Thanks again. Here is my solution in case that somebody else runs into similar problem. This isn't very elegant and I would be interested to know if somebody has a better solution. Surely many people have encountered this kind of problem where you want to force evaluation of some expression at given point of program flow. Here evaluation of sum must be forced for every update of array element or otherwise memory consumption is proportional to the number of samples (100 000) instead of being proportional to the size of the array (10).

---
import Data.Array.IO
import Test.QuickCheck
import System.Random

uniformSampler :: Gen Double
uniformSampler = choose (0,1)

withSeed sampler seed = generate 1 (mkStdGen seed) sampler

ac = 10
sc = 100000

triplet = do
    i <- uniformSampler
    s <- uniformSampler
    t <- uniformSampler
    return (round $ i * fromIntegral ac, (s,t))

sampling = sequence $ repeat triplet
samples = take sc $ withSeed sampling 1

showElems xs = foldr1 (++) [show x ++ "\n" | x <- xs]

main = do
    a1 <- newArray (0,ac) 0 :: IO (IOUArray Int Double)
    a2 <- newArray (0,ac) 0 :: IO (IOUArray Int Double)
    let addtoElem i s t = do
            s' <- readArray a1 i
            writeArray a1 i (s'+s)
            t' <- readArray a2 i
            writeArray a2 i (t'+t)
        writes = [addtoElem i s t | (i,(s,t)) <- samples]
    sequence writes
    ss <- getElems a1
    ts <- getElems a2
    putStrLn $ showElems (zip ss ts)
---

Regards,
Lauri

Yes, of course. How blind of me. Here is one more question.
If you change IOUArray to IOArray and add $! in front of the two summations in the previous code, it still works correctly.
But can you do similar trick with Array and accumArray? I have tried to put $! in different places in the first code that I posted but nothing seems to work.