Recursively discover all files using Haskell and GIO

gistfile1.hs

-- Given a File object, recursively discover all of its non-directory children
listFiles :: (FileClass f) => f -> IO ([File])
listFiles f 
    -- this file doesn't exist
    | fileQueryExists f Nothing == False = return ([])
    
    -- this file isn't a directory
    | fileQueryFileType f [FileQueryInfoNone] Nothing /= FileTypeDirectory = return ([])
    
    -- otherwise, create the enumerator and start looping
    | otherwise = do e <- fileEnumerateChildren f "standard::*" [FileQueryInfoNone] Nothing
                     enumerate f e []
                     
    -- loop; takes the current file, its enumerator, and the list of results
    -- it's not very pretty, but it gets the job done
    where enumerate f e l = do
              fileInfo <- fileEnumeratorNextFile e Nothing
              case fileInfo of
                  Nothing -> return l
                  Just i  -> do let fileName = fileInfoGetName i
                                case fileName of
                                    Nothing -> return l
                                    Just n  -> do let c = fileGetChild f n
                                                  case (fileQueryFileType c [FileQueryInfoNone] Nothing) of
                                                      FileTypeDirectory -> (++) <$> listFiles c <*> enumerate f e l
                                                      _ -> enumerate f e $ c:l