Export pseq from Control.Parallel, and use it in Control.Parallel.Strategies

[haskell-directory.git] / Control / Parallel.hs

diff --git a/Control/Parallel.hs b/Control/Parallel.hs
index 1d6a126..8e1a6eb 100644 (file)
--- a/Control/Parallel.hs
+++ b/Control/Parallel.hs
@@ -1,21 +1,20 @@
 -----------------------------------------------------------------------------
--- 
+-- |
 -- Module      :  Control.Parallel
 -- Copyright   :  (c) The University of Glasgow 2001
--- License     :  BSD-style (see the file libraries/core/LICENSE)
+-- License     :  BSD-style (see the file libraries/base/LICENSE)
 -- 
 -- Maintainer  :  libraries@haskell.org
 -- Stability   :  experimental
 -- Portability :  non-portable
 --
--- $Id: Parallel.hs,v 1.1 2001/06/28 14:15:01 simonmar Exp $
---
 -- Parallel Constructs
 --
 -----------------------------------------------------------------------------
 
 module Control.Parallel (
-          par, seq -- re-exported
+          par, pseq,
+         seq, -- for backwards compatibility, 6.6 exported this
 #if defined(__GRANSIM__)
        , parGlobal, parLocal, parAt, parAtAbs, parAtRel, parAtForNow     
 #endif
@@ -24,7 +23,7 @@ module Control.Parallel (
 import Prelude
 
 #ifdef __GLASGOW_HASKELL__
-import GHC.Conc        ( par )
+import qualified GHC.Conc      ( par, pseq )
 #endif
 
 #if defined(__GRANSIM__)
@@ -32,6 +31,8 @@ import PrelBase
 import PrelErr   ( parError )
 import PrelGHC   ( parGlobal#, parLocal#, parAt#, parAtAbs#, parAtRel#, parAtForNow# )
 
+infixr 0 `par`
+
 {-# INLINE parGlobal #-}
 {-# INLINE parLocal #-}
 {-# INLINE parAt #-}
@@ -56,7 +57,48 @@ parAtForNow (I# w) (I# g) (I# s) (I# p) v x y = case (parAtForNow# x v w g s p y
 #endif
 
 -- Maybe parIO and the like could be added here later.
-#ifndef __GLASGOW_HASKELL__
+
+-- | Indicates that it may be beneficial to evaluate the first
+-- argument in parallel with the second.  Returns the value of the
+-- second argument.
+-- 
+-- @a `par` b@ is exactly equivalent semantically to @b@.
+--
+-- @par@ is generally used when the value of @a@ is likely to be
+-- required later, but not immediately.  Also it is a good idea to
+-- ensure that @a@ is not a trivial computation, otherwise the cost of
+-- spawning it in parallel overshadows the benefits obtained by
+-- running it in parallel.
+--
+-- Note that actual parallelism is only supported by certain
+-- implementations (GHC with the @-threaded@ option, and GPH, for
+-- now).  On other implementations, @par a b = b@.
+--
+par :: a -> b -> b
+#ifdef __GLASGOW_HASKELL__
+par = GHC.Conc.par
+#else
 -- For now, Hugs does not support par properly.
 par a b = b
 #endif
+
+-- | Semantically identical to 'seq', but with a subtle operational
+-- difference: 'seq' is strict in both its arguments, so the compiler
+-- may, for example, rearrange @a `seq` b@ into @b `seq` a `seq` b@.
+-- This is normally no problem when using 'seq' to express strictness,
+-- but it can be a problem when annotating code for parallelism,
+-- because we need more control over the order of evaluation; we may
+-- want to evaluate @a@ before @b@, because we know that @b@ has
+-- already been sparked in parallel with 'par'.
+--
+-- This is why we have 'pseq'.  In contrast to 'seq', 'pseq' is only
+-- strict in its first argument (as far as the compiler is concerned),
+-- which restricts the transformations that the compiler can do, and
+-- ensures that the user can retain control of the evaluation order.
+--
+pseq :: a -> b -> b
+#ifdef __GLASGOW_HASKELL__
+pseq = GHC.Conc.pseq
+#else
+pseq = seq
+#endif