有人可以解释这种类型错误"无法匹配预期类型`IO()'与实际类型`a0 - > c0'"

Question

这是我得到的类型错误,其次是相关代码.这可能是因为函数组合的使用不正确,如果是这样,我想解释一下如何解决这个问题.如果这是别的什么,我也想谈谈这个.

frameworkDaemon.lhs:125:5:
Couldn't match expected type `IO ()' with actual type `a0 -> c0'
In the expression: popJobState . popProcessConfig . readJobFile
In an equation for `makeJobState':
    makeJobState world
      = popJobState . popProcessConfig . readJobFile
      where
          readJobFile
            = do { let ...;
                   .... }
            where
                getFileContents (x : xs) = readFile (ixiaRoot ++ "/" ++ (show x))
          popProcessConfig = undefined
          popJobState = undefined

失败,模块加载:无.

 makeJobState :: JobState -> IO ()
 makeJobState world =
   popJobState . popProcessConfig . readJobFile -- f .g . h -> f(g(h))
      where readJobFile = do
               let directoryPath = ixiaRoot ++ "/jobs"
                   processedPath = map (directoryPath </>) . filter (`notElem` [".",".."])
               jobFileNames <- processedPath <$> getDirectoryContents directoryPath
               let numStrings = map (last . splitOn "/") jobFileNames
                   sortJobNumbers = map read numStrings :: [Int]
               getFileContents $ sort $ sortJobNumbers
                       where getFileContents (x:xs) = readFile (ixiaRoot ++ "/" ++ (show x))

            popProcessConfig = undefined
            popJobState =  undefined

Answer 1

-- f .g . h -> f(g(h))

f . g . h是不是等同于f(g(h)),这是相当于\x -> f(g(h x)),因此h需要一个功能,但readJobFile(这是h在你的例子)是IO ().这就是错误消息抱怨期望你给它一个功能的原因IO ().