WAI: Web Application Interface

Getting started

You want a minimal example? Here it is!

{-# LANGUAGE OverloadedStrings #-}
import Network.Wai
import Network.HTTP.Types
import Network.Wai.Handler.Warp (run)
app :: Application
app _ respond = do
    putStrLn "I've done some IO here"
    respond $ responseLBS
        status200
        [("Content-Type", "text/plain")]
        "Hello, Web!"
main :: IO ()
main = do
    putStrLn $ "https://localhost:8080/"
    run 8080 app

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

cabal install wai warp

Run it:

runhaskell hello.hs

Point your browser to:

https://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 _ respond = respond index
index :: Response
index = responseFile
    status200
    [("Content-Type", "text/html")]
    "index.html"
    Nothing

Basic dispatching

An Application maps Requests to Responses:

ghci> :info  Application
type Application = Request -> (Response -> IO ResponseReceived) -> IO ResponseReceived

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

app3 :: Application
app3 request respond = respond $ case rawPathInfo request of
    "/"     -> index
    "/raw/" -> plainIndex
    _       -> notFound
plainIndex :: Response
plainIndex = responseFile
    status200
    [("Content-Type", "text/plain")]
    "index.html"
    Nothing
notFound :: Response
notFound = responseLBS
    status404
    [("Content-Type", "text/plain")]
    "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
import           Data.CaseInsensitive (mk)
notFound = responseLBS
    status404
    [(mk $ B8.pack "Content-Type", B8.pack "text/plain")]
    (LB8.pack "404 - Not Found")