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/
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"
}
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"
}
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"
}