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Haskell，我需要帮助，因为我似乎无法找出我做错了什么.(基本的)

分类: 技术问答 • 2022-04-05 23:15:37
问题描述：

我是Haskell的新手，正在尝试使用诸如学习Haskell之类的方法来学习.有人可以解释我的代码有什么问题吗，因为我还真的不知道如何读取错误消息.到目前为止，我只能说出let语句是不正确的，但是我需要它们以某种方式工作，因为没有它们，(show(typeOf numone/numtwo))只会显示'numone'或'numtwo'的类型，而不会显示从getLine输入的值.

I'm new to Haskell and am trying to learn using things like learn you a Haskell. Can someone explain what is wrong with my code as I don't really know how to read error messages yet. All I can tell so far is that the let statements aren't correct but I need them to work somehow because without them the (show (typeOf numone/numtwo)) only shows the type of either 'numone' or 'numtwo' and not the inputted values from the getLine.

我想做的是比较输入并显示输入的类型，但这是我在没有任何帮助的情况下所能做到的.

What I am trying to do is compare the inputs and show the types of the inputs but this is as far I can go without some help.

import Data.Typeable

main = do  
  putStrLn "Enter two statements."  
  numone <- getLine
  numtwo <- getLine

putStrLn $ ("You entered " ++ show numone ++ (show (typeOf numone)) ++ " and " ++ show numone ++ (show (typeOf numone)))

  let numone = getLine
  let numtwo = getLine

if numone == numtwo

  then  

    putStrLn $ "They are the same and their types are " ++ (show (typeOf     numone)) ++ " and " ++ (show (typeOf numtwo))

  else
putStrLn $ "They are not the same"

错误消息； 

• No instance for (Eq (IO String)) arising from a use of ‘==’
• In the expression: numone == numtwo
  In a stmt of a 'do' block:
    if numone == numtwo then
        putStrLn
          $ "They are the same and their types are "
              ++ (show (typeOf numone)) ++ " and " ++ (show (typeOf numtwo))
    else
        putStrLn $ "They are not the same"
  In the expression:
    do putStrLn "Enter two statements."
       numone <- getLine
       numtwo <- getLine
       putStrLn
         $ ("You entered "
              ++
                show numone
                  ++
                    (show (typeOf numone))
                      ++ " and " ++ show numone ++ (show (typeOf numone)))
       ....
      |
   10 |   if numone == numtwo
      |      ^^^^^^^^^^^^^^^^

输出应类似于(取决于getLine的输入)；

The output should be something like (depending on the input of getLine); 

>    You entered A123[String] and B456[String]

>    They are the same and their types are [String] and [String]     
     or 
     They are not the same

答

如果您的代码与问题完全相同，那么第一个问题就是缩进.

If your code is exactly as shown in the question, then the first problem is indentation.

除非您使用 {...;...;...} 语法.

第二个问题是 getLine 是IO-monad中的一个动作，因此您不能使用 let ，而必须使用单子绑定.

The second problem is that getLine is an action in the IO-monad, so you can't use let, but must use a monadic binding.

哦，第二个绑定将覆盖第一个.因此，虽然第二次使用该名称没有错，但这是不好的风格.

Oh, and the second binding overrides the first. So while using the name a second time is not wrong, it's bad style.

第三件事(这不是一个真正的问题)是，编写的代码会将静态类型分配给 numone 和 numtwo -这不像输入其他值那样改变他们的类型. getLine 具有类型

The third thing (it's not really a problem) is that the code as written will assign static types to numone and numtwo - it's not somehow like entering different values will change their type. getLine has type

getLine :: IO String

因此，您总是 看到 [Char] (又名 String )作为类型.

so you'll always see [Char] (aka String) as the type.

第四个问题是您在第一个输出中两次使用了 numone ，而不是 numone 和 numtwo .

The fourth problem is that you used numone twice in the first output, instead of numone and numtwo.

修改

我根据评论完全删除了第二个输入(以前的 let 语句).

I completely removed the second input (the former let-statements) according to the comments.

这是更正的程序:

import Data.Typeable

main :: IO ()
main = do  
  putStrLn "Enter two statements."  
  numone <- getLine
  numtwo <- getLine
  putStrLn $ ("You entered " ++ show numone ++ (show (typeOf numone)) ++ " and " ++ show numtwo ++ (show (typeOf numtwo)))
  if numone == numtwo then 
    putStrLn $ "They are the same and their types are " ++ (show (typeOf numone)) ++ " and " ++ (show (typeOf numtwo))
  else
    putStrLn $ "They are not the same"
  return ()

示例会话来自 ghci :

*Main> main
Enter two statements.
A123
B456
You entered "A123"[Char] and "B456"[Char]
They are not the same
*Main> main
Enter two statements.
A123
A123
You entered "A123"[Char] and "A123"[Char]
They are the same and their types are [Char] and [Char]
*Main>

那应该做你想要的.

让我再次强调:无论做什么，您都会总是获得 [Char] 作为类型.您不能基于输入分配动态类型.通常，Haskell类型系统是静态的.尽管有一些高级结构，例如 Data.Typeable ，但我不建议初学者使用.您的心理形象应该是当我编译程序时，Haskell类型检查器将为每个子表达式分配一个静态类型".您实际上可以通过在REPL中使用:t 来询问类型检查器:

Let me emphasize again: You'll always get [Char] as the type, no matter what you do. You can't assign dynamic types based on input. And in general, the Haskell typesystem is static; while there are some advanced constructions like Data.Typeable, I wouldn't recommend them for beginners. Your mental image should be "when I compile the program, the Haskell typechecker will assign a single static type to every subexpression". You can actually ask the typechecker for those by using :t in the REPL:

*Main> :t getLine
getLine :: IO String

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