Haskell : random

askForNumber.hs

import System.Random
import Control.Monad(when)
{-
This is referenced from "Learn You a Haskell for Great Good!"

> prompt sample
Which number in the range from 1 to 10 am I thinking of?
9
Sorry, it was 10
Which number in the range from 1 to 10 am I thinking of?
9
You are correct!!
-}

main = do
     gen <- getStdGen
     askForNumber gen

askForNumber :: StdGen -> IO ()
askForNumber gen = do
	     let ( randNumber , newGen ) = randomR (1,10) gen :: (Int,StdGen)
	     putStrLn $ "Which number in the range from 1 to 10 am I thinking of?"
	     numberString <- getLine
	     when ( not $ null numberString ) $ do
	     	  let number = read numberString 
		  if randNumber == number
		  then putStrLn "You are correct!!"
		  else putStrLn $ "Sorry, it was " ++ show randNumber
	     askForNumber newGen

Random2Times.hs

import System.Random
-- getStdGen , newStdGen 
-- If you use getStdGen 2 times , you can get the same result.
-- In this case , you can use newStdGen instead of getStdGen after the first time you use getStdGen. 
main = do
     gen <- getStdGen
     putStrLn $ take 20 (randomRs ('a','z') gen )
     gen' <- newStdGen
     putStrLn $ take 20 (randomRs ('a','z') gen' )

SampleRandom.hs

import System.Random
-- range from 'a' to 'z' 
take 6 $ randomRs ('a','z') (mkStdGen 11)
-- range from 1 to 6 , just like dice.
take 6 $ randomRs (1,6) (mkStdGen 11)