CODEWITHSUNDEEP
haskell
Jakob
Jakob

Reputation: 3662

get sorted list of files from a directory

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

Upvotes: 0

Views: 592

Answers (3)

Vagif Verdi
Vagif Verdi

Reputation: 4906

Others already pointed to the lack of parens.

But here's a shorter and more readable version:

import System.Directory 
import Data.List
import Control.Applicative

sortedFiles :: FilePath -> IO [FilePath]
sortedFiles =  sort <$> getDirectoryContents

Upvotes: 3

ScottWest
ScottWest

Reputation: 1936

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

Upvotes: 8

Carl
Carl

Reputation: 27023

The return function doesn't take two arguments. (There's an exception to this, but it's not important for beginners - it doesn't do anything like what you'd expect.) This is because return is a function, not syntax. All the standard syntax rules for functions apply to it. You want to pass return the result of sort files, but that's not what the syntax you're using says to do.

You want either return (sort files) or return $ sort files.

The two are exactly equivalent. The latter is slightly more idiomatic Haskell. Most people prefer using the $ operator to using parentheses, when both are equivalently readable.

Upvotes: 4

Related Questions