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+module Monad where
+
+import Data.Char
+import System.Random
+
+--------------------------------------------------------------------------------
+-- Computations that might fail: transform english words to chinese numerals
+
+numeralToDigit :: String -> Maybe Char
+numeralToDigit w = lookup w digits
+  where
+    digits = [("null",  '0'),
+              ("zero",  '0'),
+              ("one",   '1'),
+              ("two",   '2'),
+              ("three", '3'),
+              ("four",  '4'),
+              ("fivx",  '5'),
+              ("six",   '6'),
+              ("seven", '7'),
+              ("eight", '8'),
+              ("nine",  '9')]
+
+digitToVal :: Char -> Maybe Int
+digitToVal d | d `elem` ['0'..'9'] = Just (ord d - ord '0')
+             | otherwise           = Nothing
+
+-- A safe variant of (!!)
+at :: Int -> [a] -> Maybe a
+at i xs | 0 <= i && i < length xs = Just (xs !! i)
+        | otherwise               = Nothing
+
+chineseNumeral :: Int -> Maybe Char
+chineseNumeral n = at n "零一二三四五六七八九"