Writing a Game Boy Emulator in OCaml

Would anyone here assert that there's any particular programming language that's better for writing emulators, virtual machines, bytecode interpreters, etc?

Where, when I say "better", I'm not so much talking about getting results that are particularly efficient/performant; nor in making fewer implementation errors... but more in terms of the experience of implementing an emulator in this particular language, being more rewarding, intuitive, and/or teaching you more about both emulators and the language.

I ask because I know that this sort of language exists in other domains. Erlang, for example, is particularly rewarding to implement a "soft-realtime nine-nines-of-uptime distributed system" in. The language, its execution semantics, its runtime, and its core libraries, were all co-designed to address this particular problem domain. Using Erlang "for what it's for" can thus teach you a lot about distributed systems (due to the language/runtime/etc guiding your hand toward its own idiomatic answers to distributed-systems problems — which usually are "best practice" solutions in theory as well); and can lead you to a much-deeper understanding of Erlang (exploring all its corners, discovering all the places where the language designers considered the problems you'd be having and set you up for success) than you'd get by trying to use it to solve problems in some other domain.

Is there a language like that... but where the "problem domain" that the language's designers were targeting, was "describing machines in code"?

C89

sml, specifically the MLTon dialect. It's good for all the same reasons ocaml is good, it's just a much better version of the ML-language in my opinion.

I think the only thing that ocaml has that I miss in sml is applicative functors, but in the end that just translates to slightly different module styles.

Well, there’s always https://pypi.org/project/rpython/

C is probably the best language for this.

Can you expand on what makes SML better, in your eyes, than OCaml?

IMO: it's certainly "simpler" and "cleaner" (although it's been a while but IIRC the treatment of things like equality and arithmetic is hacky in its own way), which I think causes some people to prefer SML over aesthetics, but TBH I feel like many of OCaml's features missing in SML are quite useful. You mentioned applicative functors, but there's also things like labelled arguments, polymorphic variants, GADTs, even the much-maligned object system that have their place. Is there anything SML really brings to the table besides the omission of features like this?

Verilog?

...just kidding (maybe).

Assuming we're talking about a pure interpreter, pretty much anything that makes it straightforward to work with bytes and/or arrays is going to work fine. I probably wouldn't recommend Haskell, just because most operations are going to involve imperatively mutating the state of the machine, so pure FP won't win you much.

The basic process of interpretation is just: "read an opcode, then dispatch on it". You'll probably have some memory address space to maintain. And that's kind of it? Most languages can do that fine. So your preference should be based on just about everything else: how comfortable are you using it, how much do you like its abilities to interface with your host platform, how much do you like type checking, and so on.

I quite frankly disagree. From personal experience I don't think there's any mainstream programming language that in itself teaches you anything much about emulating systems like the Game Boy or NES - in fact, I'd go so far as to say that none of them even at least yield elegant and accurate implementations.

People write "production-grade" emulators in C because it's fast, not because it's uniquely suited to the domain as a language.

Haskell excels at DSLs and the sort of data manipulation needed in compilers. OCaml, Lisp, and really any language with support for ADTs and such things do the trick as well. You can even try hard with modern C++ and variant types and such, but it won't be as pretty.

Of course, if you actually want to run games on the emulator, C or C++ is where the game is. I suppose Rust would work too, but I can't speak much for its low-level memory manipulation.

one of the options for fast iterations would be Forth. in its circles, it famous for generation targets and cross compiling between archs. seaech the net you shold find plenty.

Haskell and OCaml are excellent for compilers, because - as you suggest - you end up building, walking, and transforming tree data structures where sum types are really useful. Lisp is an odd suggestion there, as it doesn’t really have any built-in support for this sort of thing.

At any rate, that’s not really the case when building an emulator or bytecode interpreter. And Haskell ends up being mostly a liability here, because most work is just going to be imperatively modifying your virtual machine’s state.

> the treatment of things like equality and arithmetic is hacky in its own way

mlton allows you to use a keyword to get the same facility for function overloading that is used for addition and equality. it's disabled by default for hygienic reasons, function overloading shouldn't be abused.

https://baturin.org/code/mlton-overload/

> labelled arguments

generally speaking if my functions are large enough for this to matter, i'd rather be passing around refs to structures so refactoring is easier.

