    {-# LANGUAGE BangPatterns #-}

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Control.Parallel.SeqStrategies
    -- Copyright   :  (c) The University of Glasgow 2001-2009
    -- License     :  BSD-style (see the file libraries/base/LICENSE)
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  experimental
    -- Portability :  portable
    --
    -- Sequential strategies provide ways to compositionally specify
    -- the degree of evaluation of a data type between the extremes of
    -- no evaluation and full evaluation.
    -- Sequential strategies may be viewed as complimentary to the parallel
    -- ones (see module "Control.Parallel.Strategies").
    --

    module Control.Seq
           (
             -- * The sequential strategy type
             Strategy

             -- * Application of sequential strategies
           , using            -- :: a -> Strategy a -> a
           , withStrategy     -- :: Strategy a -> a -> a

             -- * Basic sequential strategies
           , r0               -- :: Strategy a
           , rseq
           , rdeepseq         -- :: NFData a => Strategy a

             -- * Sequential strategies for lists
           , seqList          -- :: Strategy a -> Strategy [a]
           , seqListN         -- :: Int -> Strategy a -> Strategy [a]
           , seqListNth

             -- * Sequential strategies for foldable data types
           , seqFoldable      -- :: Foldable t => Strategy a -> Strategy (t a)
           , seqMap           -- :: Strategy k -> Strategy v -> Strategy (Map k v)
           , seqArray         -- :: Ix i => Strategy a -> Strategy (Array i a)
           , seqArrayBounds   -- :: Ix i => Strategy i -> Strategy (Array i a)

             -- * Sequential strategies for tuples

             -- | Evaluate the components of a tuple according to the given strategies.
             -- No guarantee is given as to the order of evaluation.

           , seqTuple2        -- :: Strategy a -> ... -> Strategy (a,...)
           , seqTuple3
           , seqTuple4
           , seqTuple5
           , seqTuple6
           , seqTuple7
           , seqTuple8
           , seqTuple9
           ) where

    import Prelude
    import Control.DeepSeq (NFData, deepseq)
    import Data.Foldable (Foldable, toList)
    import Data.Map (Map)
    import qualified Data.Map (toList)
    import Data.Ix (Ix)
    import Data.Array (Array)
    import qualified Data.Array (bounds, elems)

    infixl 0 `using`   -- lowest precedence and associate to the left

    -- --------------------------------------------------------------------------
    -- Sequential strategies

    -- | The type @'Strategy' a@ is @a -> ()@.
    -- Thus, a strategy is a function whose sole purpose it is to evaluate
    -- its argument (either in full or in part).
    type Strategy a = a -> ()

    -- | Evaluate a value using the given strategy.
    using :: a -> Strategy a -> a
    x `using` strat = strat x `seq` x

    -- | Evaluate a value using the given strategy.
    -- This is simply 'using' with arguments reversed.
    withStrategy :: Strategy a -> a -> a
    withStrategy = flip using

    -- --------------------------------------------------------------------------
    -- Basic sequential strategies

    -- | 'r0' performs *no* evaluation.
    r0 :: Strategy a
    r0 _ = ()

    -- | 'rseq' evaluates its argument to weak head normal form.
    rseq :: Strategy a
    rseq x = x `seq` ()

    -- | 'rdeepseq' fully evaluates its argument.
    -- Relies on class 'NFData' from module "Control.DeepSeq".
    rdeepseq :: NFData a => Strategy a
    rdeepseq x = x `deepseq` ()

    -- --------------------------------------------------------------------------
    -- Sequential strategies for lists

    -- | Evaluate each element of a list according to the given strategy.
    -- This function is a specialisation of 'seqFoldable' to lists.
    seqList :: Strategy a -> Strategy [a]
    seqList _strat []    = ()
    seqList strat (x:xs) = strat x `seq` seqList strat xs
    -- Alternative definition via seqFoldable:
    -- seqList = seqFoldable

    -- | Evaluate the first n elements of a list according to the given strategy.
    seqListN :: Int -> Strategy a -> Strategy [a]
    seqListN 0  _strat _     = ()
    seqListN !_ _strat []    = ()
    seqListN !n strat (x:xs) = strat x `seq` seqListN (n-1) strat xs

    -- | Evaluate the nth element of a list (if there is such) according to
    -- the given strategy.
    -- The spine of the list up to the nth element is evaluated as a side effect.
    seqListNth :: Int -> Strategy a -> Strategy [a]
    seqListNth 0  strat  (x:_)  = strat x
    seqListNth !_ _strat []     = ()
    seqListNth !n strat  (_:xs) = seqListNth (n-1) strat xs

    -- --------------------------------------------------------------------------
    -- Sequential strategies for foldable data types

