In my [last post][1], I talked about what BuckleScript is and how to get started. I found that when writing posts, my weakest posts come after I have been spending a lot of time on a topic and then go back and try to fit a post around it. That's what happened with that post as I had been fooling around with BuckleScript for several weeks before writing the post. This time, I'm beginning with the end in mind by starting the post at the beginning of the project.


My goals for this project are:

  1. Learn more about BuckleScript's JS interoperation
  2. Make something pretty

Why PixiJS?

I chose [PixiJS][4] pretty arbitrarily. I actually started down this whole BuckleScript path after playing around a little bit with [][2]. That site is an AI programming competition. I wanted to be able to simulate different scenarios in that game, and I found they wrote their visualization with [PixiJS][4]. The idea of extending their visualization with BuckleScript seemed like a fun project. I didn't end up doing anything with [][2] (partially because my friend introduced me to [Screeps][3] which I got hooked on). Still, I liked the visualization they used, and I've been wanting to create some more game-like visualizations that are outside the intended use case for [d3.js][5] which I've worked with before.

Drawing a Circle

First, I want to draw a circle without using BuckleScript to make sure I can get that working. What I want is a single javascript file main.js that calls the PixiJS drawing API's. I wanted to get this working without any other distractions like "webpack" and npm scripts, so I made a simple index.html that included main.js in the body.

// Set up renderer and append to body. This ends up as a canvas.
var renderer = PIXI.autoDetectRenderer(800, 600, { antialias: true });

// create the root of the scene graph
var stage = new PIXI.Container();
var graphics = new PIXI.Graphics();

// Create a function to draw a circle with our
// graphics object.
function drawCircle(x, y, r) {
    graphics.beginFill(0xFFFF0B, 0.5);
    graphics.drawCircle(x, y, r);

// Initialize variables.  We're drawing a circle
// with radius littleR and it follows the path
// centered at (centerX, centerY) with radius bigR
var t = 0;
var centerX = 300;
var centerY = 300;
var bigR = 200;
var littleR = 60;

// run the render loop

function animate() {
    var x = bigR*Math.sin(t) + centerX;
    var y = bigR*Math.cos(t) + centerY;

    drawCircle(x, y, littleR);
    t += .05;

    requestAnimationFrame( animate );

Adding BuckleScript

In the example above, I could download the PixiJS library and stick it in a "js" folder. While something similar may be possible with BuckleScript, the happier path is to use NPM. Running npm init in a new directory prompts you to fill out the fields needed for a barebones package.json. Once that's done, I installed BuckleScript with npm install --save bs-platform. This gave me version 1.4.1.

Next up is to create a bsconfig.json. Mine looks like this:

    "name": "buckle-pixi",
    "sources": { "dir": "src"},
    "generate-merlin": true

This tells the bsb command where to look for your .ml files. The generate-merlin piece generates a .merlin file. See my [last post][1] for more about Merlin and .merlin files. (I've had Merlin really barf on me a couple times. Each time it's been related to paths in my .merlin file being incorrect, once because I changed the directory and once because the ppx path was not absolute. If you get inscrutable error messages, carefully inspect your paths.)

Next, I put a simple main file in src that logs to the console like this:

let () =
    print_endline "hi"

I can then run ./node_modules/.bin/bsb which will generate a main.js file in a directory lib/js/src/main.js with these contents:

'use strict';


/*  Not a pure module */

Instead of running ./node_modules/.bin/bsb directly, I can add it to my scripts in the package.json under build so it looks like:

"scripts": {"build": "bsb"}

Then to build, I run npm run build instead.

I included this JS file in the index.html, I saw the output in the console. Exciting stuff!

Calling browser API's

The first thing the demo script I am using does is append a node to the body, so the first thing I am going to tackle is interacting with the document API's. I based my calling loosely off of the "ReasonML" bindings that I found on GitHub [here][6]. Here is what I came up with to append a simple text node to the document body:

type element

external body : element = "document.body" [@@bs.val]
external appendChild : element -> element -> unit = "appendChild" [@@bs.send]
external createTextNode : string -> element = "document.createTextNode" [@@bs.val]

let () =
    appendChild body (createTextNode "hi there!")

