Safe Haskell	Safe
Language	Haskell2010

Bento

Contents

Complete example

Description

Bento manages stateful components. It is inspired by Stuart Sierra's Component library for Clojure.

Synopsis

class Component component where
- type Dependencies component :: *
- start :: Dependencies component -> IO component
- stop :: component -> IO ()

Documentation

class Component component where #

A stateful component. Usually you will define an instance of this class to a data type that contains all of its runtime state.

data Example = Example { handle :: Handle }
instance Component Example where
    -- To be defined...

Minimal complete definition

start

Associated Types

type Dependencies component :: * #

Everything necessary to start the component. Typically this is a tuple, but you are free to use whichever data structure you want. If your component does not have any dependencies, use (), the empty tuple.

type Dependencies Example = (FilePath, IOMode)

Methods

start :: Dependencies component -> IO component #

Starts the component. This is where you should do things like open file handles, set up connections, and generally acquire resources. If anything goes wrong, just throw an exception, preferrably with throwIO.

This function should not block forever. If you need to start something that should keep running, like a server, put it on another thread with forkIO.

start (path, mode) = do
    h <- openFile path mode
    let component = Example { handle = h }
    return component

stop :: component -> IO () #

Stops the component. Generally this will do the opposite of whatever you did in start. The default implementation does nothing, which can be enough if you want the garbage collector to handle everything.

stop component = do
    hClose (handle component)

Complete example

The follow is a complete example of using Components to build a larger system, which is itself a Component.

For this simple example, we will be starting a web server. The only piece of configuration we need is the port to listen on. We will get that from the environment. If it's not available, we'll fall back to a default.

data Config = Config { port :: Int }
instance Component Config where
    type Dependencies Config = ()
    start () = do
        p <- lookupEnv "PORT"
        let config = Config { port = fromMaybe 8080 p }
        return config

Now that we have the config, we can go ahead and set up the server. It doesn't have to care how the config gets the port. We just have to list the config as a dependency. We'll also need the Application we want to run on the server.

Since start shouldn't block forever, we fire up the server on a separate thread. We keep track of the thread ID so that we can shut down the server when we stop by killing the thread.

data Server = Server { threadId :: ThreadId }
instance Component Server where
    type Dependencies Server = (Config, Application)
    start (config, application) = do
        tid <- forkIO (run (port config) application)
        let server = Server { threadId = tid }
        return server
    stop server = do
        killThread (threadId server)

With the config and server in hand, we can combine them into a larger system. The system will need the Application we want to run, but it will handle starting the config and passing it to the server.

To stop the system, we stop each Component in the reverse order that we started them.

data System = System { config :: Config, server :: Server }
instance Component System where
    type Dependencies System = (Application)
    start (application) = do
        c <- start ()
        s <- start (c, application)
        let system = System { config = c, server = s }
        return system
    stop system = do
        stop (server system)
        stop (config system)

To actually run the system, we start it just like the other Components. Once it's started, we want to wait forever until something sends us a SIGINT. Then we stop the system.

main :: IO ()
main = do
    let application _request respond = respond (responseLBS ok200 [] empty)
    system <- start (application)
    sentinel <- newEmptyMVar
    let handler _signal = putMVar sentinel ()
    installHandler sigINT handler
    takeMVar sentinel
    stop (system :: System)