管理IO monad [英] Managing the IO monad

问题描述

我正在学习一些Haskell（请原谅新手错误） -

这个例程出错了。我对do& < - 语法是它们从monad中提取非Monad类型。这样的理解是有缺陷的：这里有什么正确的理解？

  exister :: String  - > Bool 
 exister path = do $ b $ fileexist<  -  doesFileExist path 
 direxist<  -  doesDirectoryExist path 
 return fileexist || direxist 
  

错误

  ghc -o joiner joiner.hs 
 
 joiner.hs：53：2：
无法与推测类型'Bool'$ b匹配预期类型'Bool' $ b在'（||）'的第一个参数中，即`return fileexist'
在表达式中：return fileexist || direxist 
在表达式中：
 do {fileexist<  -  doesFileExist path; 
 direxist<  -  doesDirectoryExist路径; 
返回fileexist ||第一个问题：行<$ c $ <$ c $> 
  

解决方案 c> return fileexist || direxist 被解析为（return fileexist）|| direxist ，并且不能将 m Bool 作为 || 的第一个参数传递。 。将其更改为 return（fileexist || direxist）。

第二个问题：您声明返回类型 exister 是 Bool ，但编译器推断它必须是 IO Bool 。修理它。 （ do 和< - 语法可让您提取 a 值来自 ma 值，但前提是您承诺返回 ma 值。）

I'm working on learning some Haskell (please excuse the newbie error)-

This routine errors out. My understanding of the do & <- syntax is that they extract the non-Monad type from the monad. So that understanding is flawed: what's the correct understanding here?

exister :: String -> Bool
exister path = do
  fileexist <- doesFileExist path 
  direxist <- doesDirectoryExist path
  return fileexist || direxist 

error

ghc -o joiner joiner.hs

joiner.hs:53:2:
    Couldn't match expected type `Bool' against inferred type `m Bool'
    In the first argument of `(||)', namely `return fileexist'
    In the expression: return fileexist || direxist
    In the expression:
        do { fileexist <- doesFileExist path;
             direxist <- doesDirectoryExist path;
               return fileexist || direxist }

解决方案

First problem: The line return fileexist || direxist is parsed as (return fileexist) || direxist, and you can’t pass m Bool as the first argument of ||. Change it to return (fileexist || direxist).

Second problem: You claim the return type of exister is Bool, but the compiler infers that it must be IO Bool. Fix it. (The do and <- syntax let you extract the a value from an m a value, but only if you promise to return an m a value.)

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