A collection of extensions use with the Snap web application framework.
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Snap Extentions

Snap Extensions is a library which makes it easy to extend your Snap application with modular chunks of functionality such as session management, user authentication, templating and database connection pooling.

Included Extensions

Gives applications which need a HDBC `Connection` efficient pooling for free.
Integrate Heist templating into your application.
Integrate Less stylesheets into your application.

Using Snap Extensions

Every extension has an interface and at least one implementation of that interface. For some extensions, like Heist, there is only ever going to be one implementation of the interface. In these cases, the interface and the implementation thereof are exported from the same module, Snap.Extension.Heist. For something like session management though, there could be multiple implementations, one using a HDBC backend, one using a CouchDB backend and one just using a flat-file backend. In these cases, the interface is exported from Snap.Extension.Session, and the implementations live in Snap.Extension.Session.HDBC, Snap.Extension.Session.CouchDB and Snap.Extension.Session.FlatFile.

Keeping this in mind, there are a number of things you need to do to use Snap extensions in your application.

Define application State and Monad

First, we define a record type AppState for holding our application's state, including the state needed by the extensions we're using.

At the same time, we also define the monad for our application, App, as a type alias to SnapExtend AppState. SnapExtend is a MonadSnap and a MonadReader, whose environment is a given type; in our case, AppState.

module App where

import Database.HDBC.Postgresql
import Snap.Extension
import Snap.Extension.ConnectionPool
import Snap.Extension.Heist
import Snap.Types

type App = SnapExtend AppState

data AppState = AppState
    { connectionPoolState :: ConnectionPoolState Connection
    , heistState  :: HeistState App

An important thing to note is that the -State types that we use in the fields of AppState are specific to each implementation of a extension's interface. That is, Snap.Extension.Session.HDBC will export a different SessionState to Snap.Extension.Session.FlatFile, whose internal representation might be complete different.

Provide instances for "HasState" classes

So, we have a datatype that contains all the internal state needed by our application and the extensions it uses. Great! But when do we actually get to use this interface that these extensions export? What is actually being extended?

We use the interface provided by an extension inside our application's monad, App. Snap extensions extend our App with new functionality. For example, the Heist extension provides the function render :: MonadHeist m => ByteString -> m (). Is App a MonadHeist? Well, not quite yet. Any MonadReader which is also a MonadSnap whose environment contains a HeistState is a MonadSnap. That sounds a lot like App, doesn't it? We just have to tell the Heist extension how to find the HeistState in our AppState.

instance HasHeistState AppState where
    getHeistState = heistState
    setHeistState hs as = as { heistState = hs }

And similarly for our ConnectionPoolState:

instance HasConnectionPoolState AppState where
    getConnectionPoolState = connectionPoolState
    setConnectionPoolState dbps as = as { connectionPoolState = dbps }

With these instances, our application's monad App is now a MonadHeist and a MonadConnectionPool, giving it access to operations like render :: MonadHeist m => ByteString -> m () and withConnection :: MonadConnectionPool m => (Connection -> IO a) -> m a.

Define the Runner for AppState

So, our monad is now a MonadHeist and a MonadConnectionPool, but how do we actually construct our AppState and turn an App () into a Snap ()? Snap extensions has a thing called a "Runner" that does these things. Each implementation of a Snap extenion interface provides a Runner for its -State type. We must construct a runner type for our -State type, AppState. A Runner monad is provided to make it easy to do this. For your convenience, Runner is an instance of MonadIO.

appRunner :: Runner AppState
appRunner = do
    db <- connectionPoolRunner $ connectPostgreSQL "user=dbuser pass=sekrit"
    hs <- heistRunner "resources/templates"
    return $ AppState db hs

In addition to constructing the AppState, the Runner monad also constructs the init, destroy and reload functions for our application from the init, reload and destroy functions for the extensions. Although it won't cause a compile-time error, it is important to get the order of the runners correct as much as possible, otherwise they may be reloaded and destroyed in the wrong order. The "right" order is an order where every extension's dependencies are initialised before that extension. For example, Snap.Extension.Session.HDBC would depend on something which would extend the monad with MonadConnectionPool, i.e., Snap.Extension.ConnectionPool. If you had this configuration it would be important that you put the connectionPoolRunner before the sessionRunner in your appRunner.

