I'm still playing round with my random dieroll generation program. In doing so, I just hit a segmentation fault (I didn't think Haskell could *cause* a segfault!) I'm sure it's my code - I got this to compile by fiddling with types until the errors (which I didn't understand) went away. Certainly not the right way to code, I know, but never mind. The problem is that I have *no idea* how to begin debugging this. In C, Python, or any other imperative language, I'd put traces in, etc. But in Haskell, I don't even know where to start. I attach the code below. While help in the form of pointers to what I did wrong would of course be appreciated, what I'm really looking for is a pointer on how I'd find out for myself. (Hey! I just read the bit in the ghc manual which says if I am not using foreign or unsafe functions, a crash is a compiler bug. Did I find a compiler bug?) My code is below. All I did is ghc --make hist_3d6.hs, then run hist_3d6.exe. This is ghc 6.10.4 on Windows Vista Home 32-bit. Thanks for any help, Paul. import System.Random.Mersenne import qualified Data.Map as Map import Data.Map (Map) import Data.List takes :: Int -> [a] -> [[a]] takes n [] = [] takes n xs = take n xs : takes n (drop n xs) sums :: Num a => Int -> [a] -> [a] sums n xs = map sum (takes n xs) simulate :: Int -> IO [Double] simulate count = do gen <- newMTGen Nothing dice <- (randoms gen :: IO [Double]) return (take count dice) histogram :: Ord a => [a] -> [(a,Int)] histogram = Map.assocs . foldl' f Map.empty where f m k = Map.insertWith' (+) k 1 m simulation = do lst <- simulate 100000 return lst --return (histogram lst) main = do s <- simulation putStrLn (show s)