Petersburg.hs

{-# LANGUAGE NumDecimals #-}
module Main where

import           Data.List
import           Data.List.Split
import           Data.Ratio
import           System.Random

data Coin = Head | Tail deriving (Eq, Enum, Read, Show)

instance Random Coin where
    randomR (lo, hi ) g = let (x, g') = randomR (fromEnum lo, fromEnum hi) g
                          in (toEnum x, g')
    random = randomR (Head, Tail)

main :: IO ()
main = do
    coins <- randoms <$> newStdGen
    let trials = splitWhen (== Head) coins
    let outcomes = product . map (const (2 :: Integer)) <$> trials

    let average = let n = 1e7 in sum (take n outcomes) % (fromIntegral n)
    print (fromRational average)