-- Stability : experimental
-- Portability : non-portable
--
--- Parallel strategy combinators
+-- Parallel strategy combinators. See
+-- <http://www.macs.hw.ac.uk/~dsg/gph/papers/html/Strategies/strategies.html>
+-- for more information.
+--
+-- Original authors:
+-- Phil Trinder, Hans-Wolfgang Loidl, Kevin Hammond et al.
--
-----------------------------------------------------------------------------
module Control.Parallel.Strategies where
-- based on hslibs/concurrent/Strategies.lhs; see it for more detailed
--- code comments. Original authors:
---
--- Phil Trinder, Hans-Wolfgang Loidl, Kevin Hammond et al.
---
+-- code comments.
import Control.Parallel as Parallel (par, pseq)
import Data.Array
seq = Parallel.pseq
------------------------------------------------------------------------------
--- Strategy Type, Application and Semantics
+-- * Strategy Type, Application and Semantics
------------------------------------------------------------------------------
-type Done = ()
-type Strategy a = a -> Done
{-
-A strategy takes a value and returns a dummy `done' value to indicate that
-the specifed evaluation has been performed.
-
-The basic combinators for strategies are @par@ and @seq@ but with types that
+The basic combinators for strategies are 'par' and 'seq' but with types that
indicate that they only combine the results of a strategy application.
NB: This version can be used with Haskell 1.4 (GHC 2.05 and beyond), *but*
you won't get strategy checking on seq (only on par)!
-The infix fcts >| and >|| are alternative names for `seq` and `par`.
+The operators >| and >|| are alternative names for `seq` and `par`.
With the introduction of a Prelude function `seq` separating the Prelude
function from the Strategy function becomes a pain. The notation also matches
the notation for strategic function application.
-}
-{-
-par and seq have the same types as before; >| and >|| are more specific
-and can only be used when composing strategies.
--}
+type Done = ()
-(>|), (>||) :: Done -> Done -> Done
+-- | A strategy takes a value and returns a 'Done' value to indicate that
+-- the specifed evaluation has been performed.
+type Strategy a = a -> Done
+
+
+-- | Evaluates the first argument before the second.
+(>|) :: Done -> Done -> Done
{-# INLINE (>|) #-}
-{-# INLINE (>||) #-}
(>|) = Prelude.seq
+
+-- | Evaluates the first argument in parallel with the second.
+(>||) :: Done -> Done -> Done
+{-# INLINE (>||) #-}
(>||) = Parallel.par
+
+-- | Takes a value and a strategy, and applies the strategy to the
+-- value before returning the value. Used to express data-oriented
+-- parallelism. @x \`using\` s@ is a projection on @x@, i.e. both:
+--
+-- [a retraction] @x \`using\` s@ ⊑ @x@
+--
+-- [idempotent] @(x \`using\` s) \`using\` s@ = @x \`using\` s@
+--
using :: a -> Strategy a -> a
using x s = s x `seq` x
-{-
-using takes a strategy and a value, and applies the strategy to the
-value before returning the value. Used to express data-oriented parallelism
-
-x `using` s is a projection on x, i.e. both
- a retraction: x `using` s [ x
- -
- and idempotent: (x `using` s) `using` s = x `using` s
-
-demanding and sparking are used to express control-oriented
-parallelism. Their second argument is usually a sequence of strategy
-applications combined `par` and `seq`. Sparking should only be used
-with a singleton sequence as it is not necessarily excuted
--}
-
-demanding, sparking :: a -> Done -> a
+-- | Evaluates the second argument before the first.
+-- Used to express control-oriented parallelism. The second
+-- argument is usually a strategy application.
+demanding :: a -> Done -> a
demanding = flip seq
-sparking = flip Parallel.par
-{-
-sPar and sSeq have been superceded by sparking and demanding: replace
- e `using` sPar x with e `sparking` x
- e `using` sSeq x with e `demanding` x
-sPar is a strategy corresponding to par. i.e. x `par` e <=> e `using` sPar x
--}
+-- | Evaluates the second argument in parallel with the first.
