Random.hs

import System.Random
 
-- takes a random generator and returns a list of strings of 50 chars
start_population :: (RandomGen g) => g -> [[Char]]
start_population gen =
  [take 50 $ randomRs ('A', 'z') gen | x <- [0..]]
 
main = do
  randomGen <- newStdGen  -- get a random generator
 
  print $ take 2 $ start_population randomGen  -- use it as a function argument

but the problem is, that the RandomGen doesnt get updated, so every 50 char string from start_population is the same. instead you need to pull 50 char chunks from the original randomRs call like so:

import System.Random

-- from split Data.List.Split
build :: ((a -> [a] -> [a]) -> [a] -> [a]) -> [a]
build g = g (:) []

chunksOf :: Int -> [e] -> [[e]]
chunksOf i ls = map (take i) (build (splitter ls)) where
  splitter :: [e] -> ([e] -> a -> a) -> a -> a
  splitter [] _ n = n
  splitter l c n  = l `c` splitter (drop i l) c n

-- takes a random generator and returns a list of strings of 50 chars
start_population :: (RandomGen g) => g -> [[Char]]
start_population gen =
  chunksOf 50 $ map (chars !!) $ randomRs (0, l-1) gen
  where chars = ['a'..'z'] ++ ['A'..'Z'] ++ ['0'..'9']
        l = length chars - 1

main = do
  randomGen <- newStdGen  -- get a random generator

  mapM_ print $ take 2 $ start_population randomGen  -- use it as a function argument