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 = 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 ----]

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

Unlike spec, this version does not check to see if the argument has already been evaluated. This can save a small amount of work when you know the argument will always require computation.

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

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.

If 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.

 specFoldr = specFoldrN 0

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

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

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

If 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 = specFoldlN 0

class Foldable f => Speculative f where

Methods

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

specFoldr1 is to foldr1' as specFoldr is to foldr'

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

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

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

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

specFoldl1 is to foldl1' as specFoldl is to foldl'

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

Given a valid estimator g, specFoldlN n g f z xs yields the same answer as foldl f z xs.

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

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

Transform an arbitrary Foldable into a Speculative container

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

Provides a Speculative container that doesn't actually speculate.

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)