On Sat, Aug 1, 2009 at 4:44 PM, Paul Moore
PS I know my code is probably fairly clumsy - I'd appreciate style suggestions, but my main interest here is whether a beginner, with a broad programming background, a basic understanding of Haskell, and access to Google, put together a clear, efficient, program (ie, the case where my usual scripting language is too slow and I want to knock something up quickly in a high-level, high-performance language).
Here is one way to rewrite your program. It improved the speed somewhat for me. I timed both programs on my computer. I suppose one could try using an array for calculating the histogram as well, but I only tried the simples thing here. I hope someone can weigh in with a more thorough analysis. Please note how I've avoided including the IO Monad in some type signatures by extracting the data from it locally (with <-). It is quite possible to apply the histogram function to the data before going through the IO Monad as well, but it doesn't appear to change the execution speed much here. Caveat: My testing wasn't extensive. I just compiled with -O and timed the programs a couple of times. import System.Random import qualified Data.Map as Map import Data.Map (Map) import Data.List diceRolls :: Int -> IO [Int] diceRolls highVal = do generator <- getStdGen return (randomRs (1, highVal) generator) groupDice :: Int -> [Int] -> [[Int]] groupDice chunk rolls = map (take chunk) $ iterate (drop chunk) rolls simulate :: Int -> Int -> Int -> IO [Int] simulate count m n = do rolls <- diceRolls n let sums = map sum $ groupDice m rolls return (take count sums) 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 3 6 return (histogram $ lst) main = do s <- simulation putStrLn (show s)