gistfile1.hs

import Control.Applicative
import Control.Monad.ST.Safe
import qualified Data.Vector.Unboxed as IU
import qualified Data.Vector.Unboxed.Mutable as U

-- Mutable world
data World s = World
    { _souls     :: !(U.STVector s Int)
    , _positions :: !(U.STVector s (Float,Float))
    }

-- Immutable world
data IWorld = IWorld
    { _iSouls     :: !(IU.Vector Int)
    , _iPositions :: !(IU.Vector (Float,Float))
    } deriving (Show)

-- Thaw immutable world, apply update, freeze again.
worldST :: IWorld -> (World s -> ST s (World s)) -> ST s IWorld
worldST world f = do
    stWorld <- World 
        <$> IU.thaw (_iSouls world)
        <*> IU.thaw (_iPositions world)
    newWorld <- f stWorld
    IWorld 
        <$> IU.unsafeFreeze (_souls newWorld)
        <*> IU.unsafeFreeze (_positions newWorld)

main :: IO ()
main = do
    let w = IWorld (IU.replicate 100 0) (IU.replicate 100 (0,0))
    print $ runST (worldST w return)