further clarify the docs for 'evaluate'

[haskell-directory.git] / GHC / Exception.lhs

diff --git a/GHC/Exception.lhs b/GHC/Exception.lhs
index fb9e7aa..5cee08f 100644 (file)
--- a/GHC/Exception.lhs
+++ b/GHC/Exception.lhs
@@ -14,6 +14,7 @@
 -- 
 -----------------------------------------------------------------------------
 
+-- #hide
 module GHC.Exception 
        ( module GHC.Exception, 
          Exception(..), AsyncException(..), 
@@ -101,14 +102,19 @@ 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
+-- | 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 undefined `seq` return ()  ==> return ()
--- >   catch (evaluate undefined) (\e -> return ())  ==> return ()
+-- >   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
 --
--- 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