pepe mnislaih at gmail.com
Tue Nov 27 10:35:08 EST 2007

```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
>
> main1 = do
>         b <- newIORef 0
>         let loop = do
>                c <- randomRIO (1,2)
>                unless (c == 1) (modifyIORef b increment >> loop)
>         loop

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

> 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 :)

pepe "otaku!"

PS: Puedo preguntarme qué hace este hombre aprendiendo Haskell? Viva!

On 27/11/2007, at 14:27, manolo at 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
>
> _______________________________________________

```