Question

Learn You a Haskell介绍了这个作家Monad的例子：

import Control.Monad.Writer  

gcd' :: Int -> Int -> Writer (DiffList String) Int  
gcd' a b  
    | b == 0 = do  
        tell (toDiffList ["Finished with " ++ show a])  
        return a  
    | otherwise = do  
        result <- gcd' b (a `mod` b)  
        tell (toDiffList [show a ++ " mod " ++ show b ++ " = " ++ show (a `mod` b)])  
        return result

如何使用>>=而非do表示法编写此示例？

Answer 1

以下是诀窍：

gcd'' :: Int -> Int -> Writer (DiffList String) Int
gcd'' a b
    | b == 0 = tell (toDiffList ["Finished with " ++ show a]) >> return a
    | otherwise = gcd'' b (a `mod` b) >>= (\result -> tell (toDiffList [show a ++ " mod " ++ show b ++ " = " ++ show (a `mod` b)]) >> return result)

作家Monad示例 - 从`do`表示法到'＆gt;＆gt; =`表示法

1 个答案: