I'm not exactly sure why you get the error, but the easiest way to fix it is just to type it this way: runSimulation :: Word32 -> [Int] runSimulation seed = runST $ do gen <- initialize (singleton seed) whileM (do r1 <- uniformR (-1.0, 1.0 :: Double) gen if r1 > 0.0 then return True else return False) (do r2 <- uniformR (0, 10 :: Int) gen if r2 > 5 then return r2 else return 0) It has something to do with the forall s in runST, although I'm not completely sure what. On Sun, Aug 18, 2013 at 9:18 PM, Aurimas wrote:

I have the following code which does not compile due to explicit type annotation (ST s Double). Error message says usual thing about "s" type variables.

------------------------------**------------------------------** ------------------------------**---------------- import Control.Monad.ST import System.Random.MWC (initialize, uniformR, Gen) import Control.Monad.Loops (whileM) import Data.Vector (singleton) import Data.Word(Word32)

main :: IO () main = do print $ runSimulation 1

runSimulation :: Word32 -> [Int] runSimulation seed = runST $ do gen <- initialize (singleton seed) whileM (do r1 <- uniformR (-1.0, 1.0) gen :: ST s Double -- does not compile due to this if r1 > 0.0 then return True else return False) (do r2 <- uniformR (0, 10) gen if r2 > 5 then return r2 else return 0) ------------------------------**------------------------------** ------------------------------**---------------

if I rewrite runSimulation like this (below), everything is OK.

------------------------------**------------------------------** ------------------------------**--------------- runSimulation :: Word32 -> [Int] runSimulation seed = runST $ do gen <- initialize (singleton seed) whileM (do r1 <- tempFun gen if r1 > 0.0 then return True else return False) (do r2 <- uniformR (0, 10) gen if r2 > 5 then return r2 else return 0) where tempFun :: Gen s -> ST s Double -- this line automatically provides required type annotation tempFun g = uniformR (-1.0, 1.0) g ------------------------------**------------------------------** ------------------------------**---------------

Ca somebody explain what's wrong with the first version?

Best Regards, Aurimas

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