softmechanics/wai

Haskell Web Application Interface

WAI: Web Application Interface

Getting started

You want a minimal example? Here it is!

{-# LANGUAGE OverloadedStrings #-}
import Network.Wai
import Network.Wai.Enumerator (fromLBS)
import Network.Wai.Handler.SimpleServer (run)

app :: Application
app _ = return Response
    { status          = status200
    , responseHeaders = [("Content-Type", "text/plain")]
    , responseBody    = ResponseLBS "Hello, Web!"
    }

main :: IO ()
main = do
    putStrLn $ "http://localhost:8080/"
    run 8080 app

Put that code into a file named hello.hs and install wai and wai-extra from Hackage:

cabal install wai wai-extra

Run it:

runhaskell hello.hs

Point your browser to:

http://localhost:8080/

Serving static content

We can modify our previous example to serve static content. For this create a file named index.html:

<p>Hello, Web!</p>

Now we redefine responseBody to refer to that file:

app2 :: Application
app2 _ = return index

index = Response
    { status          = status200
    , responseHeaders = [("Content-Type", "text/html")]
    , responseBody    = ResponseFile "index.html"
    }

Basic dispatching

An Application maps Requests to Responses:

ghci> :info  Application
type Application = Request -> IO Response

Depending on the path info provided with each Request we can serve different Responses:

app3 :: Application
app3 request = case pathInfo request of
    "/"     -> return index
    "/raw/" -> return plainIndex
    _       -> return notFound

plainIndex = Response
    { status          = status200
    , responseHeaders = [("Content-Type", "text/plain")]
    , responseBody    = ResponseFile "index.html"
    }

notFound = Response
    { status          = status404
    , responseHeaders = [("Content-Type", "text/plain")]
    , responseBody    = ResponseLBS "404 - Not Found"
    }

Doing without overloaded strings

For the sake of efficiency, WAI uses the bytestring package. We used GHCs overloaded strings to almost hide this fact. But we can easily do without. What follows is a more verbose definition of notFound, that works without GHC extensions:

import qualified Data.ByteString.Char8 as B8
import qualified Data.ByteString.Lazy.Char8 as LB8

notFound = Response
    { status          = Status404
    , responseHeaders = [("Content-Type", B8.pack "text/plain")]
    , responseBody    = Right $ fromLBS $ LB8.pack "404 - Not Found"
    }