生成3位数加减法算式

最近在玩haskell，刚好孩子要求出三位数的加减法，所以写了个小程序。

import System.Random
import System.IO.Unsafe

drawInt :: Int -> Int -> IO Int
drawInt x y = getStdRandom (randomR (x,y))

random_list :: (Eq a, Num a) => a -> Int -> Int -> IO [Int]
random_list 0 _ _ = return []
random_list n x y = do
    a <- drawInt x y
    rest <- (random_list (n-1) x y)
    return (a : rest)




math_practice n x y = zip [1..] $ map trans_formula (mix_list (init_val (n*2) x y) (init_sym n ))
    where   mix_list [] _ = []
            mix_list (x:y:xs) (z:zs) = (x,y,z) : mix_list xs zs
            init_val n x y =  unsafePerformIO $ random_list n x y
            init_sym n     =  unsafePerformIO $ random_list n 0 1

            trans_formula (x,y,z) =
                        if z ==0
                        then ((add_str x y) , (add_str' x y))
                        else ((plus_str x y), (plus_str' x y))
            add_str x y = show x ++ " + " ++ show y ++ "=    "
            add_str' x y = show x ++ " + " ++ show y ++ "=" ++ show (x+y)

            plus_str x y = show (max x y) ++ " - " ++ show (min x y)  ++ "=    "
            plus_str' x y = show (max x y) ++ " - " ++ show (min x y)  ++ "=" ++ (show  (abs (x-y)))

gs n x y = do
    mapM_ putStrLn [ show a ++ ". " ++ b| (a,(b,c))<-temp_list]
    putStrLn " "
    mapM_ putStrLn [ show a ++ ". " ++ c| (a,(b,c))<-temp_list]
    where temp_list = math_practice n x y

结果示例：

ghci>gs 10 100 999
1. 797 + 899=
2. 711 - 163=
3. 132 - 101=
4. 270 + 292=
5. 895 + 710=
6. 805 - 329=
7. 489 - 193=
8. 796 - 179=
9. 651 - 144=
10. 491 - 476=

1. 797 + 899=1696
2. 711 - 163=548
3. 132 - 101=31
4. 270 + 292=562
5. 895 + 710=1605
6. 805 - 329=476
7. 489 - 193=296
8. 796 - 179=617
9. 651 - 144=507
10. 491 - 476=15