Developing Web Applications with Haskell and Yesod

Chapter 20. JSON Web Service

Let’s create a very simple web service: it takes a JSON request and returns a JSON response. We’re going to write the server in WAI/Warp, and the client in http-conduit. We’ll be using aeson for JSON parsing and rendering. We could also write the server in Yesod itself, but for such a simple example, the extra features of Yesod don’t add much.

Server

WAI uses the conduit package to handle streaming request bodies, and efficiently generates responses using blaze-builder. aeson uses attoparsec for parsing; by using attoparsec-conduit we get easy interoperability with WAI. This plays out as:

{-# LANGUAGE OverloadedStrings #-}
import Network.Wai (Response, responseLBS, Application, requestBody)
import Network.HTTP.Types (status200, status400)
import Network.Wai.Handler.Warp (run)
import Data.Aeson.Parser (json)
import Data.Conduit.Attoparsec (sinkParser)
import Control.Monad.IO.Class (liftIO)
import Data.Aeson (Value, encode, object, (.=))
import Control.Exception (SomeException)
import Data.ByteString (ByteString)
import Data.Conduit (ResourceT, ($$))
import Control.Exception.Lifted (handle)

main :: IO ()
main = run 3000 app

app :: Application
app req = handle invalidJson $ do
    value <- requestBody req $$ sinkParser json
    newValue <- liftIO $ modValue value
    return $ responseLBS
        status200
        [("Content-Type", "application/json")]
        $ encode newValue

invalidJson :: SomeException -> ResourceT IO Response
invalidJson ex = return $ responseLBS
    status400
    [("Content-Type" ...