Hola Manolo, What you are trying to do is very easy in Haskell, but you'd better change the approach. In short, you are trying to use b as if it was a mutable variable, which it is not! One could rewrite your program using mutable variables, as below:
import Data.IORef import Random import Control.Monad
main1 = do b <- newIORef 0 let loop = do c <- randomRIO (1,2) unless (c == 1) (modifyIORef b increment >> loop) loop readIORef b
Ugh, that's ugly (I have changed 'until' for 'unless', which is much more widely used). But as I said, this is not the right approach. What one would do in Haskell is to simply generate an infinite list of random numbers, and then operate on that, e.g. counting the number of consecutive heads of the coin.
main2 = do gen <- newStdGen let tosses = randomRs (1::Int,2) gen b = takeWhile ( /= 1) tosses return (length b)
Hope that was of help. You can find more material on Haskell in the wiki :) http://haskell.org/haskellwiki/Learning_Haskell pepe "otaku!" PS: Puedo preguntarme qué hace este hombre aprendiendo Haskell? Viva! On 27/11/2007, at 14:27, manolo@austrohungaro.com wrote:
Hello,
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
increment :: Int -> Int increment b = b + 1
main = do let b = 0 let c = randomRIO (1,2) until (c == 1) increment b return b
This is intended to print the number of consecutive heads (i.e., 2) before the first tail, but I get the following error:
ERROR "StPetersburg.hs":8 - Type error in application *** Expression : until (c == 1) increment b *** Term : c == 1 *** Type : Bool *** Does not match : Int -> Bool
I don't really see what's going on, so any help will be more than welcome. I hope this is a suitable question for the Haskell Café list.
I'm using Hugs in an Ubuntu box, in case that should be useful.
Thanks, Manolo
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