Start to actually use extensible exceptions

[ghc-base.git] / Prelude.hs

diff --git a/Prelude.hs b/Prelude.hs
index ff3902a..8eb912b 100644 (file)
--- a/Prelude.hs
+++ b/Prelude.hs
@@ -170,6 +170,8 @@ import GHC.Show
 import GHC.Err   ( error, undefined )
 #endif
 
+import qualified Control.OldException as Old
+
 #ifdef __HUGS__
 import Hugs.Prelude
 #endif
@@ -192,3 +194,27 @@ f $! x  = x `seq` f x
 seq :: a -> b -> b
 seq _ y = y
 #endif
+
+-- | 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 io handler = io `Old.catch` handler'
+    where handler' (Old.IOException ioe) = handler ioe
+          handler' e                     = throw e
+