @@ testGlobChangeMatchSubdir mtimeChange =
 testGlobMatchTopDir :: Int -> Assertion
 testGlobMatchTopDir mtimeChange =
   withFileMonitor $ \root monitor -> do
-    updateMonitor root monitor [monitorFileGlobStr "*"] () ()
+    updateMonitor root monitor (Set.singleton (monitorFileGlobStr "*")) () ()
     threadDelay mtimeChange
     touchFile root "a"
     reason <- expectMonitorChanged root monitor ()
@@ -567,47 +570,47 @@ testGlobAddNonMatch :: Int -> Assertion
 testGlobAddNonMatch mtimeChange =
   withFileMonitor $ \root monitor -> do
     touchFile root ("dir" </> "good-a")
-    updateMonitor root monitor [monitorFileGlobStr "dir/good-*"] () ()
+    updateMonitor root monitor (Set.singleton (monitorFileGlobStr "dir/good-*")) () ()
     threadDelay mtimeChange
     touchFile root ("dir" </> "bad")
     (res, files) <- expectMonitorUnchanged root monitor ()
     res @?= ()
-    files @?= [monitorFileGlobStr "dir/good-*"]
+    files @?= Set.fromList [monitorFileGlobStr "dir/good-*"]
 
 testGlobRemoveNonMatch :: Int -> Assertion
 testGlobRemoveNonMatch mtimeChange =
   withFileMonitor $ \root monitor -> do
     touchFile root ("dir" </> "good-a")
     touchFile root ("dir" </> "bad")
-    updateMonitor root monitor [monitorFileGlobStr "dir/good-*"] () ()
+    updateMonitor root monitor (Set.singleton (monitorFileGlobStr "dir/good-*")) () ()
     threadDelay mtimeChange
     removeFile root "dir/bad"
     (res, files) <- expectMonitorUnchanged root monitor ()
     res @?= ()
-    files @?= [monitorFileGlobStr "dir/good-*"]
+    files @?= Set.fromList [monitorFileGlobStr "dir/good-*"]
 
 testGlobAddNonMatchSubdir :: Int -> Assertion
 testGlobAddNonMatchSubdir mtimeChange =
   withFileMonitor $ \root monitor -> do
     touchFile root ("dir" </> "a" </> "good-a")
-    updateMonitor root monitor [monitorFileGlobStr "dir/*/good-*"] () ()
+    updateMonitor root monitor (Set.singleton (monitorFileGlobStr "dir/*/good-*")) () ()
     threadDelay mtimeChange
     touchFile root ("dir" </> "b" </> "bad")
     (res, files) <- expectMonitorUnchanged root monitor ()
     res @?= ()
-    files @?= [monitorFileGlobStr "dir/*/good-*"]
+    files @?= Set.fromList [monitorFileGlobStr "dir/*/good-*"]
 
 testGlobRemoveNonMatchSubdir :: Int -> Assertion
 testGlobRemoveNonMatchSubdir mtimeChange =
   withFileMonitor $ \root monitor -> do
     touchFile root ("dir" </> "a" </> "good-a")
     touchFile root ("dir" </> "b" </> "bad")
-    updateMonitor root monitor [monitorFileGlobStr "dir/*/good-*"] () ()
+    updateMonitor root monitor (Set.singleton (monitorFileGlobStr "dir/*/good-*")) () ()
     threadDelay mtimeChange
     removeDir root ("dir" </> "b")
     (res, files) <- expectMonitorUnchanged root monitor ()
     res @?= ()
-    files @?= [monitorFileGlobStr "dir/*/good-*"]
+    files @?= Set.fromList [monitorFileGlobStr "dir/*/good-*"]
 
 -- try and tickle a bug that happens if we don't maintain the invariant that
 -- MonitorStateGlobFiles entries are sorted
@@ -618,7 +621,7 @@ testInvariantMonitorStateGlobFiles mtimeChange =
     touchFile root ("dir" </> "b")
     touchFile root ("dir" </> "c")
     touchFile root ("dir" </> "d")
-    updateMonitor root monitor [monitorFileGlobStr "dir/*"] () ()
+    updateMonitor root monitor (Set.singleton (monitorFileGlobStr "dir/*")) () ()
     threadDelay mtimeChange
     -- so there should be no change (since we're doing content checks)
     -- but if we can get the dir entries to appear in the wrong order
@@ -634,7 +637,7 @@ testInvariantMonitorStateGlobFiles mtimeChange =
     touchFile root ("dir" </> "a")
     (res, files) <- expectMonitorUnchanged root monitor ()
     res @?= ()
-    files @?= [monitorFileGlobStr "dir/*"]
+    files @?= Set.fromList [monitorFileGlobStr "dir/*"]
 