First, I declared an abstract type element. The next line lets me use body as an element, and whenever I use it, the resulting JS will use document.body to get the value (this what the [@@bs.val] language extension does.)

Next is the appendChild definition. This uses the [@@bs.send] language extension. This means that the first argument is the object, and the next arguments are the arguments to be sent to that object. In this case, the first element has appendChild called on it with the second element as its argument and returns nothing (aka unit). A more comprehensive description of these attributes can be found on the BuckleScript wiki [here][7].

The last external definition is for createTextNode. We use [@@bs.val] again here. We could also have created an external declaration for document and used [@@bs.send] like we did for appendChild, but this way seemed simpler since there will only ever be one document (i.e. we won't benefit from having the general type for document and general functions for it).

Calling an External library

The next step is making a call to PixiJS. Up to this point, I've had an index.html with a script tag in it's <head> importing the library. This set the global PIXI variable which let me make my calls in my main.js which I included inside the <body>. (Note: I could stick my script in the <head> too, but then I would have to check that the document had loaded before appending children to the <body>.)

Eventually, I may want to manage this dependency using a require statement, but for now, I'll continue with including the script in <head>. This means I can assume I have a global PIXI object at my disposal.

The first piece of code I would like to convert this:

var renderer = PIXI.autoDetectRenderer(800, 600, {antialias: true});

I wrote a sub module to help here:

module Renderer = struct                                              
    class type _t = object
        method view: element                                          
    end [@bs]
    type t = _t Js.t

    type options = < antialias : bool > Js.t

    external autoDetectRenderer :
        int -> int -> options -> t = "PIXI.autoDetectRenderer" [@@bs.val]

The first class definition sets up the type for the return value for PIXI.autoDetectRenderer. We also add a view member since that is what gets appended to the body. We call it a method with 0 arguments in order for it to be a field. Now we can access the view of an instance of Renderer.t via the ## operator. (The normal field access operator in OCaml is #, ## is a language extension from BuckleScript.)

The second part defines a type for the options to pass to our function. We're only using the antialias flag (and to be honest I'm not sure how necessary that is, but I figured it was worthwhile to see how passing options would work.)

Finally, we have our autoDetectRenderer function which I wrote as an external declaration calling the global PIXI object. We may have to adapt this if we start including PixiJS via imports rather than letting it be set globally.

Now the code to attach the renderer to the page is this:

let () =                
    let opts = [%bs.obj {antialias = true}] in                        
    let renderer = Renderer.autoDetectRenderer 800 600 opts in        
    appendChild body renderer##view

Notably, I use the [%bs.obj] extension to create a javascript object. Opening this shows the blank black canvas.

Drawing a Circle Again

Now that we've got the renderer in place, we can do similar things to get other pieces in place. Here is what I came up with for the Graphics and Container pieces:

module Graphics = struct
    class type _t = object
        method lineStyle: int -> unit
        method beginFill: int -> float -> unit
        method drawCircle: float -> float -> float -> unit
        method endFill: unit -> unit
        method clear: unit -> unit
    end [@bs]
    type t = _t Js.t

    external create : unit -> t = "PIXI.Graphics" []

module Container = struct
    class type _t = object
        (* This should probably accept more than Graphics.t *)
        method addChild : Graphics.t -> unit
    end [@bs]
    type t = _t Js.t
    external create : unit -> t = "PIXI.Container" []

One new thing here was the [] extension to call the constructor. The new draw circle function looks very similar to the plain JS version:

let drawCircle graphics x y r =                                       
    graphics##lineStyle 0;                                            
    graphics##beginFill 0xFFFF0B 0.5;
    graphics##drawCircle x y r;
    graphics##endFill ()

Then our main function looks like this:

let () =
    let opts = [%bs.obj {antialias = true}] in                        
    let renderer = Renderer.autoDetectRenderer 800 600 opts in        
    let stage = Container.create () in                                
    let graphics = Graphics.create () in                              
    appendChild body renderer##view;
    stage##addChild graphics;
    drawCircle graphics 300. 300. 60.;
    renderer##render stage

With all this code in place, the circle is drawn on the stage!