+-- Used to express control-oriented
+-- parallelism. The second argument is usually a strategy application.
+sparking :: a -> Done -> a
+sparking = flip Parallel.par
+-- Sparking should only be used
+-- with a singleton sequence as it is not necessarily executed.
+-- | A strategy corresponding to 'par':
+-- @x \`par\` e@ = @e \`using\` sPar x@.
+--
+-- 'sPar' has been superceded by 'sparking'.
+-- Replace @e \`using\` sPar x@ with @e \`sparking\` rwhnf x@.
sPar :: a -> Strategy b
sPar x y = x `par` ()
-{-
-sSeq is a strategy corresponding to seq. i.e. x `seq` e <=> e `using` sSeq x
--}
+-- | A strategy corresponding to 'seq':
+-- @x \`seq\` e@ = @e \`using\` sSeq x@.
+--
+-- 'sSeq' has been superceded by 'demanding'.
+-- Replace @e \`using\` sSeq x@ with @e \`demanding\` rwhnf x@.
sSeq :: a -> Strategy b
sSeq x y = x `seq` ()
-----------------------------------------------------------------------------
--- Basic Strategies
+-- * Basic Strategies
-----------------------------------------------------------------------------
--- r0 performs *no* evaluation on its argument.
+-- | Performs /no/ evaluation of its argument.
r0 :: Strategy a
r0 x = ()
---rwhnf reduces its argument to weak head normal form.
+-- | Reduces its argument to weak head normal form.
rwhnf :: Strategy a
rwhnf x = x `seq` ()
class NFData a where
- -- rnf reduces its argument to (head) normal form
+ -- | Reduces its argument to (head) normal form.
rnf :: Strategy a
-- Default method. Useful for base types. A specific method is necessay for
-- constructed types
class (NFData a, Ord a) => NFDataOrd a
------------------------------------------------------------------------------
--- Strategic Function Application
+-- * Strategic Function Application
------------------------------------------------------------------------------
{-
-The two infix functions @$|@ and @$||@ perform sequential and parallel
-function application, respectively. They are parameterised with a strategy
-that is applied to the argument of the function application. This is very
-handy when writing pipeline parallelism as a sequence of @$@, @$|@ and
-@$||@'s. There is no need of naming intermediate values in this case. The
-separation of algorithm from strategy is achieved by allowing strategies
+These are very
+handy when writing pipeline parallelism asa sequence of @$@, @$|@ and
+@$||@'s. There is no need of naming intermediate values in this case. The
+separation of algorithm from strategy is achieved by allowing strategies
only as second arguments to @$|@ and @$||@.
-}
-($|), ($||) :: (a -> b) -> Strategy a -> a -> b
-
+-- | Sequential function application. The argument is evaluated using
+-- the given strategy before it is given to the function.
+($|) :: (a -> b) -> Strategy a -> a -> b
f $| s = \ x -> f x `demanding` s x
-f $|| s = \ x -> f x `sparking` s x
-
-{-
-The same thing for function composition (.| and .||) and inverse function
-composition (-| and -||) for those who read their programs from left to
-right.
--}
-(.|), (.||) :: (b -> c) -> Strategy b -> (a -> b) -> (a -> c)
-(-|), (-||) :: (a -> b) -> Strategy b -> (b -> c) -> (a -> c)
+-- | Parallel function application. The argument is evaluated using
+-- the given strategy, in parallel with the function application.
+($||) :: (a -> b) -> Strategy a -> a -> b
+f $|| s = \ x -> f x `sparking` s x
+-- | Sequential function composition. The result of
+-- the second function is evaluated using the given strategy,
+-- and then given to the first function.
+(.|) :: (b -> c) -> Strategy b -> (a -> b) -> (a -> c)
(.|) f s g = \ x -> let gx = g x
in f gx `demanding` s gx
+
+-- | Parallel function composition. The result of the second
+-- function is evaluated using the given strategy,
+-- in parallel with the application of the first function.
