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 :: 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 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 :: (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 :: (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



  • Support GHC 8
  • Support transformers 0.5


  • Build warning-free on GHC 7.10+
  • Added an HLint configuration
  • Removed a redundant constraint from the type of sequenceByA_.
  • Removed a redundant Monad m constraint from instance MonadSpec (ContT r m).


  • Removed the use of tag-bits. This enables the API to be Trustworthy.


  • Simplified MonadSpec


  • Removed old benchmark/test framework.
  • Make numSparks support mandatory.
  • Moved to Control.Concurrent from Data
  • Added MonadSpec, so we can make instances for Codensity, etc. in other packages
  • Removed the ContT r STM combinators
  • Cleaned out old issues
  • Made compatible with the removal of the Eq/Show superclasses of Num in GHC 7.3+


  • Fixed name collision with the new Distribution.Simple.testHook in Setup.lhs


  • Weakened dependencies


  • Reorganized the module hierarchy into Data.Speculation


  • Added support for numSparks


  • Released


  • Removed interim boxing in the unsafeIsEvaluated and unsafeGetTagBits check


  • Added Data.List.Foldable
  • Added Data.Traversable.Foldable
  • Fixed an off-by-one error in the arguments to the speculative fold estimators


  • changed tests and benchmarks to not build by default to work around corruption in the hackage db


  • test suite now forces build
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