+%*********************************************************
+%* *
+\subsection{Try and bracket}
+%* *
+%*********************************************************
+
+The construct @try comp@ exposes errors which occur within a
+computation, and which are not fully handled. It always succeeds.
+
+\begin{code}
+try :: IO a -> IO (Either IOError a)
+try f = catch (do r <- f
+ return (Right r))
+ (return . Left)
+
+bracket :: IO a -> (a -> IO b) -> (a -> IO c) -> IO c
+bracket before after m = do
+ x <- before
+ rs <- try (m x)
+ after x
+ case rs of
+ Right r -> return r
+ Left e -> ioError e
+
+-- variant of the above where middle computation doesn't want x
+bracket_ :: IO a -> (a -> IO b) -> IO c -> IO c
+bracket_ before after m = do
+ x <- before
+ rs <- try m
+ after x
+ case rs of
+ Right r -> return r
+ Left e -> ioError e
+\end{code}
+
+
+%*********************************************************
+%* *
+\subsection{ioError}
+%* *
+%*********************************************************
+