Anyway, on to the code. We are going to implement a simple hit counter application. First we just enable a bunch of language extensions that happstack-state requires, and import a bunch of modules.

> {-# LANGUAGE GeneralizedNewtypeDeriving, ImpredicativeTypes, TemplateHaskell, > FlexibleInstances, FlexibleContexts, MultiParamTypeClasses, UndecidableInstances, > DeriveDataTypeable, TypeFamilies, RankNTypes, TypeSynonymInstances #-} > module Main where > import Control.Concurrent (forkIO) > import Control.Monad.State (MonadState(get,put)) > import qualified Data.ByteString as S > import qualified Data.ByteString.Char8 as C (pack, unpack) > import Happstack.Data (Default(defaultValue),deriveAll) > import Happstack.State (Component(..),Proxy(..), Version(..),End, createCheckpoint, > deriveSerialize, mkMethods, shutdownSystem, startSystemState, > update, waitForTermination) > import Happstack.Server.Cron (cron) > import Hyena.Server (serve) > import Network.Wai (Application, Enumerator) > > -- * State

Next we implement a happstack-state component which stores the number of requests we have received. This code is in no way hyena specific.

First we declare a simple type for counting requests.

> > $(deriveAll [''Read, ''Show, ''Eq ,''Ord, ''Default, ''Num, ''Enum] > [d| > newtype Hits = Hits Integer > |])

Next we create Serialize and Version instances so that we can use the Hits type with happstack-state.

> $(deriveSerialize ''Hits) > instance Version Hits

Now we create a Component for the Hits type.

> instance Component Hits where > type Dependencies Hits = End > initialValue = defaultValue

Then we create a simple function which increments the number of hits, and returns to use the current (updated) count.

> inc :: (MonadState Hits m) => m Hits > inc = > do hits <- get > let hits' = succ hits > put hits' > return hits'

And, then we turn the function into a method that can be used with update.

> $(mkMethods ''Hits ['inc]) > -- * main

Next we move onto the main function. This main function is almost exactly what we would have for a happstack-server based program, except the traditional simpleHTTP conf impl is replaced with the hyena equivalent server hitApplication.

> main :: IO () > main = > do ctl <- startSystemState (Proxy :: Proxy Hits) > forkIO $ cron (60*60*24) (createCheckpoint ctl) > forkIO $ serve hitApplication > putStrLn "running..." > waitForTermination > createCheckpoint ctl > shutdownSystem ctl

An Application is a 4-tuple which consists of an HTTP return code (200, 404, etc), a string associated with that return code (OK, Not Found, etc), a list of HTTP headers, and an enumerator which generates the body of the HTTP response.

> hitApplication :: Application > hitApplication environ = do > return (200, C.pack "OK", [textPlain, closeC], showHits)

There are two important headers set for the hit application. The first is simply the Content-type: text/plain header.

> textPlain :: (S.ByteString, S.ByteString) > textPlain = (C.pack "Content-type", C.pack "text/plain")

The second header is Connection: close. This tells hyena that it should close the connection after the enumerator has finished outputting the body. This is required because HTTP 1.1 specifies that all HTTP connections are 'keep-alive' by default.

> closeC :: (S.ByteString, S.ByteString) > closeC = (C.pack "Connection", C.pack "close")

There are three ways that an HTTP connection can be closed:

  1. The HTTP server sets the Content-length header and the client closes the connection after downloading the specified number of bytes.
  2. The HTTP server sets the Connection: close header in the Response and then closes the connection after it sends the body.
  3. The client sets the Connection: close header in the Request. The server notes this and closes the connection after sending the body.

All three of these methods work in hyena. If none of the above conditions are met, then hyena assumes that you don't want the connection closed.

Finally we have our Enumerator which generates the HTTP response body:

> showHits :: Enumerator > showHits = > let yieldHits f z = do > hits <- update Inc > z' <- f z (C.pack $ "hello, you are visitor number: " ++ show hits ++ "\n") > case z' of > (Left z'') -> return z'' > (Right z'') -> return z'' > in > yieldHits

This Enumerator is very boring. It outputs one line of text, and then it is done. It is not a particular instructive example of an Enumerator, so rather than trying to understand it now, I recommend moving on to the next section where we will examine Enumerators using more instructive examples.