Control.Monad.Writer不能在haskell中工作 [英] Control.Monad.Writer not working in haskell

问题描述

我一直试图编译Haskell代码 - 再次涉及到Control.Monad.Writer。下面是一个代码示例，它不会从 学习你学习Haskell ：

  import Control.Monad.Writer 
 
 gcd':: Int  - > Int  - > Writer [String] Int 
 gcd'a b 
 | b == 0 = do 
 tell [用++显示a结束
返回
 |否则= do 
 tell [show a ++mod++ show b ++=++ show（a`mod` b）] 
 gcd'b（a`mod` b） 
  

我收到此错误：

<$ p $
可能的修正：
添加一个实例声明，用于（显示（Writer [String] Int））
由于使用`print' Show（Writer [String] Int））
在一个交互式GHCi命令中：print it

我试过编写代码，我的老师今天写的也涉及到Control.Monad.Writer，但没有任何效果。

我使用的是Ubuntu 12.04，gedit和GHC 7.4。 1

来自的所有Writer monad程序学习你一个Haskell 未能编译，而且我坚持原样。

解决方案

C> ghci中>在ghci提示符下输入gcd'12345 6789

。因此，你要求ghci输出 Writer [String] Int 类型的值，但是显示实例用于 Writer 类型，因此ghci不能打印它。您需要申请 runWriter 或类似的功能，

  ghci> runWriter $ gcd'12345 6789 
  

应该可以工作。

I have been trying to compile Haskell code all day – again – involving Control.Monad.Writer. Here is a code example that won't compile from Learn You a Haskell:

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)

I receive this error:

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

I have tried compiling code my teacher wrote today also involving Control.Monad.Writer but nothing works.

I am using Ubuntu 12.04, gedit, and GHC 7.4.1.

All the Writer monad programs from Learn You a Haskell have failed to compile, and I am pretty stuck as it is.

解决方案

You apparently entered something like

ghci> gcd' 12345 6789

at the ghci prompt. Thus you asked ghci to print a value of type Writer [String] Int, but there's no Show instance for Writer types, hence ghci can't print it. You need to apply runWriter or a similar function,

ghci> runWriter $ gcd' 12345 6789

should work.

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