{- 
   October 4, 2006   
   Preparatory Course for the Software Technology Master
   Bastiaan Heeren (bastiaan@cs.uu.nl), Universiteit Utrecht, The Netherlands
-}

module Guess where

import Data.Char (isDigit)
import System.Random (newStdGen, randomRs)

-- First, we create a random number generator. This generator is used
-- to generate a list of random numbers between 0 and 100. We are only
-- interested in the head of this list.
main :: IO ()
main = 
   do rng <- newStdGen 
      let nr:_ = randomRs (0,100) rng
      putStrLn "Which number between 0 and 100 do I have in mind?"
      guess nr 1

-- The input from the user is validated first: the string should not be
-- empty, nor should it contain characters that are not digits.
guess :: Int -> Int -> IO ()
guess nr turn = 
   do putStr "? "
      input <- getLine
      if null input || any (not . isDigit) input 
        then guess nr turn 
        else eval (read input)
 where
   eval :: Int -> IO ()
   eval i | i <  nr   = do putStrLn ("The number is larger than " ++ show i)
                           guess nr (turn+1)
          | i >  nr   = do putStrLn ("The number is smaller than " ++ show i) 
                           guess nr (turn+1)
          | otherwise = do putStrLn ("Congratulations: you got the number in " ++ show turn ++ " turns!")