[Haskell-cafe] Sparks created, but not used (par/pseq, ST monad)

4 Feb 2010

      Hello haskell-cafe,

I currently have a problem with running par/pseq in the ST monad. The function
testST is the minimal counterexample that works -- or, to be more clear, does
not work as expected for me. As a remark, the tasks/function calls in my "real
application" are much more computational expensive, but the code is too long
to post here.
...
-- File ST.lhs
module Main where
import Control.Monad.ST
import Control.Parallel
main = testST
testST :: IO ()
testST = do
   putStrLn "Starting"
   (runST $ f 10) `pseq` putStrLn "Stopping"
 where
       f :: forall s. Int -> ST s ()
       f n = do
           p $ "\nTask:" ++ show n
           if n < 0
               then return ()
               else do
                   (n1,n2) <- return (n-1,n-2)
                   q n1
                   q n2
                   a <- f n1
                   b <- f n2
                   (a `par` b) `pseq` return a
-- Some helper functions
p x = unsafeIOToST (putStrLn x)
q x = unsafeIOToST (print x)
As far as I understand, compiling with

    ghc --make -O2 -threaded ST.lhs -o st -XRankNTypes

and running with

    ./st +RTS -N -s

should create sparks that could be run in parallel. The problem I have now, is
that sparks are created but not converted, according to -s output:

    ... a lot of (uninteresting) stuff
    SPARKS: 232 (0 converted, 0 pruned)
    ...

I have not found any information on this type of behaviour on the net and would
be glad if someone could give me points or hints what's happening and how I can
improve this.

Cheers,
  Michael

-- 
Dipl.-Inf. Michael C. Lesniak
University of Kassel
Programming Languages / Methodologies Research Group
Department of Computer Science and Electrical Engineering

Wilhelmshöher Allee 73
34121 Kassel

Phone: +49-(0)561-804-6269

[Haskell-cafe] Sparks created, but not used (par/pseq, ST monad)

Michael Lesniak