speculation
===========
This package provides speculative evaluation primitives for Haskell, very loosely based on the paper
"Safe Programmable Speculative Parallelism" by Prabhu, Ramalingam, and Vaswani.
You can download it using 'cabal install speculation', if you have the Haskell Platform installed.
## Combinators
### Speculative Function Application
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
A speculative version of 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 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
-------------------
I can be reached through the user ekmett on github, as edwardk on irc.freenode.net #haskell channel, or by email to .
-Edward Kmett