 -- same thing for the subdirs case
 testInvariantMonitorStateGlobDirs :: Int -> Assertion
@@ -644,7 +647,7 @@ testInvariantMonitorStateGlobDirs mtimeChange =
     touchFile root ("dir" </> "b" </> "file")
     touchFile root ("dir" </> "c" </> "file")
     touchFile root ("dir" </> "d" </> "file")
-    updateMonitor root monitor [monitorFileGlobStr "dir/*/file"] () ()
+    updateMonitor root monitor (Set.singleton (monitorFileGlobStr "dir/*/file")) () ()
     threadDelay mtimeChange
     removeDir root ("dir" </> "a")
     removeDir root ("dir" </> "b")
@@ -656,25 +659,25 @@ testInvariantMonitorStateGlobDirs mtimeChange =
     touchFile root ("dir" </> "a" </> "file")
     (res, files) <- expectMonitorUnchanged root monitor ()
     res @?= ()
-    files @?= [monitorFileGlobStr "dir/*/file"]
+    files @?= Set.fromList [monitorFileGlobStr "dir/*/file"]
 
 -- ensure that a glob can match a directory as well as a file
 testGlobMatchDir :: Int -> Assertion
 testGlobMatchDir mtimeChange =
   withFileMonitor $ \root monitor -> do
     createDir root ("dir" </> "a")
-    updateMonitor root monitor [monitorFileGlobStr "dir/*"] () ()
+    updateMonitor root monitor (Set.singleton (monitorFileGlobStr "dir/*")) () ()
     threadDelay mtimeChange
     -- nothing changed yet
     (res, files) <- expectMonitorUnchanged root monitor ()
     res @?= ()
-    files @?= [monitorFileGlobStr "dir/*"]
+    files @?= Set.fromList [monitorFileGlobStr "dir/*"]
     -- expect dir/b to match and be detected as changed
     createDir root ("dir" </> "b")
     reason <- expectMonitorChanged root monitor ()
     reason @?= MonitoredFileChanged ("dir" </> "b")
     -- now remove dir/a and expect it to be detected as changed
-    updateMonitor root monitor [monitorFileGlobStr "dir/*"] () ()
+    updateMonitor root monitor (Set.singleton (monitorFileGlobStr "dir/*")) () ()
     threadDelay mtimeChange
     removeDir root ("dir" </> "a")
     reason2 <- expectMonitorChanged root monitor ()
@@ -683,13 +686,13 @@ testGlobMatchDir mtimeChange =
 testGlobMatchDirOnly :: Int -> Assertion
 testGlobMatchDirOnly mtimeChange =
   withFileMonitor $ \root monitor -> do
-    updateMonitor root monitor [monitorFileGlobStr "dir/*/"] () ()
+    updateMonitor root monitor (Set.singleton (monitorFileGlobStr "dir/*/")) () ()
     threadDelay mtimeChange
     -- expect file dir/a to not match, so not detected as changed
     touchFile root ("dir" </> "a")
     (res, files) <- expectMonitorUnchanged root monitor ()
     res @?= ()
-    files @?= [monitorFileGlobStr "dir/*/"]
+    files @?= Set.fromList [monitorFileGlobStr "dir/*/"]
     -- note that checking the file monitor for changes can updates the
     -- cached dir mtimes (when it has to record that there's new matches)
     -- so we need an extra mtime delay
@@ -704,14 +707,14 @@ testGlobChangeFileType mtimeChange =
   withFileMonitor $ \root monitor -> do
     -- change file to dir
     touchFile root ("dir" </> "a")
-    updateMonitor root monitor [monitorFileGlobStr "dir/*"] () ()
+    updateMonitor root monitor (Set.singleton (monitorFileGlobStr "dir/*")) () ()
     threadDelay mtimeChange
     removeFile root ("dir" </> "a")
     createDir root ("dir" </> "a")
     reason <- expectMonitorChanged root monitor ()
     reason @?= MonitoredFileChanged ("dir" </> "a")
     -- change dir to file
-    updateMonitor root monitor [monitorFileGlobStr "dir/*"] () ()
+    updateMonitor root monitor (Set.singleton (monitorFileGlobStr "dir/*")) () ()
     threadDelay mtimeChange
     removeDir root ("dir" </> "a")
     touchFile root ("dir" </> "a")
@@ -725,19 +728,19 @@ testGlobAbsolutePath mtimeChange =
     -- absolute glob, removing a file
     touchFile root ("dir/good-a")
     touchFile root ("dir/good-b")
-    updateMonitor root monitor [monitorFileGlobStr (root' </> "dir/good-*")] () ()
+    updateMonitor root monitor (Set.singleton (monitorFileGlobStr (root' </> "dir/good-*"))) () ()
     threadDelay mtimeChange
     removeFile root "dir/good-a"
     reason <- expectMonitorChanged root monitor ()
     reason @?= MonitoredFileChanged (root' </> "dir" </> "good-a")
     -- absolute glob, adding a file
-    updateMonitor root monitor [monitorFileGlobStr (root' </> "dir/good-*")] () ()
+    updateMonitor root monitor (Set.singleton (monitorFileGlobStr (root' </> "dir/good-*"))) () ()
     threadDelay mtimeChange
     touchFile root ("dir/good-a")
     reason2 <- expectMonitorChanged root monitor ()
     reason2 @?= MonitoredFileChanged (root' </> "dir" </> "good-a")
     -- absolute glob, changing a file
-    updateMonitor root monitor [monitorFileGlobStr (root' </> "dir/good-*")] () ()
+    updateMonitor root monitor (Set.singleton (monitorFileGlobStr (root' </> "dir/good-*"))) () ()
     threadDelay mtimeChange
     touchFileContent root "dir/good-b"
     reason3 <- expectMonitorChanged root monitor ()
@@ -751,16 +754,16 @@ testValueUnchanged :: Assertion
 testValueUnchanged =
   withFileMonitor $ \root monitor -> do
     touchFile root "a"
-    updateMonitor root monitor [monitorFile "a"] (42 :: Int) "ok"
+    updateMonitor root monitor (Set.singleton (monitorFile "a")) (42 :: Int) "ok"
     (res, files) <- expectMonitorUnchanged root monitor 42
     res @?= "ok"
-    files @?= [monitorFile "a"]
+    files @?= Set.fromList [monitorFile "a"]
 
