Friday, April 29, 2016

Porting a Haskell graphics framework to Rust (luminance)

I wanted to write that new article to discuss about something important I’ve been doing for several weeks. It’s actually been a month that I’ve been working on luminance, but not in the usual way. Yeah, I’ve put my Haskell experience aside to… port luminance into Rust! There are numerous reasons why I decided to jump in and I think it could be interesting for people to know about the differences I’ve been facing while porting the graphics library.

You said Rust?

Yeah, Rust. It’s a strong and static language aiming at system programming. Although it’s an imperative language, it has interesting functional conventions that caught my attention. Because I’m a haskeller and because Rust takes a lot from Haskell, learning it was a piece of cake, even though there are a few concepts I needed a few days to wrap my mind around. Having a strong C++11/14 experience, it wasn’t that hard though.

How does it compare to Haskell?

The first thing that amazed me is the fact that it’s actually not that different from Haskell! Rust has a powerful type system – not as good as Haskell’s but still – and uses immutability as a default semantic for bindings, which is great. For instance, the following is forbidden in Rust and would make rustc – the Rust compiler – freak out:

let a = "foo";
a = "bar"; // wrong; forbidden

Haskell works like that as well. However, you can introduce mutation with the mut keyword:

let mut a = "foo";
a = "bar"; // ok

Mutation should be used only when needed. In Haskell, we have the ST monad, used to introduce local mutation, or more drastically the IO monad. Under the wood, those two monads are actually almost the same type – with different warranties though.

Rust is strict by default while Haskell is lazy. That means that Rust doesn’t know the concept of memory suspensions, or thunks – even though you can create them by hand if you want to. Thus, some algorithms are easier to implement in Haskell thanks to laziness, but some others will destroy your memory if you’re not careful enough – that’s a very common problem in Haskell due to thunks piling up in your stack / heap / whatever as you do extensive lazy computations. While it’s possible to remove those thunks by optimizing a Haskell program – profiling, strictness, etc., Rust doesn’t have that problem because it gives you full access to the memory. And that’s a good thing if you need it. Rust exposes a lot of primitives to work with memory. In contrast with Haskell, it doesn’t have a garbage collector, so you have to handle memory on your own. Well, not really. Rust has several very interesting concepts to handle memory in a very nice way. For instance, objects’ memory is held by scopes – which have lifetimes. RAII is a very well known use of that concept and is important in Rust. You can glue code to your type that will be ran when an instance of that type dies, so that you can clean up memory and scarce resources.

Rust has the concept of lifetimes, used to give names to scopes and specify how long an object reference should live. This is very powerful yet a bit complex to understand in the first place.

I won’t go into comparing the two languages because it would require several articles and a lot of spare time I don’t really have. I’ll stick to what I’d like to tell you: the Rust implementation of luminance.

Porting luminance from Haskell to Rust

The first very interesting aspect of that port is the fact that it originated from a realization while refactoring some of my luminance Haskell code. Although it’s functional, stateless and type-safe, a typical use of luminance doesn’t really require laziness nor a garbage collector. And I don’t like using a tool – read language – like a bazooka. Haskell is the most powerful language ever in terms of abstraction and expressivity over speed ratio, but all of that power comes with an overhead. Even though you’ll find folks around stating that Haskell is pretty okay to code a video game, I think it will never compete with languages that are made to solve real time computations or reactive programming. And don’t get me wrong: I’m sure you can write a decent video game in Haskell – I qualify myself as a Haskeller and I’ve not been writing luminance just for the joy of writing it. However, the way I use Haskell with luminance shouldn’t require all the overhead – and profiling got me right, almost no GC was involved.

So… I looked into Rust and discovered and learned the language in only three days. I think it’s due to the fact that Rust, which is simpler than Haskell in terms of type system features and has almost everything taken from Haskell, is, to me, an imperative Haskell. It’s like having a Haskell minus a few abstraction tools – HKT (but they’ll come soon), GADTs, fundeps, kinds, constraints, etc. – plus a total control of what’s happening. And I like that. A lot. A fucking lot.

Porting luminance to Rust wasn’t hard as a Haskell codebase might map almost directly to Rust. I had to change a few things – for instance, Rust doesn’t have the concept of existential quantification as-is, which is used intensively in the Haskell version of luminance. But most Haskell modules map directly to their respective Rust modules. I changed the architecture of the files to have something clearer. I was working on loose coupling in Haskell for luminance. So I decided to directly introduce loose coupling into the Rust version. And it works like a charm.

So there are, currently, two packages available: luminance, which is the core API, exporting the whole general interface, and luminance-gl, an OpenGL 3.3 backend – though it will contain more backends as the development goes on. The idea is that you need both the dependencies to have access to luminance’s features.

I won’t say much today because I’m working on a demoscene production using luminance. I want it to be a proof that the framework is usable, works and acts as a first true example. Of course, the code will be open-source.

The documentation is not complete yet but I put some effort documenting almost everything. You’ll find both the packages here:

luminance-0.1.0

luminance-gl-0.1.0

I’ll write another article on how to use luminance as soon as possible!

Keep the vibe!

3 comments:

  1. Hey, that's cool. I'm eager to see the demo!

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  2. s/Under the wood/Under the hood/

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  3. interesting functional conventions
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