Control.Monad.Writer不在haskell中工作

Question

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 `print'
    Possible fix:
      add an instance declaration for (Show (Writer [String] Int))
    In a stmt of an interactive GHCi command: print it

我已经尝试编译我的老师今天写的代码也涉及Control.Monad.Writer,但没有任何作用.

我使用的是Ubuntu 12.04,gedit和GHC 7.4.1.

来自Learn You a Haskell的所有Writer monad程序都无法编译,而且我很困惑.

Answer 1

你显然输入了类似的东西

ghci> gcd' 12345 6789

在ghci提示.因此你要求ghci打印一个类型的值Writer [String] Int,但是没有类型的Show实例Writer,因此ghci无法打印它.你需要申请runWriter或类似的功能,

ghci> runWriter $ gcd' 12345 6789

应该管用.