\begin{code}
-{-# OPTIONS -fno-implicit-prelude #-}
+{-# OPTIONS_GHC -fno-implicit-prelude #-}
-----------------------------------------------------------------------------
-- |
-- Module : GHC.Exception
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
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}