How to use MonadRandom?

Question

Can someone provide \"for-dummies\" example of how to use `MonadRandom\'?

Currently I have code that does stuff like passing around the generator variable, all the w

Answer 1

Basically all the extra g parameters can just be dropped. You then get random numbers using the functions from Control.Monad.Random (such as getRandomR). Here is your example (I added some args to make it compile):

import Control.Monad.Random

main = do
    g <- getStdGen
    let r = evalRand (myFunc 1 2 3) g :: Double
    -- or use runRand if you want to do more random stuff:
    -- let (r,g') = runRand (myFunc 1 2 3) g :: (Double,StdGen)
    putStrLn $ "Result is : " ++ show r

--my complicated func
myFunc x y z = afunc x y z
afunc x y z = bfunc x y
bfunc x y = cfunc x
cfunc x = do
    ret <- getRandomR (0.0,1.0)
    return ret

Answer 2

You just run something in the RandT monad transformer with runRandT or evalRandT, and for the pure Rand monad, with runRand or evalRand:

main = do
  g <- getStdGen
  r = evalRand twoEliteNumbers g
  putStrLn $ "Result is: " ++ show r

twoEliteNumbers :: (RandomGen g) => Rand g (Double, Double)
twoEliteNumbers = do
  -- You can call other functions in the Rand monad
  number1 <- eliteNumber
  number2 <- eliteNumber
  return $ (number1, number2)

eliteNumber :: (RandomGen g) => Rand g Double
eliteNumber = do
  -- When you need random numbers, just call the getRandom* functions
  randomNumber <- getRandomR (0.0, 1.0)
  return $ randomNumber * 1337