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, [EMAIL PROTECTED] 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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