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diff --git a/Utility/FileMode.hs b/Utility/FileMode.hs
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+{- File mode utilities.
+ -
+ - Copyright 2010 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Utility.FileMode where
+
+import System.Posix.Files
+import System.Posix.Types
+import Foreign (complement)
+
+{- Removes a FileMode from a file.
+ - For example, call with otherWriteMode to chmod o-w -}
+unsetFileMode :: FilePath -> FileMode -> IO ()
+unsetFileMode f m = do
+	s <- getFileStatus f
+	setFileMode f $ fileMode s `intersectFileModes` complement m
+
+{- Removes the write bits from a file. -}
+preventWrite :: FilePath -> IO ()
+preventWrite f = unsetFileMode f writebits
+	where
+		writebits = foldl unionFileModes ownerWriteMode
+					[groupWriteMode, otherWriteMode]
+
+{- Turns a file's write bit back on. -}
+allowWrite :: FilePath -> IO ()
+allowWrite f = do
+	s <- getFileStatus f
+	setFileMode f $ fileMode s `unionFileModes` ownerWriteMode