    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Control.Parallel
    -- Copyright   :  (c) The University of Glasgow 2001
    -- License     :  BSD-style (see the file libraries/base/LICENSE)
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  experimental
    -- Portability :  non-portable
    --
    -- Parallel Constructs
    --
    -----------------------------------------------------------------------------

    module Control.Parallel (
              par, pseq,
    	  seq, -- for backwards compatibility, 6.6 exported this
    #if defined(__GRANSIM__)
    	, parGlobal, parLocal, parAt, parAtAbs, parAtRel, parAtForNow
    #endif
        ) where

    import Prelude

    #ifdef __GLASGOW_HASKELL__
    import qualified GHC.Conc	( par, pseq )
    #endif

    #if defined(__GRANSIM__)
    import PrelBase
    import PrelErr   ( parError )
    import PrelGHC   ( parGlobal#, parLocal#, parAt#, parAtAbs#, parAtRel#, parAtForNow# )

    infixr 0 `par`

    {-# INLINE parGlobal #-}
    {-# INLINE parLocal #-}
    {-# INLINE parAt #-}
    {-# INLINE parAtAbs #-}
    {-# INLINE parAtRel #-}
    {-# INLINE parAtForNow #-}
    parGlobal   :: Int -> Int -> Int -> Int -> a -> b -> b
    parLocal    :: Int -> Int -> Int -> Int -> a -> b -> b
    parAt	    :: Int -> Int -> Int -> Int -> a -> b -> c -> c
    parAtAbs    :: Int -> Int -> Int -> Int -> Int -> a -> b -> b
    parAtRel    :: Int -> Int -> Int -> Int -> Int -> a -> b -> b
    parAtForNow :: Int -> Int -> Int -> Int -> a -> b -> c -> c

    parGlobal (I# w) (I# g) (I# s) (I# p) x y = case (parGlobal# x w g s p y) of { 0# -> parError; _ -> y }
    parLocal  (I# w) (I# g) (I# s) (I# p) x y = case (parLocal#  x w g s p y) of { 0# -> parError; _ -> y }

    parAt       (I# w) (I# g) (I# s) (I# p) v x y = case (parAt#       x v w g s p y) of { 0# -> parError; _ -> y }
    parAtAbs    (I# w) (I# g) (I# s) (I# p) (I# q) x y = case (parAtAbs#  x q w g s p y) of { 0# -> parError; _ -> y }
    parAtRel    (I# w) (I# g) (I# s) (I# p) (I# q) x y = case (parAtRel#  x q w g s p y) of { 0# -> parError; _ -> y }
    parAtForNow (I# w) (I# g) (I# s) (I# p) v x y = case (parAtForNow# x v w g s p y) of { 0# -> parError; _ -> y }

    #endif

    -- Maybe parIO and the like could be added here later.

    -- | 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