How to do parallel "either-side" short-circuiting with "and" and "or"

Question

Does haskell have a parallel "and" method ``` parAnd :: Bool -> Bool -> Bool ``` such that ``` (a `parAnd` b) ``` will spark the evaluation of a and b in parallel and return false as soon as either a or b evaluates to false (and not wait for the other one)? Is there some way to implement such a thing?

Accepted Answer

Normally, this is not possible. You can do something like

```
a `par` b `pseq` (a && b)
```

but if `b` evaluates to `False`, `a` is still fully evaluated.

However, this is possible with the unambiguous choice operator created by Conal Elliott for his Functional Reactive Programming (FRP) implementation. It's available on Hackage as unamb package and does exactly what you want. In particular, it contains

```
-- | Turn a binary commutative operation into one that tries both orders in
-- parallel. Useful when there are special cases that don't require
-- evaluating both arguments.
-- ...
parCommute :: (a -> a -> b) -> a -> a -> b
```

and also directly defines `pand`,`por` and other similar commutative functions, such that

```
pand undefined False   -> False
pand False undefined   -> False
```

How to do parallel "either-side" short-circuiting with "and" and "or"

Solution

Problem