Example of ReaderT monad transformer

MonadTransformersExample.hs

module Main where

import Control.Monad (replicateM)
import Control.Monad.Trans (lift)
import Control.Monad.Reader (ReaderT, ask, runReaderT)
import Control.Monad.Random (Rand, getRandomR, evalRand)
import System.Random (getStdGen, StdGen)
import Text.Printf

op :: ReaderT Int (Rand StdGen) String
op = do
    x <- ask -- Get read-only state from ReaderT
    y <- lift $ getRandomR (0, 1 :: Double) -- Lift Rand computation into ReaderT
    return $ printf "%d : %f" x y -- Build String value to return

main :: IO ()
main = do
    rand <- getStdGen -- Get random generator from IO
    let ops = replicateM 3 op -- ReaderT Int (Rand StdGen) [String]
    let ranReader = runReaderT ops 5 -- Pass '5' in as read-only state. Rand StdGen [String]
    let ranRandom = evalRand ranReader rand -- [String]
    print ranRandom

Transformer Example Output

ghci> main
["5 : 0.6494038665120211","5 : 0.325141434228958","5 : 0.41333205510519855"]