X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=GHC%2FException.lhs;h=fb9e7aa5c42727b0ec37a69b1e1afb83790778d2;hb=28fb12f4e4059674e9396fc76a2783e0ae8798cd;hp=3e26cdc53c38d76f552eb22f7ca84a4f01d0bf7c;hpb=a3e56f82973d7117fe4544e5702d1c0180ae7d79;p=ghc-base.git

diff --git a/GHC/Exception.lhs b/GHC/Exception.lhs
index 3e26cdc..fb9e7aa 100644
--- a/GHC/Exception.lhs
+++ b/GHC/Exception.lhs
@@ -1,5 +1,5 @@
 \begin{code}
-{-# OPTIONS -fno-implicit-prelude #-}
+{-# OPTIONS_GHC -fno-implicit-prelude #-}
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  GHC.Exception
@@ -14,7 +14,6 @@
 -- 
 -----------------------------------------------------------------------------
 
-#ifndef __HUGS__
 module GHC.Exception 
 	( module GHC.Exception, 
 	  Exception(..), AsyncException(..), 
@@ -24,8 +23,6 @@ module GHC.Exception
 
 import GHC.Base
 import GHC.IOBase
-
-#endif
 \end{code}
 
 %*********************************************************
@@ -48,12 +45,26 @@ have to work around that in the definition of catchException below).
 
 \begin{code}
 catchException :: IO a -> (Exception -> IO a) -> IO a
-#ifdef __HUGS__
-catchException m k =  ST (\s -> unST m s `primCatch'` \ err -> unST (k err) s)
-#else
 catchException (IO m) k =  IO $ \s -> catch# m (\ex -> unIO (k ex)) s
-#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 m k	=  catchException m handler
   where handler (IOException err)   = k err
@@ -85,13 +96,22 @@ block :: IO a -> IO a
 -- be disabled again.
 unblock :: IO a -> IO a
 
-#ifndef __HUGS__
 block (IO io) = IO $ blockAsyncExceptions# io
 unblock (IO io) = IO $ unblockAsyncExceptions# io
-#else
-unblock :: IO a -> IO a
-unblock (IO io) = IO io
-#endif
 \end{code}
 
-
+\begin{code}
+-- | Forces its argument to be evaluated, and returns the result in
+-- the 'IO' monad.  It can be used to order evaluation with respect to
+-- other 'IO' operations; its semantics are given by
+--
+-- >   evaluate undefined `seq` return ()  ==> return ()
+-- >   catch (evaluate undefined) (\e -> return ())  ==> return ()
+--
+-- NOTE: @(evaluate a)@ is /not/ the same as @(a \`seq\` return a)@.
+evaluate :: a -> IO a
+evaluate a = IO $ \s -> case a `seq` () of () -> (# s, a #)
+        -- NB. can't write  
+        --      a `seq` (# s, a #)
+        -- because we can't have an unboxed tuple as a function argument
+\end{code}