emptyP def to gHC.PArr

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

diff --git a/GHC/Exception.lhs b/GHC/Exception.lhs
index a98ca19..852f465 100644 (file)
--- a/GHC/Exception.lhs
+++ b/GHC/Exception.lhs
@@ -1,5 +1,6 @@
 \begin{code}
-{-# OPTIONS -fno-implicit-prelude #-}
+{-# OPTIONS_GHC -fno-implicit-prelude #-}
+{-# OPTIONS_HADDOCK hide #-}
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  GHC.Exception
@@ -14,11 +15,12 @@
 -- 
 -----------------------------------------------------------------------------
 
-module GHC.Exception 
-       ( module GHC.Exception, 
-         Exception(..), AsyncException(..), 
-         IOException(..), ArithException(..), ArrayException(..),
-         throw, throwIO, ioError ) 
+-- #hide
+module GHC.Exception
+        ( module GHC.Exception,
+          Exception(..), AsyncException(..),
+          IOException(..), ArithException(..), ArrayException(..),
+          throw, throwIO, ioError )
   where
 
 import GHC.Base
@@ -26,9 +28,9 @@ import GHC.IOBase
 \end{code}
 
 %*********************************************************
-%*                                                     *
+%*                                                      *
 \subsection{Primitive catch}
-%*                                                     *
+%*                                                      *
 %*********************************************************
 
 catchException used to handle the passing around of the state to the
@@ -47,28 +49,52 @@ 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
+catch m k       =  catchException m handler
   where handler (IOException err)   = k err
-       handler other               = throw other
+        handler other               = throw other
 \end{code}
 
 
 %*********************************************************
-%*                                                     *
+%*                                                      *
 \subsection{Controlling asynchronous exception delivery}
-%*                                                     *
+%*                                                      *
 %*********************************************************
 
 \begin{code}
 -- | 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
+-- attempts to raise an exception in the current thread with 'Control.Exception.throwTo' 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'.
+--
+-- Threads created by 'Control.Concurrent.forkIO' inherit the blocked
+-- state from the parent; that is, to start a thread in blocked mode,
+-- use @block $ forkIO ...@.  This is particularly useful if you need to
+-- establish an exception handler in the forked thread before any
+-- asynchronous exceptions are received.
 block :: IO a -> IO a
 
 -- | To re-enable asynchronous exceptions inside the scope of
@@ -82,4 +108,23 @@ block (IO io) = IO $ blockAsyncExceptions# io
 unblock (IO io) = IO $ unblockAsyncExceptions# io
 \end{code}
 
-
+\begin{code}
+-- | Forces its argument to be evaluated when the resultant 'IO' action
+-- is executed.  It can be used to order evaluation with respect to
+-- other 'IO' operations; its semantics are given by
+--
+-- >   evaluate x `seq` y    ==>  y
+-- >   evaluate x `catch` f  ==>  (return $! x) `catch` f
+-- >   evaluate x >>= f      ==>  (return $! x) >>= f
+--
+-- /Note:/ the first equation implies that @(evaluate x)@ is /not/ the
+-- same as @(return $! x)@.  A correct definition is
+--
+-- >   evaluate x = (return $! x) >>= return
+--
+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}