Parallel strategy applied to Array

parr.hs

import qualified Data.Array as A
import Control.Parallel.Strategies

ack 0 n = n+1
ack m 0 = ack (m-1) 1
ack m n = ack (m-1) (ack m (n-1))

main = do
  let arr = A.listArray (0,999) [1..] :: A.Array Int Int
  let res = (fmap (ack 2) arr) `using` parTraversable rdeepseq
  print res

{-
Speed-up ≈ 1.9. Works also with Data.Array.Unboxed.

$ ghc --make -threaded -O2 parr.hs 
[1 of 1] Compiling Main             ( parr.hs, parr.o )
Linking parr ...

$ time ./parr +RTS -N1 >/dev/null

real	0m7.058s
user	0m6.870s
sys	0m0.030s

$ time ./parr +RTS -N2 >/dev/null

real	0m3.649s
user	0m7.140s
sys	0m0.000s

-}