* manolo@austrohungaro.com wrote:
I'm trying to program an implementation of the St. Petersburg game in Haskell. There is a coin toss implied, and the random-number generation is driving me quite mad. So far, I've tried this:
import Random
import System.Random -- time goes on, interfaces change
increment :: Int -> Int increment b = b + 1
main = do let b = 0 let c = randomRIO (1,2) until (c == 1) increment b return b
In Haskell you take it the other side around: - Given a random number generator System.Random.newStdGen :: IO StdGen - you generate an infinite list of coin flip results System.Random.randoms :: (RandomGen g) => g -> [a] System.Random.randomRs :: (RandomGen g) => (a,a) -> g -> [a] - you are interested in the the first elements of a given value takeWhile :: (a -> Bool) -> [a] -> [a] - and need to compute the length of this list length :: [a] -> Int To model the result of a coin flip, you need two possible values. Your choice [1,2] is possible, but the boolean values are much easier. Let's choose True for number up and False otherwise. Put it together: main :: IO () main = do rnd <- newStdGen let result = computeResult rnd print result computeResult :: (RandomGen g) => g -> Int computeResult = length . takeWhile not . randoms Or in short: main = print . length . takeWhile not . randoms =<< newStdGen