Developing Web Applications with Haskell and Yesod

Chapter 19. Wiki: Markdown, Chat Subsite, Event Source

This example will tie together a few different ideas. We’ll start with a chat subsite, which allows us to embed a chat widget on any page. We’ll use the HTML5 event source API to handle sending events from the server to the client.

-- @Chat.hs
{-# LANGUAGE OverloadedStrings, TypeFamilies, QuasiQuotes,
             TemplateHaskell, FlexibleInstances, MultiParamTypeClasses,
             FlexibleContexts
  #-}
-- | This module defines a subsite that allows you to insert a chat box on
-- any page of your site. It uses eventsource for sending the messages from
-- the server to the browser.
module Chat where

import Yesod
import Control.Concurrent.Chan (Chan, dupChan, writeChan)
import Data.Text (Text)
import Network.Wai.EventSource (ServerEvent (..), eventSourceAppChan)
import Language.Haskell.TH.Syntax (Type (VarT), Pred (ClassP), mkName)
import Blaze.ByteString.Builder.Char.Utf8 (fromText)
import Data.Monoid (mappend)

-- | Our subsite foundation. We keep a channel of events that all connections
-- will share.
data Chat = Chat (Chan ServerEvent)

-- | We need to know how to check if a user is logged in and how to get
-- his/her username (for printing messages).
class (Yesod master, RenderMessage master FormMessage)
        => YesodChat master where
    getUserName :: GHandler sub master Text
    isLoggedIn :: GHandler sub master Bool

-- Now we set up our subsite. The first argument is the subsite, very similar
-- to how we've used mkYesod in the past. The second argument ...