[Haskell-cafe] Unexpected results with simple IO

17 Feb 2006

      Dear Haskell users,

   I have a problem using IO. The small test program below asks the user 
to guess from a list of random numbers between 1 and 10. Everything 
works well excepts for one problem: all the messages ("Guess a 
number...", "Right..." and "Wrong...") are printed after the program 
finishes, i.e., I have to use it blind. I'm afraid I misunderstand 
something important about lazyness or monads... What am I doing wrong?

   Thanks,
   Maurício

module Main where
import Random

main = do
    r_gen <- getStdGen --random generator
    let r_list = (randomRs (1,10) r_gen) --random list
    guess_loop (r_list)

guess_loop (r:r_others) = do
    putStrLn "Guess a number between 1 and 10:"
    n <- readLn
    if n==r
       then do
          putStrLn "Right! :)"
          return ()
       else do
          putStrLn "Wrong... :("
          guess_loop r_others