daemons

Daemons in Haskell made fun and easy

Example

Here's AddOne, a simple daemon that waits for a number and responds with the incremented number.

import Data.Default ( def )
import System.Environment ( getArgs )
import System.Daemon

addOne :: Int -> IO Int
addOne n = return (n + 1)

main :: IO ()
main = do
    startDaemon "addOne" def addOne
    [n] <- getArgs
    res <- runClient "localhost" 5000 ((read n) :: Int)
    print (res :: Maybe Int)

Running it, we see:

% addone 22
Just 23
% addone 41
Just 42

The two important functions above are startDaemon, which checks if a daemon named addOne is already running, and starts it if not, and runClient which connects to the daemon running on localhost:5000, passes it a number, and waits for the response.

For a less trivial example, see the in-memory key-value store, Memo.

For an example that uses the streaming interface of daemons, see PMTQ (Poor Man's Task Queue).

Installation

This package is on Hackage. To install it, run:

cabal update
cabal install daemons

Modules

• Control.Pipe.C3 provides simple RPC-like wrappers for pipes.

• Control.Pipe.Serialize provides pipes to serialize and deserialize streams of strict ByteStrings using cereal.

• Control.Pipe.Socket provides functions to setup strict ByteString pipes around sockets.

• System.Daemon provides a high-level interface to starting daemonized programs that are controlled through sockets.

• System.Posix.Daemon provides a low-level interface to starting, and controlling detached jobs.

See also

• pipes The Pipes Tutorial

• C3 Wikipedia