Question

I want a sorted list of files from a directory. How do I apply the sort function to a list with IO monad?

import System.Directory 
import Data.List

sortedFiles :: FilePath -> IO [FilePath]
sortedFiles path = do
    files <- getDirectoryContents "."
    return sort files                   -- this does not work

Answer 1

The original problem is just lack of parentheses, as currently return is being applied to two arguments (sort and files), just fix that up:

sortedFiles path = do
    files <- getDirectoryContents "."
    return (sort files)

If you want you can fmap the sort function over the directory contents. It kind of has a nice, direct feel to it, basically lifting the sort function up into the IO monad:

sortedFiles path = sort `fmap` getDirectoryContents path