> polymorphic variants

haven't really missed them.

> GADTs

afaik being able to store functors inside of modules would fix this (and I think sml/nj supports this), but SML's type system is more than capable of expressing virtual machines in a comfortable way with normal ADTs. if i wanted to get that cute with the type system, i'd probably go the whole country mile and reach for idris.

> even the much-maligned object system that have their place

never used it.

> Is there anything SML really brings to the table besides the omission of features like this?

mlton is whole-program optimizing (and very good at it)[1], has a much better FFI[2][3], is much less opinionated as a language, and the parallelism is about 30 years ahead[4]. the most important feature to me is that sml is more comfortable to use over ocaml. being nicer syntactically matters, and that increases in proportion with the amount of code you have to read and write. you dont go hiking in flip flops. as a knock-on effect, that simplicitly in sml ends up with a language that allows for a lot more mechanical sympathy.

all of these things combine for me, as an engineer, to what's fundamentally a more pragmatic language. the french have peculiar taste in programming languages, marseille prolog is also kind of weird. ocaml feels quirky in the same way as a french car, and i don't necessarily want that from a tool.

[1] - http://www.mlton.org/Performance

[2] - http://www.mlton.org/ForeignFunctionInterface

[3] - http://www.mlton.org/MLNLFFIGen

[4] - https://sss.cs.purdue.edu/projects/multiMLton/mML/Documentat...

C isn't really the best language for anything anymore. Maybe as a compilation target for other languages.

I love, love, love StandardML.

I respect the sheer power of what mlton does. The language itself is clean, easy to understand, reads better than anything else out there, and is also well-formalised. I read (enjoyed!) the tiger book before I knew anything about SML.

Sadly, this purism (not as in Haskell but as a vision) is what probably killed it. MLTon or not, the language needed to evolve, expand, rework the stdlib, etc.

But authors were just not interested in the boring part of language maintenance.

What are your thoughts on basis[1] and successorml[2]?

[1] - http://www.mlton.org/MLBasis

[2] - https://smlfamily.github.io/successor-ml/

I’d also point out, that even in the compiler space, there are basically no production compilers written in Haskell and OCaml.

I believe those two languages themselves self-host. So not saying it’s impossible. And I have no clue about the technical merits.

But if you look around programming forums, there’s this ideas that”Ocaml is one of the leading languages for compiler writers”, which seems to be a completely made up statistic.

I don't know that many production compilers are in them, but how much of that is compilers tending towards self hosting once they get far enough along these days? My understanding is early Rust compilers were written in Ocaml, but they transitioned to Rust to self-host.

> And Haskell ends up being mostly a liability here, because most work is just going to be imperatively modifying your virtual machine’s state.

That sounds odd to me. Haskell is great for managing state, since it makes it possible to do so in a much more controlled manner than non-pure languages.

Yeah, I don't understand what the "liability" here is. I never claimed it was going to be optimal, and I already pointed out C/C++ as the only reasonable choice if you actually want to run games on the thing and get as much performance as possible. But manipulating the machine state in Haskell is otherwise perfect. Code will look like equations, everything becomes trivially testable and REPLable, and you'd even get a free time machine from the immutability of the data, which makes debugging easy.

I remember working through Appel's compiler textbook in school, in SML and Java editions side by side, and the SML version was of course laughably more concise. It felt like cheating, because it practically was.

If you're effectively always in a stateful monad, Haskell's purity offers nothing. Code doesn't look like equations, things aren't trivially testable and REPLable, you don't get a free time machine, and there's syntactic overhead from things like lifting or writes to deeply nested structures and arrays, since the language doesn't have built-in syntactic support for them.

Haskell isn't a liability for that lol

On the other hand, it does have support for things like side-effectful traversals, folds, side effects conditional on value existing, etc. In most other languages you have to write lower-level code to accomplish the same thing.

OCaml doesn't seem like a bad choice here. Haven't played with it much, but I wonder if Racket might be a good choice as well?

Also another option for fun in the browser Elm. check out similar older project https://github.com/Malax/elmboy