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https://github.com/ekmett/speculation

Safe, programmable, speculative evaluation for Haskell
https://github.com/ekmett/speculation

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Safe, programmable, speculative evaluation for Haskell

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README 

        
speculation

===========



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



*  Prakash Prabhu, G. Ramalingam, and Kapil Vaswani, "*Safe Programmable Speculative Parallelism*",

   In the proceedings of Programming Language Design and Implementation (PLDI) Vol 45, Issue 6 (June 2010) pp 50-61.

   



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 .



-Edward Kmett