add some utility functions for later

Will need to update the DirMap to add or remove subdirs.

Utility/Kqueue.hs

@@ -8,8 +8,10 @@
 {-# LANGUAGE ForeignFunctionInterface #-}
 
 module Utility.Kqueue (
+	scanRecursive,
+	addSubDir,
+	removeSubDir,
 	waitChange,
-	scanRecursive
 ) where
 
 import Common
@@ -23,9 +25,37 @@ import qualified Data.Map as M
 
 type DirMap = M.Map Fd FilePath
 
+{- Builds a map of directories in a tree, possibly pruning some.
+ - Opens each directory in the tree. -}
+scanRecursive :: FilePath -> (FilePath -> Bool) -> IO DirMap
+scanRecursive dir prune = M.fromList <$> (mapM opendir =<< dirTree dir prune)
+	where
+		opendir d = (,)
+			<$> openFd d ReadOnly Nothing defaultFileFlags
+			<*> pure d
+
+{- Adds a subdirectory (and all its subdirectories, unless pruned) to a
+ - directory map. -}
+addSubDir :: DirMap -> FilePath -> (FilePath -> Bool) -> IO DirMap
+addSubDir dirmap dir prune = M.union dirmap <$> scanRecursive dir prune
+
+{- Removes a subdirectory (and all its subdirectories) from a directory map. -}
+removeSubDir :: FilePath -> DirMap -> DirMap
+removeSubDir dir = M.filter (not . dirContains dir)
+
 foreign import ccall unsafe "libkqueue.h waitchange" c_waitchange
 	:: Ptr Fd -> IO Fd
 
+{- Waits for a change in a map of directories, and returns the directory
+ - where the change took place. 
+ -
+ - The kqueue interface does not tell what type of change took place in
+ - the directory; it could be an added file, a deleted file, a renamed
+ - file, a new subdirectory, or a deleted subdirectory, or a moved
+ - subdirectory.
+ -
+ - Note that if subdirectories have changed, the caller will want to
+ - update the map before calling this again. -}
 waitChange :: DirMap -> IO (Maybe FilePath)
 waitChange dirmap = withArray (M.keys dirmap) $ \c_fds -> do
 	changed <- c_waitchange c_fds
@@ -35,10 +65,3 @@ waitChange dirmap = withArray (M.keys dirmap) $ \c_fds -> do
 		)
 	where
 		safeErrno (Errno v) = v == 0
-
-scanRecursive :: FilePath -> (FilePath -> Bool) -> IO DirMap
-scanRecursive dir prune = M.fromList <$> (mapM opendir =<< dirTree dir prune)
-	where
-		opendir d = (,)
-			<$> openFd d ReadOnly Nothing defaultFileFlags
-			<*> pure d