Felipe Almeida Lessa wrote:

(Sorry if this is a newbie question, couldn't find the answer anywhere)

Suppose I have an expensive function (such that even to be reduced to WHNF it takes a long processing time)

...

expensive :: Foo -> Maybe Bar

and I want to calculate it on multiple processors, like in

...

calculate = parMap rnf expensive

Well, fine. But suppose that then I give the result to a function like

...

final xs = reverse $ final' xs [] where final' [] r = r final' (x:xs) r = case x of Nothing -> [] Just x -> final' xs (x:r)

Given list :: [Foo], a sequential calculation as in

...

resultS = final $ map expensive list

would not call expensive after finding a Nothing. But

...

resultP = final $ calculate list

will keep calculating all the other expensive's because they were sparked?

Right. This is an example of speculation: you don't know in advance how much work you really have to do, so any work you do in parallel may or may not be necessary. If we fire off the whole list in parallel, we might end up doing a lot of work on the elements at the end of the list, which are less likely to be required than the elements at the front of the list. That's a bad strategy: we need to prioritise the elements near the front. Fortunately GHC's built-in strategy for picking sparks to evaluate picks the oldest ones first, so this shouldn't be a problem. The problem occurs when you've found the Nothing, but the rest of the list has already been sparked. You really want to throw away all those sparks, but there's no way to do that currently. One way you could improve the situation though is to only spark the next N elements in the list, limiting the amount of speculation. Hope this helps... Cheers, Simon