[project @ 2003-08-30 12:26:56 by ross]

[ghc-base.git] / GHC / Exception.lhs

diff --git a/GHC/Exception.lhs b/GHC/Exception.lhs
index a98ca19..08f5e97 100644 (file)
--- a/GHC/Exception.lhs
+++ b/GHC/Exception.lhs
@@ -47,6 +47,24 @@ have to work around that in the definition of catchException below).
 catchException :: IO a -> (Exception -> IO a) -> IO a
 catchException (IO m) k =  IO $ \s -> catch# m (\ex -> unIO (k ex)) s
 
+-- | The 'catch' function establishes a handler that receives any 'IOError'
+-- raised in the action protected by 'catch'.  An 'IOError' is caught by
+-- the most recent handler established by 'catch'.  These handlers are
+-- not selective: all 'IOError's are caught.  Exception propagation
+-- must be explicitly provided in a handler by re-raising any unwanted
+-- exceptions.  For example, in
+--
+-- > f = catch g (\e -> if IO.isEOFError e then return [] else ioError e)
+--
+-- the function @f@ returns @[]@ when an end-of-file exception
+-- (cf. 'System.IO.Error.isEOFError') occurs in @g@; otherwise, the
+-- exception is propagated to the next outer handler.
+--
+-- When an exception propagates outside the main program, the Haskell
+-- system prints the associated 'IOError' value and exits the program.
+--
+-- Non-I\/O exceptions are not caught by this variant; to catch all
+-- exceptions, use 'Control.Exception.catch' from "Control.Exception".
 catch           :: IO a -> (IOError -> IO a) -> IO a 
 catch m k      =  catchException m handler
   where handler (IOException err)   = k err