Developing Web Apps with Haskell and Yesod, 2nd Edition

Chapter 24. 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           Control.Exception        (SomeException)
import           Control.Exception.Lifted (handle)
import           Control.Monad.IO.Class   (liftIO)
import           Data.Aeson               (Value, encode, object, (.=))
import           Data.Aeson.Parser        (json)
import           Data.ByteString          (ByteString)
import           Data.Conduit             (($$))
import           Data.Conduit.Attoparsec  (sinkParser)
import           Network.HTTP.Types       (status200, status400)
import           Network.Wai              (Application, Response, responseLBS)
import           Network.Wai.Conduit      (sourceRequestBody)
import           Network.Wai.Handler.Warp (run)

main :: IO ()
main = run 3000 app

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

invalidJson :: SomeException -> Response
invalidJson ...