Get list of the executables in the current directory

Question

This is what I've done;

import Control.Applicative
import Control.Monad
import qualified System.Directory as SD
main = do
 files <- SD.getDirectoryContents =<< SD.getCurrentDirectory
 exes <- filterM (liftA SD.executable <$> SD.getPermissions) files
 print exes

This works fine, but I can't help feeling that this code is somewhat redundant --- with filterM, liftA, <$> and =<< all present at once, things seem unnecessarily complicated.

How can I improve this piece of code?

For a quick reference:

SD.getCurrentDirectory :: IO FilePath
SD.getDirectoryContents :: FilePath -> IO [FilePath]
SD.executable :: Permissions -> Bool
SD.getPermissions :: FilePath -> IO Permissions

and I want to filter the contents of the current directory based on its being executable or not.

Answer 1

I wouldn't import System.Directory qualified, it doesn't export anything that could clash with standard names.

import Control.Applicative
import Control.Monad
import System.Directory
main :: IO ()
main = do
 files <- getDirectoryContents =<< getCurrentDirectory
 exes <- filterM (liftA executable <$> getPermissions) files
 print exes

You can toss out the Functor and Applicative stuff and just compose Monads.

import Control.Monad
import System.Directory
main :: IO ()
main = do
 files <- getDirectoryContents =<< getCurrentDirectory
 exes <- filterM (liftM executable . getPermissions) files
 print exes

You could also get rid of reverse bind and use the forward version, it's maybe more familiar?

import Control.Monad
import System.Directory
main :: IO ()
main = do
 files <- getCurrentDirectory >>= getDirectoryContents
 exes <- filterM (liftM executable . getPermissions) files
 print exes

I'd eliminate the explicit use of bind entirely though, then it becomes easier to turn this into a function that can work on directories other than the current one.

import Control.Monad
import System.Directory
main :: IO ()
main = do
 dir <- getCurrentDirectory
 printExecutables dir
printExecutables :: FilePath -> IO ()
printExecutables dir = do
 files <- getDirectoryContents dir
 exes <- filterM (liftM executable . getPermissions) files
 print exes