更改函数以支持IO String而不是String [英] Change a function to support IO String instead of String

问题描述

  import Data.Char 
 import Network.HTTP 
 import Text.HTML.TagSoup 
 
 openURL :: String  - > IO字符串
 openURL x = getResponseBody =<< simpleHTTP（getRequest x）
 
 crawlType :: String  - > IO字符串
 crawlType pkm = do 
 src<  -  openURL url 
 return。 fromBody $ parseTags src 
其中
 fromBody = unwords。下降6。拿7。单词。 innerText。 dropWhile（〜/ =< p>）
 url =http://pokemon.wikia.com/wiki/++ pkm 
  

我想通过以下方式解析其数据：

  getType ::字符串 - > （String，String）
 getType pkmType =（dropWhile（=='/'）$ fst b，dropWhile（=='/'）$ snd b）
 where b = break（=='/ '）pkmType 
  

但是就像你看到的那样， getType 不支持IO字符串。

我是IO新手，那么如何使它工作？
我也尝试了解将IO字符串赋给该函数时的错误，但对于我来说这太复杂了：/ $ / $>

解决方案

，强调： IO字符串不是字符串。这是一个IO动作，当你将它绑定在 main 动作中的某个地方时，将会产生类型结果 String ，但你不应该把它看作某种字符串类型的变体。相反，它是 IO a 类型

的特殊实例化。

由于这个原因，你几乎可以肯定地做，而不是想要改变一个函数来支持 IO字符串而不是字符串&rdquo ;. 而不是，您希望将此字符串接受函数应用于 crawlType 动作的结果。正如我所说的那样，这样的结果是输入 String ，所以你很好。例如，

  main :: IO（）
 main = do 
 pkm =blablabla
 typeString<  -  crawlType pkm 
 let typeSpec = getType typeString 
 print typeSpec  - 或者您希望用它做的任何事情。 
  

您可以通过编写省略 typeString 变量

  typeSpec<  -  getType< $> crawlType pkm 
  

如果您愿意;这对应于过程语言中可能看起来像什么样的内容。

  var typeSpec = getType（crawlType（pkm））; 
  

或者，您当然可以在 crawlType ：

  crawlType':: String  - > IO（String，String）
 crawlType'pkm = do 
 src<  -  openURL url 
 return。 getType。 fromBody $ parseTags src 
其中
 fromBody = unwords。下降6。拿7。单词。 innerText。 dropWhile（〜/ =< p>）
 url =http://pokemon.wikia.com/wiki/++ pkm 
  

---

^† _{如果您好奇 < $> 运算符确实：这不是像 do / < - 标记的内置语法。相反，它只是 fmap 的中缀版本，您可能在其列表中找到特殊版本 map 。列表 [] 和 IO 都是函子，这意味着你可以通过普通函数来拉动它们，只改变元素/结果值，但不改变IO动作/列表脊柱结构。}

I get an IO String via:

import Data.Char
import Network.HTTP
import Text.HTML.TagSoup

openURL :: String -> IO String
openURL x = getResponseBody =<< simpleHTTP (getRequest x)

crawlType :: String -> IO String
crawlType pkm = do
  src <- openURL url
  return . fromBody $ parseTags src
  where
    fromBody = unwords . drop 6 . take 7 . words . innerText . dropWhile (~/= "<p>")
    url = "http://pokemon.wikia.com/wiki/" ++ pkm

and I want to parse its data via:

getType :: String -> (String, String)
getType pkmType = (dropWhile (== '/') $ fst b, dropWhile (== '/') $ snd b)
                  where b = break (== '/') pkmType

But like you see, getType doesn't support the IO String yet.

I'm new to IO, so how to make it working? I also tryed to understand the error when giving the IO String to that function, but it's too complicated for me up to now :/

解决方案

First, to emphasize: an IO String is not a string. It's an IO action which, when you bind it somewhere within the main action, will yield a result of type String, but you should not think of it as some sort of “variation on the string type”. Rather, it's a special instantiation of the IO a type.

For this reason, you almost certainly do not want to “change a function to support IO String instead of String”. Instead, you want to apply this string-accepting function, as it is, to an outcome of the crawlType action. Such an outcome, as I said, has type String, so you're fine there. For instance,

main :: IO ()
main = do
   pkm = "blablabla"
   typeString <- crawlType pkm
   let typeSpec = getType typeString
   print typeSpec -- or whatever you wish to do with it.

You can omit the typeString variable by writing^†

   typeSpec <- getType <$> crawlType pkm

if you prefer; this corresponds to what in a procedural language might look like

   var typeSpec = getType(crawlType(pkm));

Alternatively, you can of course include the parsing right in crawlType:

crawlType' :: String -> IO (String, String)
crawlType' pkm = do
  src <- openURL url
  return . getType . fromBody $ parseTags src
  where
    fromBody = unwords . drop 6 . take 7 . words . innerText . dropWhile (~/= "<p>")
    url = "http://pokemon.wikia.com/wiki/" ++ pkm

---

^†_{If you're curious what the <$> operator does: this is not built-in syntax like do/<- notation. Instead, it's just an infix version of fmap, which you may better know in its list-specialised version map. Both list [] and IO are functors, which means you can pull them through ordinary functions, changing only the element/outcome values but not the structure of the IO action / list spine.}

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