download hs

download.hs

#!/usr/bin/env stack
-- stack --resolver lts-12.0 script
import Network.HTTP.Simple (parseRequest, httpLbs, getResponseBody)
import qualified Data.ByteString.Lazy as BL (writeFile)
import System.Environment (getArgs)
import Data.Foldable (for_, traverse_)
import Control.Concurrent.Async

-- Get the following program working to download the 4 URLs and save
-- their contents in the given files.
--
-- Bonus points: use the async package to do the downloads in
-- parallel.

downloads :: [(String, FilePath)]
downloads =
  [ ("https://raw.githubusercontent.com/haskell-lang/haskell-lang/master/static/tutorial/package-async.md", "async.md")
  , ("https://raw.githubusercontent.com/haskell-lang/haskell-lang/master/static/tutorial/package-binary.md", "binary.md")
  , ("https://raw.githubusercontent.com/haskell-lang/haskell-lang/master/static/tutorial/package-conduit.md", "conduit.md")
  , ("https://raw.githubusercontent.com/haskell-lang/haskell-lang/master/static/tutorial/package-containers.md", "containers.md")
  ]

download :: String -> FilePath -> IO ()
download url fp = do
  req <- parseRequest url
  res <- httpLbs req
  BL.writeFile fp $ getResponseBody res

downloadAll :: [(String, FilePath)] -> IO ()
downloadAll = mapConcurrently_ (uncurry download)

main :: IO ()
main = downloadAll downloads

Thanks to @snoyberg for suggesting mapConcurrently_.