+(.||) :: (b -> c) -> Strategy b -> (a -> b) -> (a -> c)
(.||) f s g = \ x -> let gx = g x
in f gx `sparking` s gx
+-- | Sequential inverse function composition,
+-- for those who read their programs from left to right.
+-- The result of the first function is evaluated using the
+-- given strategy, and then given to the second function.
+(-|) :: (a -> b) -> Strategy b -> (b -> c) -> (a -> c)
(-|) f s g = \ x -> let fx = f x
in g fx `demanding` s fx
+
+-- | Parallel inverse function composition,
+-- for those who read their programs from left to right.
+-- The result of the first function is evaluated using the
+-- given strategy, in parallel with the application of the
+-- second function.
+(-||) :: (a -> b) -> Strategy b -> (b -> c) -> (a -> c)
(-||) f s g = \ x -> let fx = f x
in g fx `sparking` s fx
-----------------------------------------------------------------------------
-----------------------------------------------------------------------------
--- Tuples
+-- * Tuples
-----------------------------------------------------------------------------
{-
rnf x8 `seq`
rnf x9
-
+-- | Apply two strategies to the elements of a pair sequentially
+-- from left to right.
seqPair :: Strategy a -> Strategy b -> Strategy (a,b)
seqPair strata stratb (x,y) = strata x `seq` stratb y
+-- | Apply two strategies to the elements of a pair in parallel.
parPair :: Strategy a -> Strategy b -> Strategy (a,b)
parPair strata stratb (x,y) = strata x `par` stratb y `par` ()
+-- The reason for the last 'par' is so that the strategy terminates
+-- quickly. This is important if the strategy is used as the 1st
+-- argument of a seq
-{-
-The reason for the second `par` is so that the strategy terminates
-quickly. This is important if the strategy is used as the 1st argument of a seq
--}
-
+-- | Apply three strategies to the elements of a triple in sequentially
+-- from left to right.
seqTriple :: Strategy a -> Strategy b -> Strategy c -> Strategy (a,b,c)
seqTriple strata stratb stratc p@(x,y,z) =
strata x `seq`
stratb y `seq`
stratc z
+-- | Apply three strategies to the elements of a triple in parallel.
parTriple :: Strategy a -> Strategy b -> Strategy c -> Strategy (a,b,c)
parTriple strata stratb stratc (x,y,z) =
strata x `par`
instance NFData ()
-----------------------------------------------------------------------------
--- Lists
+-- Lists
----------------------------------------------------------------------------
instance NFData a => NFData [a] where
rnf (x:xs) = rnf x `seq` rnf xs
----------------------------------------------------------------------------
--- Lists: Parallel Strategies
+-- * Lists: Parallel Strategies
----------------------------------------------------------------------------
--- | Applies a strategy to every element of a list in parallel
+-- | Applies a strategy to every element of a list in parallel.
parList :: Strategy a -> Strategy [a]
parList strat [] = ()
parList strat (x:xs) = strat x `par` (parList strat xs)
--- | Applies a strategy to the first n elements of a list in parallel
+-- | Applies a strategy to the first @n@ elements of a list in parallel.
parListN :: (Integral b) => b -> Strategy a -> Strategy [a]
parListN n strat [] = ()
parListN 0 strat xs = ()
parListN n strat (x:xs) = strat x `par` (parListN (n-1) strat xs)
--- | Evaluates N elements of the spine of the argument list and applies
--- the given strategy to the Nth element (if there is one) in parallel with
--- the result. e.g. parListNth 2 [e1, e2, e3] evaluates e2
+-- | Evaluates @n@ elements of the spine of the argument list and applies
+-- the given strategy to the @n@th element (if there is one) in parallel with
+-- the result. E.g. @parListNth 2 [e1, e2, e3]@ evaluates @e3@.
parListNth :: Int -> Strategy a -> Strategy [a]
parListNth n strat xs
| null rest = ()
where
rest = drop n xs
--- | 'parListChunk' sequentially applies a strategy to chunks
--- (sub-sequences) of a list in parallel. Useful to increase grain size
+-- | Sequentially applies a strategy to chunks
+-- (sub-sequences) of a list in parallel. Useful to increase grain size.
parListChunk :: Int -> Strategy a -> Strategy [a]
parListChunk n strat [] = ()
parListChunk n strat xs = seqListN n strat xs `par`
parListChunk n strat (drop n xs)
--- | 'parMap' applies a function to each element of the argument list in
--- parallel. The result of the function is evaluated using the given
--- strategy.
