monad-par-0.3.4.7: A library for parallel programming based on a monad

Safe Haskell | None
Language | Haskell98

Control.Monad.Par

Contents

- The Par Monad
- Communication: IVars
- Operations

Description

The `monad-par` package provides a family of `Par` monads, for speeding up pure computations using parallel processors. (for a similar programming model for use with `IO`, see Control.Monad.Par.IO.)

The result of a given `Par` computation is always the same - i.e. it is deterministic, but the computation may be performed more quickly if there are processors available to share the work.

For example, the following program fragment computes the values of `(f x)` and `(g x)` in parallel, and returns a pair of their results:

     runPar $ do
         fx <- spawnP (f x)  -- start evaluating (f x)
         gx <- spawnP (g x)  -- start evaluating (g x)
         a  <- get fx        -- wait for fx
         b  <- get gx        -- wait for gx
         return (a,b)        -- return results

`Par` can be used for specifying pure parallel computations in which the order of the computation is not known beforehand. The programmer specifies how information flows from one part of the computation to another, but not the order in which computations will be evaluated at runtime. Information flow is described using "variables" called `IVar`s, which support `put` and `get` operations. For example, suppose you have a problem that can be expressed as a network with four nodes, where `b` and `c` require the value of `a`, and `d` requires the value of `b` and `c`:

                          a
                         / \
                        b   c
                         \ /
                          d

Then you could express this in the `Par` monad like this:

      runPar $ do
          [a,b,c,d] <- sequence [new,new,new,new]
          fork $ do x <- get a; put b (x+1)
          fork $ do x <- get a; put c (x+2)
          fork $ do x <- get b; y <- get c; put d (x+y)
          fork $ do put a (3 :: Int)
          get d

The result of the above computation is always 9. The `get` operation waits until its input is available; multiple `put`s to the same `IVar` are not allowed, and result in a runtime error. Values stored in `IVar`s are usually fully evaluated (although there are ways provided to pass lazy values if necessary).

In the above example, `b` and `c` will be evaluated in parallel. In practice the work involved at each node is too small here to see the benefits of parallelism though: typically each node should involve much more work. The granularity is completely under your control - too small and the overhead of the `Par` monad will outweigh any parallelism benefits, whereas if the nodes are too large then there might not be enough parallelism to use all the available processors.

Unlike `Control.Parallel`, in `Control.Monad.Par` parallelism is not combined with laziness, so sharing and granulairty are completely under the control of the programmer. New units of parallel work are only created by `fork` and a few other combinators.

The default implementation is based on a work-stealing scheduler that divides the work as evenly as possible between the available processors at runtime. Other schedulers are available that are based on different policies and have different performance characteristics. To use one of these other schedulers, just import its module instead of Control.Monad.Par:

- Control.Monad.Par.Scheds.Trace
- Control.Monad.Par.Scheds.Sparks

For more information on the programming model, please see these sources:

- The wiki/tutorial (http://www.haskell.org/haskellwiki/Par_Monad:_A_Parallelism_Tutorial)
- The original paper (http://www.cs.indiana.edu/~rrnewton/papers/haskell2011_monad-par.pdf)
- Tutorial slides (http://community.haskell.org/~simonmar/slides/CUFP.pdf)
- Other slides: (http://www.cs.ox.ac.uk/ralf.hinze/WG2.8/28/slides/simon.pdf, http://www.cs.indiana.edu/~rrnewton/talks/2011_HaskellSymposium_ParMonad.pdf)

Synopsis

- data Par a
- runPar :: Par a -> a
- runParIO :: Par a -> IO a
- fork :: Par () -> Par ()
- data IVar a
- new :: Par (IVar a)
- newFull :: NFData a => a -> Par (IVar a)
- newFull_ :: a -> Par (IVar a)
- get :: IVar a -> Par a
- put :: NFData a => IVar a -> a -> Par ()
- put_ :: IVar a -> a -> Par ()
- spawn :: NFData a => Par a -> Par (IVar a)
- spawn_ :: Par a -> Par (IVar a)
- spawnP :: NFData a => a -> Par (IVar a)
- module Control.Monad.Par.Combinator
- class NFData a

# The Par Monad

data Par a Source

Instances

Monad Par Source |
Functor Par Source |
Applicative Par Source |

runPar :: Par a -> a Source

runParIO :: Par a -> IO a Source

A version that avoids an internal `unsafePerformIO` for calling contexts that are already in the `IO` monad.

fork :: Par () -> Par () Source

forks a computation to happen in parallel. The forked computation may exchange values with other computations using `IVar`s.

