say

Send textual messages to a Handle in a thread-friendly way.

The motivation for this package is described in a blog post on Haskell’s Missing Concurrency Basics. The simple explanation is, when writing a line of textual data to a Handle - such as sending some messages t o ther terminal - we’d like to have the following properties:

• Properly handle character encoding settings on the Handle
• For reasonably sized messages, ensure that the entire message is written in one chunk to avoid interleaving data with other threads
  • This includes the trailing newline character
• Avoid unnecessary memory allocations and copies
• Minimize locking
• Provide a simple API

On the last point: for the most part, you can make the following substitutions in your API usage:

• Replace putStrLn with say
• Replace print with sayShow
• If you’re using a String instead of Text, replace putStrLn with sayString

In addition, sayErr, sayErrString and sayErrShow work on standard error instead, and hSay, hSayString and hSayShow work on arbitrary Handles.

#!/usr/bin/env stack
-- stack --install-ghc --resolver lts-6.23 runghc --package async --package say
import Control.Concurrent.Async (mapConcurrently)
import Control.Monad            (forM_, void)
import Say                      (sayString)

worker :: Int -> IO ()
worker ident = forM_ [1..1000] $ \msg -> sayString $ concat
    [ "Hello, I am worker #"
    , show ident
    , ", and this is message #"
    , show msg
    ]

main :: IO ()
main = void $ mapConcurrently worker [1..100]