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

    module Control.Parallel (
              par, pseq
        ) where

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

    infixr 0 `par`, `pseq`
    #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