On Nov 27, 2007 1:27 PM,
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:
Yeah, random number generation is one of those things in Haskell that can be tricky. But it looks like you're struggling more with the idea of monadic programming. That is expected :-)
import Random
increment :: Int -> Int increment b = b + 1
This is unnecessary; it can just be written (+1). (I.e. wherever you said "increment" you could write "(+1)" instead)
main = do let b = 0 let c = randomRIO (1,2) until (c == 1) increment b return b
You can think of this block as four "statements", one after the other. the "do-until" thing doesn't delimit anything, i.e. doesn't work the way you think it does. Let me rewrite this so it's clearer what's going on: main = do { let b = 0; let c = randomRIO (1,2); until (c == 1) increment b; return b; } In particular, until is a function, and you've given it three arguments: c == 1 (which is False), increment, and b. To solve this problem you'll probably want to use recursion, since it is a loop. There are "higher-order" ways to loop, but they all boil down to recursion in the end. So let's write a function which does this, call it count: count :: Int -> IO Int That is the type. It takes an integer representing the current count, does some IO and returns an integer. Specifically, it should take the current count and flip a coin. If the coin comes up tails, it should just return the current count. It it comes up heads, it should call itself again with 1 + the current count as an argument. I'll get you started count currentCount = do coin <- randomRIO (1,2) ... We use <- to run an action and get its result; we use let .. = to define the meaning of a symbol (but nothing is run). Using let just gives a shorter name for an expression. Why don't you try to write the rest? main will look like: main = do flips <- count 0 print flips I also recommend going through a tutorial which others will doubtless recommend to you until you get to monads (or skip ahead to monads and see if you understand). Luke