RankNTypes.hs https://ocharles.org.uk/blog/guest-posts/2014-12-18-rank-n-types.html

RankNTypes.hs

{-# LANGUAGE RankNTypes #-}
module Main where

import Control.Monad.State
import System.Random

type GenAction m = forall a. (Random a) => m a
type GenActionR m = forall a. (Random a) => (a, a) -> m a

data Player = Player
  { playerName :: String
  , playerPos :: (Double, Double)
  } deriving (Eq, Ord, Show)

genRandom :: (RandomGen g) => GenAction (State g)
genRandom = state random

genRandomR :: (RandomGen g) => GenActionR (State g)
genRandomR range = state (randomR range)

randomPlayer :: (MonadIO m) => GenActionR m -> m Player
randomPlayer genR = do
  liftIO $ putStrLn "Generating random player..."
  len <- genR (8, 12)
  name <- replicateM len $ genR ('a', 'z')
  x <- genR (-100, 100)
  y <- genR (-100, 100)
  liftIO $ putStrLn "Done."
  return $ Player name (x, y)

main :: IO ()
main = randomPlayer randomRIO >>= print