Control.Concurrent.Speculation

Documentation

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 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.

 spec a f a = a `seq` 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 -----------]

Compared to the unspeculated timeline of

 [---- a -----]
              [---- f a ----]

spec' :: Eq a => a -> (a -> b) -> a -> b

spec' g f a evaluates a function f using a cheap guess g at the argument in parallel with forcing the argument.

This is one way to induce parallelism in an otherwise sequential task. Unlike spec this version does not check to see if the argument has already been evaluated before evaluating the speculated version. This is useful when you know evaluated will always return False.

The following identity holds:

 spec' a f a = a `seq` f a

evaluated :: a -> Bool

Returns a guess as to whether or not a value has been evaluated. This is an impure function that relies on GHC internals and will return false negatives, but (hopefully) no false positives.

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

Compute a right biased fold. The estimator function provides a guess at the value of the suffix

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

Compute a left-biased fold. The estimator function provides a guess at the value of the prefix

class Foldable f => Speculative f where

Methods

specFoldr1 :: Eq a => (Int -> a) -> (a -> a -> a) -> f a -> a

Compute a right-biased fold. The estimator function is a guess at the value of the prefix

specFoldrN :: Eq b => Int -> (Int -> b) -> (a -> b -> b) -> b -> f a -> b

specFoldl1 :: Eq a => (Int -> a) -> (a -> a -> a) -> f a -> a

Compute a left biased fold. the estimator function is a guess at the value of the prefix

specFoldlN :: Eq b => Int -> (Int -> b) -> (b -> a -> b) -> b -> f a -> b

Instances

Speculative []

Speculative Maybe

Speculative IntMap

Speculative Set

Speculative Seq

Ix i => Speculative (Array i)

Speculative (Map a)

Foldable f => Speculative (WithoutSpeculation f)

Foldable f => Speculative (WrappedFoldable f)

newtype WrappedFoldable f a

Constructors

WrappedFoldable

Fields

getWrappedFoldable :: f a

Instances

Functor f => Functor (WrappedFoldable f)

Typeable1 f => Typeable1 (WrappedFoldable f)

Foldable f => Foldable (WrappedFoldable f)

Traversable f => Traversable (WrappedFoldable f)

Foldable f => Speculative (WrappedFoldable f)

(Typeable1 f, Data (f a), Data a) => Data (WrappedFoldable f a)

newtype WithoutSpeculation f a

Constructors

WithoutSpeculation

Fields

getWithoutSpeculation :: f a

Instances

Functor f => Functor (WithoutSpeculation f)

Typeable1 f => Typeable1 (WithoutSpeculation f)

Foldable f => Foldable (WithoutSpeculation f)

Traversable f => Traversable (WithoutSpeculation f)

Foldable f => Speculative (WithoutSpeculation f)

(Typeable1 f, Data (f a), Data a) => Data (WithoutSpeculation f a)