# Communication: IVars

data IVar a Source

Instances

ParFuture IVar ParIO |
ParIVar IVar ParIO |
Eq (IVar a) Source | Equality for IVars is physical equality, as with other reference types.
NFData (IVar a) Source |

new :: Par (IVar a) Source

creates a new `IVar`

creates a new `IVar`

newFull :: NFData a => a -> Par (IVar a) Source

creates a new `IVar` that contains a value

creates a new `IVar` that contains a value

newFull_ :: a -> Par (IVar a) Source

creates a new `IVar` that contains a value (head-strict only)

creates a new `IVar` that contains a value (head-strict only)

get :: IVar a -> Par a Source

read the value in a `IVar`. The `get` can only return when the value has been written by a prior or parallel `put` to the same `IVar`.

read the value in an `IVar`. `get` can only return when the value has been written by a prior or parallel `put` to the same `IVar`.

put :: NFData a => IVar a -> a -> Par () Source

put a value into a `IVar`. Multiple `put`s to the same `IVar` are not allowed, and result in a runtime error.

`put` fully evaluates its argument, which therefore must be an instance of `NFData`. The idea is that this forces the work to happen when we expect it, rather than being passed to the consumer of the `IVar` and performed later, which often results in less parallelism than expected.

Sometimes partial strictness is more appropriate: see `put_`.

put a value into a `IVar`. Multiple `put`s to the same `IVar` are not allowed, and result in a runtime error.

`put` fully evaluates its argument, which therefore must be an instance of `NFData`. The idea is that this forces the work to happen when we expect it, rather than being passed to the consumer of the `IVar` and performed later, which often results in less parallelism than expected.

Sometimes partial strictness is more appropriate: see `put_`.

put_ :: IVar a -> a -> Par () Source

like `put`, but only head-strict rather than fully-strict.

like `put`, but only head-strict rather than fully-strict.

# Operations

spawn :: NFData a => Par a -> Par (IVar a) Source

Like `fork`, but returns a `IVar` that can be used to query the result of the forked computataion. Therefore `spawn` provides futures or promises.

     spawn p = do
       r <- new
       fork (p >>= put r)
       return r

spawn_ :: Par a -> Par (IVar a) Source

Like `spawn`, but the result is only head-strict, not fully-strict.

spawnP :: NFData a => a -> Par (IVar a) Source

Spawn a pure (rather than monadic) computation. Fully-strict.

     spawnP = spawn . return

module Control.Monad.Par.Combinator

This module also reexports the Combinator library for backwards compatibility with version 0.1.

class NFData a

A class of types that can be fully evaluated.