This Runner AppState can then be passed to runRunner, whose type signature is Runner s -> SnapExtend s () -> IO (Snap (), IO (), IO [(ByteString, Maybe ByteString)]). The other arguments it takes is an App (), and the tuple it returns is a Snap action (which can be passed to httpServe), a cleanup action (which you run after httpServe) and a "reload" action (which you may want to use in your handler for the path "admin/reload"; the list it returns is for error reporting - there is one tuple in the list for each Snap extension; the first element of the tuple is the name of the Snap extension and the second is a Maybe which contains Nothing if there was no error reloading that extension and a Just with the ByteString containing the error message if there was) and a cleanup action which you would run after httpServe. The following is an example of how you might use this in main:

main :: IO ()
main = do
    (snap,cleanup,reload) <- runRunner appRunner site
    quickHttpServe $ snap
                  <|> path "admin/reload" $ defaultReloadHandler reload

You'll notice we're using defaultReloadHandler. This is a function exported by Snap.Extension with the type signature MonadSnap m => IO [(ByteString, Maybe ByteString)] -> m (). It takes the reload action returned by runRunner and returns a Snap action which renders a simple page showing how the reload went.


One of the big features of Snap 0.3 is the "development" mode which is implemented using Hint. To use Hint in its current form, you need to call a Template Haskell function loadSnapTH which resembles an imaginary function loadSnap :: IO AppState -> (AppState -> IO ()) -> (AppState -> Snap ()) -> IO (IO (), Snap ()). The tuple it returns contains a cleanup function and a Snap action which can be served with httpServe.

To make it easier to use Snap extensions with the Hint project template in 0.3, runRunnerHint :: Runner s -> SnapExtend s () -> (IO [(ByteString, Maybe ByteString)] -> SnapExtend s ()) -> IO (IO s, s -> IO (), s -> Snap ()) is provided. The first arguments are the same as in runRunner. The additional third argument is to do with reload handling; if you would like the same behaviour as the above example using runRunner, then you would pass path "admin/reload" . defaultReloadHandler as that argument. If you don't want to a web handler for reloading Snap Extensions at all, just pass nullReloadHandler as the argument.

The tuple it returns contains all the arguments you need to pass to loadSnapTH. An example of the use of runRunnerHint is given below:

import Snap.Http.Server

main :: IO ()
main = do
    (state, mkCleanup, mkSnap) <-
        runRunnerHint appRunner site $ path "admin/reload" . defaultReloadHandler

    (cleanup, snap) <- $(loadSnapTH 'state 'mkCleanup 'mkSnap)
    quickHttpServe snap

Simplified Snap Extension server

All of the above is kinda messy, so Snap Extensions provides another module, Snap.Extension.Server, which provides an interface mimicking that of Snap.Http.Server (their function names clash, so if you need to use both of them in the same module, use a qualified import). Using this module, the above application becomes:

import Snap.Extension.Server

main :: IO ()
main = quickHttpServe appRunner site

That's much better, isn't it? All it needs is a Runner AppState and an App () and it's ready to go. You might be wondering what happened to all the reload handler nonsense we had before? That stuff has been absorbed into the config for the server. quickHttpServe of course doesn't take a config, it uses the defaults augmented with any options specified on the command-line. The default reload handler is path "admin/reload" . defaultReloadHandler. If you wanted to change this to nullReloadHandler, this is what you would do:

import Snap.Extension.Server

main :: IO ()
main = do
    config <- commandLineConfig emptyConfig
    httpServe (setReloadHandler nullReloadHandler config) appRunner site

This behaves exactly as the above example apart from the reload handler.

Finally, if you wanted to turn the above application into one that used Hint, simply import Snap.Extension.Server.Hint instead of Snap.Extension.Server. The modules' interfaces are identical and no other changes to your code are needed.

Developing Snap Extensions

See src/Snap/Extensions/Timer.hs for a simple example of a Snap extension.