Requesting Animation Frame

The last piece is requestAnimationFrame. The new parts about this is that it makes a recursive call and it depends on mutability (for the t value). For the mutability, I used a ref, and for the recursive call I used the rec keyword both which you can read more about in Real World OCaml ([Ref cells][8], [recursive functions][9]) Here's what I came up with:

external requestAnimationFrame :
    (unit -> unit) -> unit = "requestAnimationFrame" [@@bs.val]
external sin : float -> float = "Math.sin" [@@bs.val]
external cos : float -> float = "Math.cos" [@@bs.val]

let startAnimation graphics renderer stage =
    let t = ref 0.0 in
    let centerX = 300. in
    let centerY = 300. in
    let bigR = 200. in
    let littleR = 60. in
    let deltaT = 0.05 in
    let rec animate () =
        let x = (bigR *. (sin !t)) +. centerX in
        let y = bigR *. (cos !t) +. centerY in
        graphics##clear ();
        drawCircle graphics x y littleR;
        t := !t +. deltaT;
        renderer##render stage;
        requestAnimationFrame animate
    in requestAnimationFrame animate

One piece which caught me up (but would probably be old news to an experienced OCaml-er) is that floating point and integer operators are different. For integer addition, you use +, but for float addition you use +.. Similarly, you have to specify floats expliclitly with the trailing decimal point, they won't be inferred. My first draft, I left this out, so my variables were ints. The syntax error was pretty verbose because it told me that the type for the Graphics.t I was passing in didn't match the expected type. This was because the actual Graphics.t had methods that worked on floats while I was asking for one with methods that used ints.

Stopping Exports

BuckleScript follows OCaml's logic about what is exported in a module. In the case of this module, it did this by adding these lines to the bottom of the JavaScript output:

exports.Graphics       = Graphics;
exports.Container      = Container;
exports.Renderer       = Renderer;
exports.drawCircle     = drawCircle;
exports.startAnimation = startAnimation

When I include this file directly in index.html, I get an error that exports is not defined. In order to prevent these exports statements from being included, I can add a .mli file. These are OCaml's interface files which define what is available to other modules. I created a main.mli file with just a comment and this got rid of the export statements (when it was totally empty, I was still seeing the export statements). If I ever want to use this module as a library, I'll need to modify that .mli file to include what I actually want to export.

The final JS output

Here is the final output:

'use strict';

function drawCircle(graphics, x, y, r) {
  graphics.beginFill(16776971, 0.5);
  graphics.drawCircle(x, y, r);
  return graphics.endFill();

function startAnimation(graphics, renderer, stage) {
  var t = [0.0];
  var animate = function () {
    var x = 200 * Math.sin(t[0]) + 300;
    var y = 200 * Math.cos(t[0]) + 300;
    drawCircle(graphics, x, y, 60);
    t[0] += 0.05;
    return /* () */0;
  return /* () */0;

var opts = {
  antialias: /* true */1

var renderer = PIXI.autoDetectRenderer(800, 600, opts);

var stage = new PIXI.Container();

var graphics = new PIXI.Graphics();



startAnimation(graphics, renderer, stage);

/* opts Not a pure module */

Very readable, and very similar to the original plain JavaScript. Notably, it does not mention any of the modules I created (Graphics, Container, Renderer). I don't export them and they were mostly just declarations of functionality in the actual classes (rather than implementations), so they didn't need to be included.


Overall, the experience of working with BuckleScript has been pretty good. The documentation isn't exhaustive (for instance, I had hard time figuring out what options are available in bsconfig.json), but it gives a lot of useful examples. There was the additional learning curve of OCaml, for which [Real World OCaml][10] has been indispensable.

Going forward, it will be interesting to see where the ecosystem goes in terms of interoperability. The bsb tool has an experimental feature to output the .d.ts type definitions needed for TypeScript. A really interesting piece would be to parse .d.ts files in order to create OCaml with the right external declarations.

[1]:{% post_url 2017-01-02-bucklescript-1 %}