+-- | Applies a function to each element of a list and
+-- and evaluates the result list in parallel,
+-- using the given strategy for each element.
parMap :: Strategy b -> (a -> b) -> [a] -> [b]
-parMap strat f xs = map f xs `using` parList strat
+parMap strat f xs = map f xs `using` parList strat
--- | 'parFlatMap' uses 'parMap' to apply a list-valued function to each
--- element of the argument list in parallel. The result of the function
--- is evaluated using the given strategy.
+-- | Uses 'parMap' to apply a list-valued function to each
+-- element of a list in parallel, and concatenates the results.
parFlatMap :: Strategy [b] -> (a -> [b]) -> [a] -> [b]
parFlatMap strat f xs = concat (parMap strat f xs)
--- | 'parZipWith' zips together two lists with a function z in parallel
+-- | Zips together two lists using a function,
+-- and evaluates the result list in parallel.
parZipWith :: Strategy c -> (a -> b -> c) -> [a] -> [b] -> [c]
parZipWith strat z as bs =
zipWith z as bs `using` parList strat
----------------------------------------------------------------------------
--- Lists: Sequential Strategies
+-- * Lists: Sequential Strategies
----------------------------------------------------------------------------
--- | Sequentially applies a strategy to each element of a list
+-- | Sequentially applies a strategy to each element of a list.
seqList :: Strategy a -> Strategy [a]
seqList strat [] = ()
seqList strat (x:xs) = strat x `seq` (seqList strat xs)
--- | Sequentially applies a strategy to the first n elements of a list
+-- | Sequentially applies a strategy to the first n elements of a list.
seqListN :: (Integral a) => a -> Strategy b -> Strategy [b]
seqListN n strat [] = ()
seqListN 0 strat xs = ()
seqListN n strat (x:xs) = strat x `seq` (seqListN (n-1) strat xs)
--- | 'seqListNth' applies a strategy to the Nth element of it's argument
--- (if there is one) before returning the result. e.g. seqListNth 2 [e1,
--- e2, e3] evaluates e2
+-- | Applies a strategy to the @n@th element of a list
+-- (if there is one) before returning the result.
+-- E.g. @seqListNth 2 [e1, e2, e3]@ evaluates @e3@.
seqListNth :: Int -> Strategy b -> Strategy [b]
seqListNth n strat xs
| null rest = ()
-}
------------------------------------------------------------------------------
--- Arrays
+-- * Arrays
------------------------------------------------------------------------------
instance (Ix a, NFData a, NFData b) => NFData (Array a b) where
rnf x = rnf (bounds x) `seq` seqList rnf (elems x) `seq` ()
--- | Apply a strategy to all elements of an array in parallel. This can be done
--- either in sequentially or in parallel (same as with lists, really).
+-- | Apply a strategy to all elements of an array sequentially.
seqArr :: (Ix b) => Strategy a -> Strategy (Array b a)
seqArr s arr = seqList s (elems arr)
+-- | Apply a strategy to all elements of an array in parallel.
parArr :: (Ix b) => Strategy a -> Strategy (Array b a)
parArr s arr = parList s (elems arr)
--- Associations maybe useful even without mentioning Arrays.
-
+-- | Associations maybe useful even without mentioning Arrays.
data Assoc a b = a := b deriving ()
instance (NFData a, NFData b) => NFData (Assoc a b) where
rnf (x := y) = rnf x `seq` rnf y `seq` ()
------------------------------------------------------------------------------
--- Some strategies specific for Lolita
+-- * Some strategies specific for Lolita
------------------------------------------------------------------------------
fstPairFstList :: (NFData a) => Strategy [(a,b)]
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