Control.Concurrent.Speculation

Speculative application

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.

specBy :: (a -> a -> Bool) -> a -> (a -> b) -> a -> b

spec with a user defined comparison function

specBy' :: (a -> a -> Bool) -> a -> (a -> b) -> a -> b

spec' with a user defined comparison function

specOn :: Eq c => (a -> c) -> a -> (a -> b) -> a -> b

spec comparing by projection onto another type

specOn' :: Eq c => (a -> c) -> a -> (a -> b) -> a -> b

spec' comparing by projection onto another type

Detecting closure evaluation

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.