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

[Michael Lesniak]

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