# The speculation package

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

A framework for safe, programmable, speculative parallelism, loosely based on http://research.microsoft.com/pubs/118795/pldi026-vaswani.pdf

This package provides speculative function application and speculative folds. And speculative STM actions 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 $! a: [---- a -----] [---- f a ----] specSTM provides a similar time table for STM actions, but also rolls back side-effects. Changes in 0.3.0: • Speculative folds moved to Data.Foldable.Speculation and expanded to cover all of the Data.Foldable combinators. • specBy and specOn variants added. [Skip to Readme] ## Properties Versions 0.0.0, 0.0.1, 0.0.2, 0.1.0, 0.2.0, 0.3.0, 0.4.0, 0.5.0, 0.5.1, 0.6.0, 0.7.0, 0.8.0, 0.8.0.1, 0.8.0.2, 0.8.1.0, 0.8.2.0, 0.9.0.0, 1.0.0.0, 1.1.0.0, 1.2.0.0, 1.2.0.1, 1.2.0.2, 1.3, 1.4, 1.4.1, 1.4.1.1, 1.4.1.2, 1.5, 1.5.0.1, 1.5.0.2, 1.5.0.3 base (>=4 && <6), parallel (==2.2.*), stm (==2.1.*) [details] BSD3 (c) 2010 Edward A. Kmett Edward A. Kmett Edward A. Kmett Concurrency http://github.com/ekmett/speculation Sun Jun 27 15:04:52 UTC 2010 by EdwardKmett LTSHaskell:1.5.0.3, NixOS:1.5.0.3, Stackage:1.5.0.3, Tumbleweed:1.5.0.3 6795 total (47 in the last 30 days) 2.0 (1 ratings) [clear rating] λ λ λ Docs uploaded by userBuild status unknown Hackage Matrix CI ## Modules [Index] ## Downloads #### Maintainer's Corner For package maintainers and hackage trustees ## Readme for speculation-0.3.0 [back to package description] # speculation This package provides speculative evaluation primitives for Haskell, very loosely based on the paper "Safe Programmable Speculative Parallelism" by Prabhu, Ramalingam, and Vaswani. http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.19.4622 ## Combinators ### speculative function application #### spec spec :: Eq a => a -> (a -> b) -> a -> b  spec g f a evaluates f g while forcing a, if g == a then f g is returned. Otherwise f a is evaluated. Furthermore, if the argument has already been evaluated, we avoid sparking the parallel computation at all. If g is a good guess at the value of a, this is one way to induce parallelism in an otherwise sequential task. However, if g isn't available more cheaply than a, then this saves no work, and if g is wrong, you risk evaluating the function twice. spec a f a = f$! 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.

### folds

A number of speculative folds are also provided.

These take an extra argument which is a function that guesses the result of of the fold up to a given point.

#### specFoldr

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


Given a valid estimator g, 'specFoldr g f z xs yields the same answer as 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 fold, then this can provide increased opportunities for parallelism.

#### specFoldl

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


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