Implementing Elm in Wasm via C
This repo is part of a project to compile Elm to WebAssembly, using C as an intermediate language.
EXPERIMENTAL! DEFINITELY NOT PRODUCTION READY!
It implements parts of Elm's core libraries needed for any compiled code to work, including:
+,
-,
*,
/)
I also have a fork of the Elm compiler that generates C instead of JavaScript. It's not fully debugged yet.
Here's roughly how I see the project progressing from here, as of April 2020. (Unless some big unknown bites me, which it might!)
I have built a few demos here: https://brian-carroll.github.io/elmcwasm/
This project is very much in the early phases of development. It's not even close to being production ready.
The C compilation stage often fails because there are so many kernel modules still not written yet. And the compiler has not really been debugged so it generates bad C code sometimes.
You have been warned!
This is an installation guide for anyone who might want to play with the code and maybe contribute.
Install Emscripten
Clone and build the forked Elm compiler
cdto the root directory
stack init
stack build
Clone this repo
Run
npm install
http-serverto serve assets including the .wasm file
Ensure you're using the forked Elm binary
elm
Build the demo
cd demos/wrapper
make
Run a development server and open the demo
npx http-server(still inside
demos/wrapper)
Currently Wasm does not have direct support for any Web APIs such as DOM, XmlHttpRequest, etc. You have to call out from WebAssembly to JavaScript to use them. That means that effectful modules like
VirtualDom,
Browser, and
Http, cannot be fully ported to WebAssembly yet. A few effects could be ported, but not enough to justify the effort of porting lots of complex kernel code to C (like Scheduler.js and Platform.js), so I think it's better to leave that for later.
In the meantime, I think a good goal is to compile the pure part of the Elm program to WebAssembly and leave the effectful Kernel code in JavaScript. There would be a "wrapper"
Programtype that acts as an interface between the Elm runtime and the WebAssembly module.
There are two ways for JavaScript and WebAssembly to talk to each other
ArrayBuffer. JavaScript can write bytes into the memory and then call an exported function when the data is ready (perhaps passing the offset and size of the data written). More details here.
The wrapper
Programwould convert the
msgand
modelto a byte-level representation, write them to the WebAssembly module's memory, and then call an
updatefunction exposed by the compiled WebAssembly module.
The
updatewill return a new
modeland
Cmd, which will be decoded from bytes to the JavaScript structures that the Elm runtime kernel code uses.
VirtualDomis a unique effect in Elm in that it has no
Cmd. I think the best approach will be to implement the diff algorithm in pure Elm and use ports to apply the patches, as Jakub Hampl demonstrated in Elm 0.18 a couple of years back.
It would be possible to compile Elm code directly to WebAssembly. In fact that's what I did in my first attempt. But there are several drawbacks.
Debugging the output of the compiler is not fun if it's WebAssembly. It's a lot easier to spot bugs in generated C code than in generated WebAssembly.
Writing Kernel code in WebAssembly is even worse. I even tried building a little DSL to generate Wasm kernel code from Haskell, but it was practically impossible to debug. I gave up on it. In theory you could compile Elm to WebAssembly but have kernel code in C, but this makes everything very hard.
Having both the compiled code and kernel in C makes everything vastly more do-able. I don't think this project would actually go anywhere if it was direct-to-Wasm.
As for performance, you might expect things to be faster if you skip the C "layer" but I think at this early stage it's more likely the other way around!
C compilers have had a lot of work put into them by hundreds of experts for over 50 years. They have a lot of advanced low-level optimisations built-in. I would have to figure out how to manually do a lot of the basic optimisations before even getting started on Elm-specific ones. And I'm just one person doing a hobby project to punch in some time in the evenings.
In summary I think that using an intermediate language is a massive productivity gain, and that any downsides are more theoretical than practical.
Rust has great WebAssembly support and I tried to use it early on. But it didn't work out well for me.
By the time the Elm compiler actually gets to the code generation phase, the program has already been validated by Elm's type checker. So I know it's OK but now I have to convince Rust that it's OK, and I found this was very difficult.
Rust is very good for hand-written code that does manual memory management, but not so much for auto-generated code running with a garbage collector. I found it was throwing errors about a lot of things that were actually good ideas in that context.
Also, a language implementation, particularly one with a GC, is going to involve a lot of
unsafeRust. That's quite advanced Rust programming. It would have been too hard for me to know when to go against the normal rules and when not to, on my very first Rust project. Someone who was proficient in Rust may have made a different decision.
For all these reasons, I very reluctantly realised that I had exhausted the alternatives and C was the only tool for this job. It's my first C project in about 10 years.