There's a new version of this as v2 - Adding a duplicate entry randomly into a list in haskell using random monad
I wrote this trying to set up a Haskell testcase. The aim is to take a list and add a single duplicate from any place in the list to anywhere else in the list.
I'm trying to learn to use the Random Monad properly so the main aims should be clear, simple, idiomatic and pure code. However any recommendations for improvement are appreciated.
-- duplicate-to-list-randmonad.hs by Michael De La Rue 2014 -- licensed to StackEschange under cc by-sa 3.0 -- may also be used under AGPLv3 -- N.B. Trivial copying of code fragments does not normally require any license. import Data.List import Control.Monad.Random main :: IO () main = do putStrLn $ "list comparison " ++ prettyList list g <- getStdGen let shuffled = evalRand (infiniteDuplicateLists list) g putStrLn $ "lists after \n" ++ intercalate "\n" ( map prettyList (take 5 shuffled)) where list = ["a","b","c"] infiniteDuplicateLists genlist = do firstelt <- addRandomDuplicate genlist restoflist <- infiniteDuplicateLists genlist return $ firstelt : restoflist prettyList :: (Show a) => [a] -> String prettyList list = " [ " ++ intercalate "," (map show list) ++ " ] " addRandomDuplicate :: MonadRandom m => [a] -> m [a] addRandomDuplicate genlist = do frompos <- getRandomR (0 ,length genlist - 1) topos <- getRandomR (0 ,length genlist) let newlist = listEntryDuplicate (fromIntegral frompos) (fromIntegral topos) genlist return newlist listEntryDuplicate from to list = start ++ [repeat] ++ end where repeat = list !! from (start, end) = splitAt to list
(edited to move fixed code to a new question version for neatness)