X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=GHC%2FException.lhs;h=fb9e7aa5c42727b0ec37a69b1e1afb83790778d2;hb=28fb12f4e4059674e9396fc76a2783e0ae8798cd;hp=7979d4dd90b67f951a2d80fe4ed52bbf5d2fdb0c;hpb=b706340c451952adf230b5b8daecad8a1f34d714;p=ghc-base.git

diff --git a/GHC/Exception.lhs b/GHC/Exception.lhs
index 7979d4d..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
@@ -14,20 +14,15 @@
 -- 
 -----------------------------------------------------------------------------
 
-#ifndef __HUGS__
 module GHC.Exception 
 	( module GHC.Exception, 
 	  Exception(..), AsyncException(..), 
 	  IOException(..), ArithException(..), ArrayException(..),
-	  throw, ioError ) 
+	  throw, throwIO, ioError ) 
   where
 
-import Data.Either
-
 import GHC.Base
 import GHC.IOBase
-
-#endif
 \end{code}
 
 %*********************************************************
@@ -50,79 +45,73 @@ 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
 
-catch           :: IO a -> (Exception -> IO a) -> IO a 
+-- | 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 err@(IOException _) = k err
+  where handler (IOException err)   = k err
 	handler other               = throw other
 \end{code}
 
 
 %*********************************************************
 %*							*
-\subsection{Try and bracket}
-%*							*
-%*********************************************************
-
-The construct @try comp@ exposes errors which occur within a
-computation, and which are not fully handled.  It always succeeds.
-
-These are the IO-only try/bracket.  For the full exception try/bracket
-see hslibs/lang/Exception.lhs.
-
-\begin{code}
-try            :: IO a -> IO (Either Exception 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{Controlling asynchronous exception delivery}
 %*							*
 %*********************************************************
 
 \begin{code}
-#ifndef __HUGS__
+-- | Applying 'block' to a computation will
+-- execute that computation with asynchronous exceptions
+-- /blocked/.  That is, any thread which
+-- attempts to raise an exception in the current thread will be
+-- blocked until asynchronous exceptions are enabled again.  There\'s
+-- no need to worry about re-enabling asynchronous exceptions; that is
+-- done automatically on exiting the scope of
+-- 'block'.
 block :: IO a -> IO a
-block (IO io) = IO $ blockAsyncExceptions# io
 
+-- | To re-enable asynchronous exceptions inside the scope of
+-- 'block', 'unblock' can be
+-- used.  It scopes in exactly the same way, so on exit from
+-- 'unblock' asynchronous exception delivery will
+-- be disabled again.
 unblock :: IO a -> IO a
-unblock (IO io) = IO $ unblockAsyncExceptions# io
-#else
--- Not implemented yet in Hugs.
-block :: IO a -> IO a
-block (IO io) = IO io
 
-unblock :: IO a -> IO a
-unblock (IO io) = IO io
-#endif
+block (IO io) = IO $ blockAsyncExceptions# io
+unblock (IO io) = IO $ unblockAsyncExceptions# io
 \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}