 testValueChanged :: Assertion
 testValueChanged =
   withFileMonitor $ \root monitor -> do
     touchFile root "a"
-    updateMonitor root monitor [monitorFile "a"] (42 :: Int) "ok"
+    updateMonitor root monitor (Set.singleton (monitorFile "a")) (42 :: Int) "ok"
     reason <- expectMonitorChanged root monitor 43
     reason @?= MonitoredValueChanged 42
 
@@ -770,7 +773,7 @@ testValueAndFileChanged mtimeChange =
     touchFile root "a"
 
     -- we change the value and the file, and the value change is reported
-    updateMonitor root monitor [monitorFile "a"] (42 :: Int) "ok"
+    updateMonitor root monitor (Set.singleton (monitorFile "a")) (42 :: Int) "ok"
     threadDelay mtimeChange
     touchFile root "a"
     reason <- expectMonitorChanged root monitor 43
@@ -780,12 +783,12 @@ testValueAndFileChanged mtimeChange =
     -- then it's reported as MonitoredValueChanged
     let monitor' :: FileMonitor Int String
         monitor' = monitor{fileMonitorCheckIfOnlyValueChanged = True}
-    updateMonitor root monitor' [monitorFile "a"] 42 "ok"
+    updateMonitor root monitor' (Set.singleton (monitorFile "a")) 42 "ok"
     reason2 <- expectMonitorChanged root monitor' 43
     reason2 @?= MonitoredValueChanged 42
 
     -- but if a file changed too then we don't report MonitoredValueChanged
-    updateMonitor root monitor' [monitorFile "a"] 42 "ok"
+    updateMonitor root monitor' (Set.singleton (monitorFile "a")) 42 "ok"
     threadDelay mtimeChange
     touchFile root "a"
     reason3 <- expectMonitorChanged root monitor' 43
@@ -803,7 +806,7 @@ testValueUpdated =
             , fileMonitorKeyValid = Set.isSubsetOf
             }
 
-    updateMonitor root monitor' [monitorFile "a"] (Set.fromList [42, 43]) "ok"
+    updateMonitor root monitor' (Set.singleton (monitorFile "a")) (Set.fromList [42, 43]) "ok"
     (res, _files) <- expectMonitorUnchanged root monitor' (Set.fromList [42])
     res @?= "ok"
 
@@ -870,7 +873,7 @@ expectMonitorUnchanged
   => RootPath
   -> FileMonitor a b
   -> a
-  -> IO (b, [MonitorFilePath])
+  -> IO (b, Set MonitorFilePath)
 expectMonitorUnchanged root monitor key = do
   res <- checkChanged root monitor key
   case res of
@@ -890,7 +893,7 @@ updateMonitor
   :: (Binary a, Structured a, Binary b, Structured b)
   => RootPath
   -> FileMonitor a b
-  -> [MonitorFilePath]
+  -> Set MonitorFilePath
   -> a
   -> b
   -> IO ()
@@ -902,7 +905,7 @@ updateMonitorWithTimestamp
   => RootPath
   -> FileMonitor a b
   -> MonitorTimestamp
-  -> [MonitorFilePath]
+  -> Set MonitorFilePath
   -> a
   -> b
   -> IO ()