[Haskell-beginners] Changing evalList to parList creates a space leak?

[Travis Erdman]

Ok, since I last visited, as a result of the help from contributors on this list, I've reached a nice spot.  The serial implementation of my processing algorithm works quite quickly (including the tree pruning that I mentioned last time) and the space leaks have been eliminated without resort to the dreadful foldl'rnf.

Here are simplified versions of key components of my program:

data Tree a = Node a [Tree a]

calcTree :: Tree MyData -> InputData -> Tree MyData
calcTree (Node cargo subtrees) input = Node (process cargo input) recurseChildren
  where
    process a b = ....
    recurseChildren = map (\t -> calcTree t input) subtrees `using` evalList rseq

newTree = foldl' calcTree starttree (take n inputdatalist)


The <`using` evalList rseq> term at the end of recurseChildren was key to solving a space leak without resorting to foldl'rnf (which is a performance killer).  This is also the place in my code where it makes the most sense to introduce some parallelism (ie to traverse the subtrees in parallel).

However, when I simply change the evalList to a parList, a space leak is created anew, which I didn't anticipate at all.  (of course, I also made the change of compiling with -threaded and -feager-blackholing flags ... but if I compile with those flags and leave it as evalList, there is no space leak).

I'm using parallel-3.1.0.0, so it's not the space leak from parallel-1.1.x.  

Any ideas why this simple change should introduce a space leak?  How might I get to where I want to go?

thanks again,

Travis