Since: 1.1.0.0

Instances

NFData Bool |
NFData Char |
NFData Double |
NFData Float |
NFData Int |
NFData Int8 |
NFData Int16 |
NFData Int32 |
NFData Int64 |
NFData Integer |
NFData Word |
NFData Word8 |
NFData Word16 |
NFData Word32 |
NFData Word64 |
NFData TypeRep | NOTE: Only defined for `base-4.8.0.0` and later Since: 1.4.0.0
NFData () |
NFData Void | Defined as ``rnf` = `absurd``. Since: 1.4.0.0
NFData Unique | Since: 1.4.0.0
NFData Natural | Since: 1.4.0.0
NFData Version | Since: 1.3.0.0
NFData ThreadId | Since: 1.4.0.0
NFData CChar | Since: 1.4.0.0
NFData CSChar | Since: 1.4.0.0
NFData CUChar | Since: 1.4.0.0
NFData CShort | Since: 1.4.0.0
NFData CUShort | Since: 1.4.0.0
NFData CInt | Since: 1.4.0.0
NFData CUInt | Since: 1.4.0.0
NFData CLong | Since: 1.4.0.0
NFData CULong | Since: 1.4.0.0
NFData CLLong | Since: 1.4.0.0
NFData CULLong | Since: 1.4.0.0
NFData CFloat | Since: 1.4.0.0
NFData CDouble | Since: 1.4.0.0
NFData CPtrdiff | Since: 1.4.0.0
NFData CSize | Since: 1.4.0.0
NFData CWchar | Since: 1.4.0.0
NFData CSigAtomic | Since: 1.4.0.0
NFData CClock | Since: 1.4.0.0
NFData CTime | Since: 1.4.0.0
NFData CUSeconds | Since: 1.4.0.0
NFData CSUSeconds | Since: 1.4.0.0
NFData CFile | Since: 1.4.0.0
NFData CFpos | Since: 1.4.0.0
NFData CJmpBuf | Since: 1.4.0.0
NFData CIntPtr | Since: 1.4.0.0
NFData CUIntPtr | Since: 1.4.0.0
NFData CIntMax | Since: 1.4.0.0
NFData CUIntMax | Since: 1.4.0.0
NFData All | Since: 1.4.0.0
NFData Any | Since: 1.4.0.0
NFData TyCon | NOTE: Only defined for `base-4.8.0.0` and later Since: 1.4.0.0
NFData Fingerprint | Since: 1.4.0.0
NFData a => NFData [a] |
(Integral a, NFData a) => NFData (Ratio a) |
NFData (StableName a) | Since: 1.4.0.0
NFData a => NFData (Identity a) | Since: 1.4.0.0
NFData (Fixed a) | Since: 1.3.0.0
NFData a => NFData (Complex a) |
NFData a => NFData (ZipList a) | Since: 1.4.0.0
NFData a => NFData (Dual a) | Since: 1.4.0.0
NFData a => NFData (Sum a) | Since: 1.4.0.0
NFData a => NFData (Product a) | Since: 1.4.0.0
NFData a => NFData (First a) | Since: 1.4.0.0
NFData a => NFData (Last a) | Since: 1.4.0.0
NFData a => NFData (Down a) | Since: 1.4.0.0
NFData a => NFData (Maybe a) |
NFData a => NFData (Digit a) |
NFData a => NFData (Node a) |
NFData a => NFData (Elem a) |
NFData a => NFData (FingerTree a) |
NFData a => NFData (Set a) |
NFData a => NFData (Seq a) |
NFData (Vector a) |
NFData (Vector a) |
NFData (IVar a) |
NFData (IVar a) |
NFData (a -> b) | This instance is for convenience and consistency with `seq`. This assumes that WHNF is equivalent to NF for functions. Since: 1.3.0.0
(NFData a, NFData b) => NFData (Either a b) |
(NFData a, NFData b) => NFData (a, b) |
(Ix a, NFData a, NFData b) => NFData (Array a b) |
NFData a => NFData (Const a b) | Since: 1.4.0.0
NFData (Proxy * a) | Since: 1.4.0.0
(NFData k, NFData a) => NFData (Map k a) |
NFData (MVector s a) |
NFData (MVector s a) |
(NFData a, NFData b, NFData c) => NFData (a, b, c) |
(NFData a, NFData b, NFData c, NFData d) => NFData (a, b, c, d) |
(NFData a1, NFData a2, NFData a3, NFData a4, NFData a5) => NFData (a1, a2, a3, a4, a5) |
(NFData a1, NFData a2, NFData a3, NFData a4, NFData a5, NFData a6) => NFData (a1, a2, a3, a4, a5, a6) |
(NFData a1, NFData a2, NFData a3, NFData a4, NFData a5, NFData a6, NFData a7) => NFData (a1, a2, a3, a4, a5, a6, a7) |
(NFData a1, NFData a2, NFData a3, NFData a4, NFData a5, NFData a6, NFData a7, NFData a8) => NFData (a1, a2, a3, a4, a5, a6, a7, a8) |
(NFData a1, NFData a2, NFData a3, NFData a4, NFData a5, NFData a6, NFData a7, NFData a8, NFData a9) => NFData (a1, a2, a3, a4, a5, a6, a7, a8, a9) |

(0.3) Reexport `NFData` for fully-strict operators.