Okay, beginner encountering first major bug. I'm getting a stack overflow on a program that uses a lot of laziness to construct and modify lists---but the overflow happens even if I take just one element from the final list! The overflow seems to have started when I added the functions 'gauss' and 'gaussList' below, but I am at a loss to know why---these functions work perfectly in test cases and can even generate a long list. The following program is distilled (with a lot of work!) down to the essence of what triggers the problem. import Control.Monad import Control.Monad.Random import Data.List -- Here I put a moderate-sized list of filenames which just generic -- text or code. The longer the list, -- the more likely to overflow stack. Which is curious, because if IO were -- lazy here, it shouldn't be reading more than the first character of the -- first file. Methinks a hint to the problem. fileList = ... data TextGroup = Single Char Float deriving (Show) textGroup_addDelay :: TextGroup -> Float -> TextGroup textGroup_addDelay (Single c del) del2 = Single c (del+del2) mkTextGroup :: Char -> Rand StdGen TextGroup mkTextGroup c = liftM (Single c) gauss -- Here's the kinda weird thing I'm doing with Rand. I want to -- use fromList as the first computation to randomly choose -- a second Rand computation. It turns out if you replace -- this funkiness with -- gauss = getRandomR (0,1) -- the whole thing works. gaussList :: Rand StdGen (Rand StdGen Float) gaussList = fromList [ (getRandomR( -1.0 ,-0.8 ), 2) , (getRandomR( -0.8 ,-0.6 ), 2) , (getRandomR( -0.6 ,-0.4 ), 4) , (getRandomR( -0.4 ,-0.2 ), 8) , (getRandomR( -0.2 , 0.0 ), 12) , (getRandomR( 0.0 , 0.2 ), 12) , (getRandomR( 0.2 , 0.4 ), 8) , (getRandomR( 0.4 , 0.6 ), 4) , (getRandomR( 0.6 , 0.8 ), 2) , (getRandomR( 0.8 , 1.0 ), 2) ] gauss :: Rand StdGen Float gauss = do m <- gaussList m main = do gen0 <- newStdGen bufs <- mapM readFile fileList let buf = concat bufs let tgs = evalRand (do orig <- mapM mkTextGroup buf addBreaks orig) gen0 writeFile "output.ank" (show $ take 1 tgs)