haskell - 如何映射和连接文件路径？

Question

我仍在研究 Haskell，尤其是 IO monad。

我有一个目录路径列表，例如，

["/some/path", "/another/path", "/yet/another/path", "/still/more"]


我想将此列表映射到每个路径的完全限定内容列表中(没有 . 和 .. )，如下所示:

["/some/path/file.1", "/some/path/somedir", "/some/path/file.2", "/another/path/file.a", "/another/path/a/directory", "/another/path/file.b", "/still/more/file.alpha", ...]


我想我可以使用某种双 map 来做到这一点，如下所示:

pathItems <- mapM (\pd -> MapM (\contents -> (pd </>) contents) (getDirectoryContents pd) pathDirs


但这不起作用。我收到的错误是这样的:

program.hs:27:56:    Couldn't match type `[]' with `IO'    Expected type: IO Char      Actual type: FilePath    In the expression: (pd </>) contents    In the first argument of `mapM', namely      `(\ contents -> (pd ) contents)'program.hs:27:84:    Couldn't match expected type `[FilePath]'                with actual type `IO [FilePath]'    In the second argument of `mapM', namely      `(getDirectoryContents pathDir)'    In the expression:      mapM        (\ contents -> (pd </>) contents)        (getDirectoryContents pd)

Answer 1

可能的解决方案(导入System.IO System.Directory System.FilePath Control.Applicative):

concat <$> mapM (\pd -> map (pd </>) <$> getDirectoryContents pd) pathDirs
                                   -- ^ this is to work under IO (..)
                    --  ^ this is to affect what's under IO [..]
        -- ^ this returns IO [[FilePath]]

可能有一些方法可以进一步简化它。