# speculation

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

```
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 [email protected].

-Edward Kmett