    -- | Evaluate the elements of a foldable data structure according to
    -- the given strategy.
    seqFoldable :: Foldable t => Strategy a -> Strategy (t a)
    seqFoldable strat = seqList strat . toList
    -- Alternative definition via foldl':
    -- seqFoldable strat = foldl' (const strat) ()

    {-# SPECIALISE seqFoldable :: Strategy a -> Strategy [a] #-}

    -- | Evaluate the elements of an array according to the given strategy.
    -- Evaluation of the array bounds may be triggered as a side effect.
    seqArray :: Ix i => Strategy a -> Strategy (Array i a)
    seqArray strat = seqList strat . Data.Array.elems

    -- | Evaluate the bounds of an array according to the given strategy.
    seqArrayBounds :: Ix i => Strategy i -> Strategy (Array i a)
    seqArrayBounds strat = seqTuple2 strat strat . Data.Array.bounds

    -- | Evaluate the keys and values of a map according to the given strategies.
    seqMap :: Strategy k -> Strategy v -> Strategy (Map k v)
    seqMap stratK stratV = seqList (seqTuple2 stratK stratV) . Data.Map.toList

    -- --------------------------------------------------------------------------
    -- Sequential strategies for tuples

    seqTuple2 :: Strategy a -> Strategy b -> Strategy (a,b)
    seqTuple2 strat1 strat2 (x1,x2) =
      strat1 x1 `seq` strat2 x2

    seqTuple3 :: Strategy a -> Strategy b -> Strategy c -> Strategy (a,b,c)
    seqTuple3 strat1 strat2 strat3 (x1,x2,x3) =
      strat1 x1 `seq` strat2 x2 `seq` strat3 x3

    seqTuple4 :: Strategy a -> Strategy b -> Strategy c -> Strategy d -> Strategy (a,b,c,d)
    seqTuple4 strat1 strat2 strat3 strat4 (x1,x2,x3,x4) =
      strat1 x1 `seq` strat2 x2 `seq` strat3 x3 `seq` strat4 x4

    seqTuple5 :: Strategy a -> Strategy b -> Strategy c -> Strategy d -> Strategy e -> Strategy (a,b,c,d,e)
    seqTuple5 strat1 strat2 strat3 strat4 strat5 (x1,x2,x3,x4,x5) =
      strat1 x1 `seq` strat2 x2 `seq` strat3 x3 `seq` strat4 x4 `seq` strat5 x5

    seqTuple6 :: Strategy a -> Strategy b -> Strategy c -> Strategy d -> Strategy e -> Strategy f -> Strategy (a,b,c,d,e,f)
    seqTuple6 strat1 strat2 strat3 strat4 strat5 strat6 (x1,x2,x3,x4,x5,x6) =
      strat1 x1 `seq` strat2 x2 `seq` strat3 x3 `seq` strat4 x4 `seq` strat5 x5 `seq` strat6 x6

    seqTuple7 :: Strategy a -> Strategy b -> Strategy c -> Strategy d -> Strategy e -> Strategy f -> Strategy g -> Strategy (a,b,c,d,e,f,g)
    seqTuple7 strat1 strat2 strat3 strat4 strat5 strat6 strat7 (x1,x2,x3,x4,x5,x6,x7) =
      strat1 x1 `seq` strat2 x2 `seq` strat3 x3 `seq` strat4 x4 `seq` strat5 x5 `seq` strat6 x6 `seq` strat7 x7

    seqTuple8 :: Strategy a -> Strategy b -> Strategy c -> Strategy d -> Strategy e -> Strategy f -> Strategy g -> Strategy h -> Strategy (a,b,c,d,e,f,g,h)
    seqTuple8 strat1 strat2 strat3 strat4 strat5 strat6 strat7 strat8 (x1,x2,x3,x4,x5,x6,x7,x8) =
      strat1 x1 `seq` strat2 x2 `seq` strat3 x3 `seq` strat4 x4 `seq` strat5 x5 `seq` strat6 x6 `seq` strat7 x7 `seq` strat8 x8

    seqTuple9 :: Strategy a -> Strategy b -> Strategy c -> Strategy d -> Strategy e -> Strategy f -> Strategy g -> Strategy h -> Strategy i -> Strategy (a,b,c,d,e,f,g,h,i)
    seqTuple9 strat1 strat2 strat3 strat4 strat5 strat6 strat7 strat8 strat9 (x1,x2,x3,x4,x5,x6,x7,x8,x9) =
      strat1 x1 `seq` strat2 x2 `seq` strat3 x3 `seq` strat4 x4 `seq` strat5 x5 `seq` strat6 x6 `seq` strat7 x7 `seq` strat8 x8 `seq` strat9 x9