# The speculation package

[ Tags: bsd3, concurrency, library ] [ Propose Tags ]

A framework for safe, programmable, speculative parallelism, loosely based on:

This package provides speculative function application and speculative folds. Speculative STM transactions take the place of the transactional rollback machinery from the paper.

For example:

spec g f a evaluates f g while forcing a, if g == a then f g is returned, otherwise f a is evaluated and returned. Furthermore, if the argument has already been evaluated, we skip the f g computation entirely. If a good guess at the value of a is available, this is one way to induce parallelism in an otherwise sequential task. However, if the guess isn't available more cheaply than the actual answer, then this saves no work and if the guess is wrong, you risk evaluating the function twice.

The best-case timeline looks like:

[---- f g ----]
[----- a -----]
[-- spec g f a --]

The worst-case timeline looks like:

[---- f g ----]
[----- a -----]
[---- f a ----]
[------- spec g f a -----------]

Compare these to the timeline of f $! a: [---- a -----] [---- f a ----] specSTM provides a similar time table for STM actions, but also rolls back side-effects. Changes in 0.6.0: • Upgraded the comparisons used by the STM combinators STM actions, so they can check for other STM side-effects. Changes in 0.5.1: • Exposed unsafeGetTagBits and unsafeIsEvaluated [Skip to Readme] ## Properties Versions 0.0.0, 0.0.1, 0.0.2, 0.1.0, 0.2.0, 0.3.0, 0.4.0, 0.5.0, 0.5.1, 0.6.0, 0.7.0, 0.8.0, 0.8.0.1, 0.8.0.2, 0.8.1.0, 0.8.2.0, 0.9.0.0, 1.0.0.0, 1.1.0.0, 1.2.0.0, 1.2.0.1, 1.2.0.2, 1.3, 1.4, 1.4.1, 1.4.1.1, 1.4.1.2, 1.5, 1.5.0.1, 1.5.0.2, 1.5.0.3 base (>=4 && <6), parallel (==2.2.*), stm (==2.1.*) [details] BSD3 (c) 2010 Edward A. Kmett Edward A. Kmett Edward A. Kmett Concurrency http://github.com/ekmett/speculation Sun Jun 27 22:58:05 UTC 2010 by EdwardKmett LTSHaskell:1.5.0.3, NixOS:1.5.0.3, Stackage:1.5.0.3, openSUSE:1.5.0.3 10425 total (83 in the last 30 days) 2.0 (votes: 1) [estimated by rule of succession] λ λ λ Docs uploaded by userBuild status unknown Hackage Matrix CI ## Modules [Index] ## Downloads #### Maintainer's Corner For package maintainers and hackage trustees ## Readme for speculation-0.6.0 [back to package description] # speculation A framework for safe, programmable, speculative parallelism, loosely based on: This package provides speculative function application and speculative folds. Speculative STM transactions take the place of the transactional rollback machinery from the paper. You can download it using cabal install speculation, if you have the Haskell Platform installed. ## Speculative Function Application (Control.Concurrent.Speculation) Various speculative function application combinators are provided. Two fairly canonical samples are described here. #### spec spec :: Eq a => a -> (a -> b) -> a -> b  spec g f a evaluates f g while forcing a, if g == a then f g is returned. Otherwise f a is evaluated. Furthermore, if the argument has already been evaluated, we avoid sparking the parallel computation at all. If g is a good guess at the value of a, this is one way to induce parallelism in an otherwise sequential task. However, if g isn't available more cheaply than a, then this saves no work, and if g is wrong, you risk evaluating the function twice. spec a f a = f$! a

The best-case timeline looks like: [---- f g ----] [----- a -----] [-- spec g f a --]

The worst-case timeline looks like: [---- f g ----] [----- a -----] [---- f a ----] [------- spec g f a -----------]

Compare these to the timeline of @f \$! a@: [---- a -----] [---- f a ----]

#### specSTM

specSTM provides a similar compressed timeline for speculated STM actions, but also rolls back side-effects.

## Speculative Folds (Data.Foldable.Speculation)

A speculative version of the combinators from Data.Foldable is provided as Data.Foldable.Speculation.

Each combinator therein takes an extra argument that is used to speculate on the value of the list.

#### foldr

foldr :: (Foldable f, Eq b) => (Int -> b) -> (a -> b -> b) -> b -> f a -> b


Given a valid estimator g, foldr g f z xs yields the same answer as Foldable.foldr' f z xs.

g n should supply an estimate of the value returned from folding over the last n elements of the container.

As with spec, if the guess g n is accurate a reasonable percentage of the time and faster to compute than the ensuing fold, then this can provide increased opportunities for parallelism.

#### foldl

foldl :: (Foldable f, Eq b) => (Int -> b) -> (b -> a -> b) -> b -> f a -> b


foldl works similarly to Foldable.foldl', except that g n should provide an estimate for the first n elements.

## Contact Information

Contributions and bug reports are welcome!

I can be reached through the user ekmett on github, as edwardk on irc.freenode.net #haskell channel, or by email at ekmett@gmail.com.

-Edward Kmett