Opa Plugin Development with a PubNub Example

This will be a two part series of posts on writing plugins for Opa. One of Opa’s greatest features is you write everything in the functional, staticly typed Opa language. This even includes the frontend code you’d usually do in Javascript. This means your code is less error-prone which significantly reduces the amount of time spent on debugging code.

But how do you use Javascript library X within your Opa code?

While you can include Javascript code in a resource, like we do here for Google Analytics:

do Resource.register_external_js("/resources/js/google_analytics.js")

This isn’t what we want to do if we want to call external Javascript functions from Opa code. For this we create an Opa plugin. In this post I’ll be showing how I built the Opa client side bindings for the PubNub Javascript API, fork it on github and see all the other language bindings. PubNub is a cloud hosted real-time publish and subscribe interface for all types of applications.

Opa allows binding to external functions through its Binding System Library (BSL). In order to use this to create a plugin for some external Javascript, we create a file pubnub.js that we will compile to a plugin using opa-plugin-builder. Below is an example of one of the external functions that needs to be defined to use PubNub’s publish.

##register publish : string, string -> void
##args(c, m)
                       channel : c, message : m

    return js_void;

The first line registers the function publish and specifies its arguments to be two strings and its return value to be void. The next line names the arguments. You can think of ##args like Javascript’s function keyword. In the function, we can use the PUBNUB library just like we would normally. Here we pass to publish the channel and the message to be published. Note that the message can be any Javascript object, even though its a string to the plugin. We simply must serialize the JSON before passing to publish, which we’ll see shortly.

Now after defining external functions for all PubNub api calls in the pubnub.js file we can compile it:

$ opa-plugin-builder src/plugin/pubnub.js -o pubnub

This creates the plugin pubnub.opp.

To make using the plugin easier, we create a package in pubnub.opa that will contain a module PubNub. Below is the contents of pubnub.opa for the publish function that will show how to register the external functions and create a module named PubNub around them.

package pubnub

pubnub_publish = %%pubnub.publish%%

@client publish_client = pubnub_publish

PubNub = {{
  publish(channel: string, j : RPC.Json.json) =
    publish_client(channel, Json.to_string(j))

do Resource.register_external_js("http://cdn.pubnub.com/pubnub-3.1.min.js")

The first thing to notice is setting the pubnub_publish variable. The %% syntax is specifying that pubnub.publish is the key of an external primitive. pubnub is the name of the plugin (pubnub.opp) and publish is the function we registered in the plugin.

After this we create another variable publish_client that specifies that that this function (variables can contain functions) is to be run on the client side. Lastly, we define a module PubNub and a publish function inside the module to be used by our Opa programs.

At the end we make sure that the needed PubNub Javascript file is includes in all resources by registering it and giving the URL to the PubNub CDN.

Now we can compile it all together with the opa-plugin-builder command from above and use the plugin.

import pubnub

broadcast(author, msg) =
   username : RPC.Json.json = {String = author}
   message : RPC.Json.json = {String = msg}
   record_json : RPC.Json.json = {Record = [("username", username), ("message", message)]}
   do PubNub.publish("chat", record_json)

The broadcast function is part of pubnub_chat.opa. broadcast takes 2 arguments, the username of the user sending the message and the message itself. We then construct three variables of Opa’s json type, username, message and record_json. username and message are simply Json strings and they are then combined into a Json Record type that is a list of string/Json pairs. Since our PubNub.publish function converts the Opa Json to a string before sending to the Javascript binding we can simply pass the Opa Json variable record_json.

Lastly, lets take a quick look at the subscribe function. Defining it in pubnub.js look like:

##register subscribe : string, (string -> void) -> void

Since it takes a callback function you’ll notice that the second argument’s type looks different from in publish. (string -> void) is defining that the function takes a variable that is a function which takes a single argument of type string and has a void return type.

Now in pubnub.opa we have a subscribe function that is hardly different from the publish function we described above.

subscribe(channel: string, callback) =
  subscribe_client(channel, callback)

But how do we define a function we can pass to subscribe? That is the great thing, it can be any Opa function!

PubNub.subscribe("chat", (x -> user_update(x)))

Above we are subscribing to the channel chat and passing an anonymous function that takes a single argument and sends it to the function user_update.

user_update(x) =
  match Json.deserialize(x) with
         | {some={Record = [("username", username), ("message", message)]}} ->
                    line = <div class="line"> <div class="user">{Json.to_string(username)}:</>
                             <div class="message">{Json.to_string(message)}</>
                    do Dom.transform([#conversation +<- line ])
         | _ -> void

Now every time a message is posted to the chat channel, our callback, the anonymous function, will be run and call the function user_update which we see deserializes the Json string to get an Opa Json Record. The rest is basic Opa DOM manipulation to add the new message to the messages in the conversation element.

In my next post on Opa and PubNub, I’ll be describing an Opa server side API for PubNub.


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