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Hello, World!

Your first app!

Our first happstack application is a simple server that responds to all requests with the string, Hello, World!.

> module Main where
>
> import Happstack.Server (nullConf, simpleHTTP, toResponse, ok)
>
> main :: IO ()
> main = simpleHTTP nullConf $ ok "Hello, World!"

[Source code for the app is here.]

If you have not already installed Happstack -- you will need to do that first. You can find instructions on how to install Happstack here.

To build the application run:

$ ghc --make -threaded HelloWorld.hs -o helloworld

The executable will be named helloworld.

Alternatively, you can use runhaskell and avoid the compilation step.

$ runhaskell HelloWorld.hs

Run this app and point your browser at http://localhost:8000/. (assuming you are building the program on your local machine.)

If we point curl at the app we get the following output:

  $ curl http://localhost:8000/
 Hello, World!

How it works

Listening for HTTP requests

The top-level function simpleHTTP is what actually starts the program listening for incoming HTTP requests.

> simpleHTTP :: (ToMessage a) => Conf -> ServerPartT IO a -> IO ()

Configuring the HTTP listener

The first argument is some simple server configuration information. It is defined as:

> data Conf = Conf { port       :: Int
>                  , validator  :: Maybe (Response -> IO Response)
>                  , logAccess  :: forall t. FormatTime t =>
>                       Maybe (String -> String -> t -> String -> Int ->
>                              Integer -> String -> String -> IO ())
>                  , timeout    :: Int
>                  }
port
the TCP port to listen on for incoming connection
validator
on-the-fly validation of output during development
logAccess
logging function for HTTP requests
timeout
number of seconds to wait before killing an inactive connection

The default config is nullConf which is simply defined as:

> -- | Default configuration contains no validator and the port is set to 8000
> nullConf :: Conf
> nullConf = Conf { port      = 8000
>                 , validator  = Nothing
>                 , logAccess = Just logMAccess
>                 }

Processing a Request

The second argument is a bit more interesting. The ServerPartT IO a is the code that actually processes an incoming HTTP Request and generates a Response. The ServerPartT IO monad is essentially a fancy way of writing a function with the type:

> Request -> IO Response

simpleHTTP processes each incoming request in its own thread. It will parse the Request, call your ServerPartT handler, and then return the Response to the client. When developing your server part, it is natural to think about things as if you are writing a program which processes a single Request, generates a Response, and exits. However it is important when doing I/O, such as writing files to disk, or talking to a database to remember that there may be other threads running simultaneously.

Setting the HTTP response code

In this example, our server part is simply:

> ok "Hello, World!"

ok is one of several combinators which can be used to set the HTTP response code. In this case, it will set the response code to 200 OK. Happstack.Server.SimpleHTTP contains similar functions for the common HTTP response codes including, notFound, seeOther, badRequest and more. These functions all act like the normal return function, except they also set the response code.

Creating a Response

The body of the response will be "Hello, World!".

The String "Hello, World!" is turned into a Response because simpleHTTP calls toResponse from the ToMessage class on it. Often times we will opt to make this call explicit rather than implicit. For example:

> main :: IO ()
> main = simpleHTTP nullConf $ ok (toResponse "Hello, World!")

The toResponse function takes care of setting the Content-type and converting the value into a lazy ByteString which will be sent back to the client. Happstack comes with pre-defined ToMessage instances for many types such as Text.Html.Html, Text.XHtml.Html, String, the types from HSP, and more.

Next: routing incoming requests