一个类型为Haskell的函数：IO String->串 [英] A Haskell function of type: IO String-> String

问题描述

我在Haskell中编写了一堆代码来创建一个文本索引。顶部功能如下所示：

  index :: String  - > [（String，[Integer]）] 
 index a = [...] 
  

现在我想给这个函数从文件中读取一个字符串：

$ $ p $ $ $ c $ index index readFile input.txt

这是不行的，因为readFile的类型是FilePath - > IO String。

无法匹配预期类型'字符串'
对比推断类型'IO字符串'

我看到这个错误，但是我找不到任何类型的函数：

  IO String  - >字符串
  

我猜成功的关键在于某些Monad下的某处，但我找不到办法解决我的问题。您可以轻松地编写一个调用readFile操作的函数，并将结果传递给您的索引函数。

  readAndIndex fileName = do 
 text<  -  readFile fileName 
 return $ index text 
  

但是，IO monad会污染使用它的所有内容，所以此函数的类型为：

  readAndIndex :: FilePath  - > IO [（String，[Integer]）] 
  

I wrote a bunch of code in Haskell to create an index of a text. The top function looks like this:

index :: String -> [(String, [Integer])]
index a = [...]

Now I want to give this function a String read from a file:

index readFile "input.txt"

Which won't work because readFile is of type FilePath -> IO String.

Couldn't match expected type 'String' against inferred type 'IO String'

I see the error, but I can't find any function with type:

IO String -> String

I guess the key to success lies somewhere under some Monads, but I could not find a way to solve my problem.

解决方案

You can easily enough write a function that calls the readFile action, and passes the result to your index function.

readAndIndex fileName = do
    text <- readFile fileName
    return $ index text

However, the IO monad taints everything that uses it, so this function has the type:

readAndIndex :: FilePath -> IO [(String, [Integer])]

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