Add IO functions for game of nim

Author: evturn

10-interactive-programming/10.8-nim.hs

@@ -0,0 +1,43 @@
+import Data.Char
+
+next :: Int -> Int
+next 1 = 2
+next 2 = 1
+
+type Board = [Int]
+
+initial :: Board
+initial = [5,4,3,2,1]
+
+finished :: Board -> Bool
+finished = all (== 0)
+
+valid :: Board -> Int -> Int -> Bool
+valid board row num = board !! (row-1) >= num
+
+move :: Board -> Int -> Int -> Board
+move board row num = [update r n | (r, n) <- zip [1..] board]
+  where
+    update r n = if r == row then n - num else n
+
+putRow :: Int -> Int -> IO ()
+putRow row num = do putStr (show row)
+                    putStr ": "
+                    putStrLn (concat (replicate num "* "))
+
+putBoard :: Board -> IO ()
+putBoard [a,b,c,d,e] = do putRow 1 a
+                          putRow 2 b
+                          putRow 3 c
+                          putRow 4 d
+                          putRow 5 e
+
+getDigit :: String -> IO Int
+getDigit prompt = do putStr prompt
+                      x <- getChar
+                      newline
+                      if isDigit x the
+                         return (digitToInt x)
+                      else
+                         do putStrLn "ERROR: Invalid digit"
+                            getDigit prompt