My first Haskell script (organizes files in a directory by extension)

organize.hs

module Main where

import qualified System.Directory as Directory
import qualified System.FilePath as FilePath
import Control.Monad (forM, forM_, liftM)
import qualified Data.HashMap.Strict as HashMap
import Text.Printf (printf)

isFile :: FilePath -> IO Bool
isFile = Directory.doesFileExist

getDirectoryFiles :: FilePath -> IO [FilePath]
getDirectoryFiles filePath = do
  directoryContents <- Directory.getDirectoryContents filePath
  files <- forM directoryContents $ \entry -> do
    b <- isFile entry
    if b
      then return [entry]
      else return []
  return (concat files)

filesByExtension :: [FilePath] -> HashMap.HashMap [Char] [FilePath]
filesByExtension files =
  let
    addFileEntry h file =
      if FilePath.hasExtension file
      then
        let
          ext = tail $ FilePath.takeExtension file
          extFiles = HashMap.lookupDefault [] ext h
        in
         HashMap.insert ext (file : extFiles) h
      else
        h
  in
   foldl addFileEntry HashMap.empty files

main :: IO ()
main = do
  currentDirectory <- Directory.getCurrentDirectory
  extensions <- liftM filesByExtension $ getDirectoryFiles currentDirectory
  forM_ (HashMap.toList extensions) $ \(ext, files) ->
    do
      Directory.createDirectoryIfMissing True ext
      forM_ files $ \file ->
        do
          let path = FilePath.joinPath [ext, file]
          Directory.renameFile file path
          printf "%s -> %s\n" file path