{- |@'copyFile' old new@ copies the existing file from /old/ to /new/.
If the /new/ file already exists, it is atomically replaced by the /old/ file.
-Neither path may refer to an existing directory.
+Neither path may refer to an existing directory. The permissions of /old/ are
+copied to /new/, if possible.
+-}
+
+{- NOTES:
+
+It's tempting to try to remove the target file before opening it for
+writing. This could be useful: for example if the target file is an
+executable that is in use, writing will fail, but unlinking first
+would succeed.
+
+However, it certainly isn't always what you want.
+
+ * if the target file is hardlinked, removing it would break
+ the hard link, but just opening would preserve it.
+
+ * opening and truncating will preserve permissions and
+ ACLs on the target.
+
+ * If the destination file is read-only in a writable directory,
+ we might want copyFile to fail. Removing the target first
+ would succeed, however.
+
+ * If the destination file is special (eg. /dev/null), removing
+ it is probably not the right thing. Copying to /dev/null
+ should leave /dev/null intact, not replace it with a plain
+ file.
+
+ * There's a small race condition between removing the target and
+ opening it for writing during which time someone might
+ create it again.
-}
copyFile :: FilePath -> FilePath -> IO ()
copyFile fromFPath toFPath =