entries.hs

import           Control.Applicative ((<$>))
import           Control.Monad
import           System.Directory    (doesDirectoryExist, doesFileExist,
                                      getCurrentDirectory, getDirectoryContents)

entries :: FilePath -> IO [FilePath]
entries path = do
    contents       <- getDirectoryContents path
    files         <- filterM doesFileExist' contents
    directories   <- filter isSubDirectory <$> filterM doesDirectoryExist' contents
    subdirEntries <- mapM entries (map expandPath directories)

    return $ files ++ concat subdirEntries
  where
    expandPath x        = path ++ "/" ++ x
    doesFileExist'      = doesFileExist . expandPath
    doesDirectoryExist' = doesDirectoryExist . expandPath

isSubDirectory :: FilePath -> Bool
isSubDirectory "."  = False
isSubDirectory ".." = False
isSubDirectory _    = True

main :: IO ()
main = do
    p <- getCurrentDirectory
    contents <- entries p
    forM_ contents putStrLn

$ ./entries

Main.hs
Hi.hs
Compiler.hs
Config.hs
Context.hs
Directory.hs
FilePath.hs
Flag.hs
Option.hs
Template.hs
Types.hs
Version.hs

$ tree                                                                                                                                                       ~/work/hi/src master
.
├── Distribution
│   ├── Hi
│   │   ├── Compiler.hs
│   │   ├── Config.hs
│   │   ├── Context.hs
│   │   ├── Directory.hs
│   │   ├── FilePath.hs
│   │   ├── Flag.hs
│   │   ├── Option.hs
│   │   ├── Template.hs
│   │   ├── Types.hs
│   │   └── Version.hs
│   └── Hi.hs
└── Main.hs

2 directories, 12 files