我怎样才能改进这个Haskell源的结构？

Question

必须说我发现在Haskell中编程需要比我尝试过的任何其他语言更多的认知强度。我不确定我是否非常关心懒惰的评估或monad，但我确实感谢某些功能方面，静态类型检查以及不需要运行大型VM。

我一直在编写一个简短的程序来重命名基于EXIF的图像，现在有了一些有用的东西。我希望Haskell有一些关于源代码整体结构的专家意见，无论我做错了什么，以及如何改进和使代码更简洁。我不打算发布整个程序，但很多整体结构如下。欣赏它。

import Control.Applicative
import Control.Monad.Error
import Data.Char
import Data.Either
import Data.List
import Data.Maybe
import Data.Time.Format
import Data.Time.LocalTime
import System.Console.GetOpt
import System.Directory
import System.Environment
import System.FilePath
import System.Locale
import System.IO.Error
import Text.Printf

import Album.Utils
import Album.Exif
import Album.Error

-- -----------------------------------------------------------------------
-- Main

main = do
  r <- try main0
  case r of
    Left err -> do
      printf "Error type:     %s\n" $ (show $ ioeGetErrorType err)
      printf "Error string:   %s\n" $ (ioeGetErrorString err)
      printf "Error filename: %s\n" $ (maybe "None" id $ ioeGetFileName err)
      printf "Error handle:   %s\n" $ (maybe "None" show $ ioeGetHandle err)
    Right _  -> return ()

-- Process arguments
main0 = do
  args <- getArgs
  case processArgs args desc of
    (True, _, usage) -> showUsage usage
    (False, (flags, fns, []), usage) -> do
      rv <- runErrorT $ main1 flags fns
      either (\e -> showErrs [show e] usage) (const $ return ()) rv
    (False, (_, _, errs), usage) -> showErrs errs usage
  where
    desc = "Rename and catalog media.\n" ++
           "Usage: album -n <albumname> <options> <media files>\n"

-- Sanity check
main1 flags fns = do

    -- Check name
    albName <- maybe (throwError AlbumNameError) return $ albumName flags

    -- Check for duplicate media
    let dups = nub $ fns \\ nub fns
    unless (null dups) $ throwError $ MediaDuplicateError dups

    -- Check for valid filenames
    (haves, havenots) <- liftIO $ filesExist fns
    unless (null havenots) $ throwError $ MediaNotFoundError havenots
    when (null haves)      $ throwError MediaNotSpecifiedError

    -- Check exiftool existence
    tool0 <- liftIO $ findExecutable "exiftool"
    tool  <- maybe (throwError ExifToolNotFoundError) return tool0

    -- Get EXIF attributes
    exifs0 <- liftIO $ getExifs tool fns
    exifs  <- either (throwError . ExifParseError) return exifs0

    -- Check for exiftool errors
    let bads = findExifs "ExifTool:Error" exifs
    unless (null bads) $ throwError $ MediaBadError $ map fst bads

    -- Check for timestamps
    let infos0 = map (\(n,e) -> (n, exifToDateTime e)) exifs
    let nodates = filter (isNothing . snd) infos0
    unless (null nodates) $ throwError $ MediaDateTimeTagError $ map fst nodates

    let infos = map (\(n,Just dt) -> (n,dt)) infos0
    main2 albName infos

-- Do renames
main2 albName infos = do

    -- Album folder name
    let mints     = minimum $ map snd infos
    let mintsiso  = formatTime defaultTimeLocale "%Y%m%d" mints
    let albFolder = printf "%s - %s" mintsiso albName

    -- Get list of existing media
    albExist <- liftIO $ doesDirectoryExist albFolder
    (albCreate, existing) <- case albExist of
        False -> return (True, [])
        True -> do
            e <- liftIO $ filter ((/= ".") . nub) <$> getDirectoryContents albFolder
            return (False, e)

    -- Rename list
    let rens0 = mediaNames albName infos existing
    let rens1 = map (\(a,b) -> (a, combine albFolder b)) rens0
    let len = maximum $ map (length . fst) rens1

    if albCreate
      then do
        liftIO $ printf "Creating folder: %s\n" albFolder
        liftIO $ createDirectory albFolder
      else 
        return ()

    forM_ rens1 $ \(oldf, newf) -> do
        liftIO $ printf "%*s >>> %s\n" len oldf newf
        liftIO $ renameFile oldf newf

    return ()

showErrs errs usage = do
    putStrLn $ concatMap ("Error: " ++ ) errs
    return ()

showUsage usage = do
    putStrLn usage
    return ()

-- -----------------------------------------------------------------------
-- Rename

mediaNames albName infos existing = go existing (map cands infos)
    where
    go es [] = []
    go es ((fn,cs):css) = let p = unused cs es in (fn,p):go (p:es) css
    unused cs es = fromJust $ find (`notElem` es) cs
    cands (fn,dt) = (fn, map (++ ext) (pref:alts))
        where
        pref  = printf "%s - %s" (ft dt) albName
        ft dt = formatTime defaultTimeLocale "%Y%m%dT%H%M%S" dt
        alts  = map (printf "%s (%02d)" pref) ([1..] :: [Int])
        ext   = map toLower (takeExtension fn)

-- -----------------------------------------------------------------------
-- Arguments

data Option = OptionAlbumName String
            | OptionHelp
            deriving (Eq, Show)

processArgs args desc = (elem OptionHelp flags, opts, usage)
  where
    opts@(flags, fns, errs)  = getOpt RequireOrder conf args
    usage = usageInfo desc conf
    conf  = [
      Option "n" ["name"] (ReqArg OptionAlbumName "NAME") "Album name",
      Option "h" ["help"] (NoArg OptionHelp) "Help"]

albumName (OptionAlbumName n:xs) = Just n
albumName (x:xs)                 = albumName xs
albumName []                     = Nothing