相关疑难解决方法(0)

我一直在尝试编译Haskell代码 - 再次 - 涉及Control.Monad.Writer.这是一个不能从Learn You a Haskell编译的代码示例:

import Control.Monad.Writer  

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

我收到此错误:

No instance for (Show (Writer [String] Int))
      arising from a use of …