相关疑难解决方法(0)

我正在研究一些Haskell(请原谅新手错误) -

这个例程出错了.我对do&< - 语法的理解是它们从monad中提取非Monad类型.所以理解是有缺陷的:这里的正确理解是什么？

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

错误

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 }