WASM Imports/Exports · beginners

Stream: beginners

Topic: WASM Imports/Exports

theduke (Sep 24 2022 at 16:25):

Hey there.

Just starting to play around with Roc.
It's great to see Webassembly support.

Is there already a good way to define WASM exports and imports?

Folkert de Vries (Sep 24 2022 at 16:27):

is that the wasm/JS FFI? I think is is something that the host (so zig/rust/?) code would do

theduke (Sep 24 2022 at 16:35):

It's basically the equivalent of external function definitions or publically exported functions (on a linker level).
But they need to be defined in the compiled .wasm file.

How does the Wasm compilation pipeline work?
Will both host code and the Roc code be compiled do Wasm and somehow combined?

Folkert de Vries (Sep 24 2022 at 16:37):

yes

theduke (Sep 24 2022 at 16:56):

Is there an example for writing a custom platform in Rust?

Brian Carroll (Sep 24 2022 at 16:57):

The concept of Wasm imports/exports is not really visible inside Roc itself. Rather in the host language you define externs for whatever you expect to be "dynamically linked" with JS at runtime.

Brian Carroll (Sep 24 2022 at 16:58):

The examples directory in the repo has several platforms in Rust.

Folkert de Vries (Sep 24 2022 at 16:58):

in particular https://github.com/roc-lang/roc/tree/main/examples/platform-switching/rust-platform

Folkert de Vries (Sep 24 2022 at 16:58):

which passes a string between the roc and rust code

Folkert de Vries (Sep 24 2022 at 16:59):

and https://github.com/roc-lang/roc/tree/main/examples/interactive/cli-platform which does a lot more

Brian Carroll (Sep 24 2022 at 16:59):

One of the examples there is a WebAssembly platform and there's an HTML page to run it in, but the host is in Zig rather than Rust.

Folkert de Vries (Sep 24 2022 at 17:00):

https://github.com/roc-lang/roc/blob/main/examples/false-interpreter/platform/src/lib.rs is perhaps a good intermediate rust platform

theduke (Sep 24 2022 at 17:00):

Yeah, I sort of got that.
So I would have to:

For imports:

define the extern fn in Rust
Provide those to the guest

For exports:

Receive the functions form the guest, I guess that would work like with main
Export those functions. I guess I will need some global variables to store function pointers that I can then fill in the platform initialization code.

Right?

Brian Carroll (Sep 24 2022 at 17:00):

The false interpreter doesn't compile to wasm currently though. One of the Rust crates doesn't support that target.

Folkert de Vries (Sep 24 2022 at 17:01):

we're also working on (more) codegen to make passing stuff across the boundary easier, and making all of the manual weird function signatures go away

theduke (Sep 24 2022 at 17:01):

PS: is there a WASI platform yet?
If not I could look into contributing that.

Brian Carroll (Sep 24 2022 at 17:03):

I'm not sure your description of exports is right... are you referring to guest as Roc code or JS code?

Brian Carroll (Sep 24 2022 at 17:03):

I would imagine Roc

Brian Carroll (Sep 24 2022 at 17:03):

But then that's not a Wasm export because both Roc and Rust compile to wasm

Brian Carroll (Sep 24 2022 at 17:04):

So you have two boundaries here. There's the Roc app/host boundary and then within the host there's a Rust/JS boundary.

theduke (Sep 24 2022 at 17:04):

Well, I would need to have a Webassembly exported function that then calls into Roc code.

theduke (Sep 24 2022 at 17:05):

But I think with the examples I have enough to work something out.

Brian Carroll (Sep 24 2022 at 17:09):

theduke said:

Well, I would need to have a Webassembly exported function that then calls into Roc code.

I don't quite follow what you mean here.
Hmm.. maybe this is a good way to say it: From Rust's point of view, JS code and Roc code are both "extern C".

Brian Carroll (Sep 24 2022 at 17:10):

Also worth noting that the Roc app and host end up as a single Wasm module, not two separate ones.

theduke (Sep 24 2022 at 17:13):

From Rust's point of view, JS code and Roc code are both "extern C".

Ah, I see.

#[link_name = "roc__mainForHost_1_exposed_generic"]
    fn roc_main(_: &mut RocStr);

So Roc exports are also just extern fns.
That makes it easy then to forward those as Wasm-level exports.

Brian Carroll (Sep 24 2022 at 17:17):

Right yeah I suppose so!

theduke (Sep 24 2022 at 17:19):

Is there some concept in place or plans around creating alignment between platforms?
Or make code run on multiple platforms?

Easy example: file handles

theduke (Sep 24 2022 at 17:19):

Would be very awkward to duplicate code that works with files between WASI and eg the standard CLI.

Brian Carroll (Sep 24 2022 at 17:20):

The same Roc platform can compile to many targets.

Brian Carroll (Sep 24 2022 at 17:21):

So you can write a Rust platform that supports a WASI target and a Linux target and a Windows target.

Brian Carroll (Sep 24 2022 at 17:22):

Currently we don't have one of those though

Brendan Hansknecht (Sep 24 2022 at 17:23):

Just linking this in case it is useful as a reference: https://github.com/bhansconnect/functional-mos6502-web-performance/tree/master/implementations/roc-effectful

This is an emulator for a cpu in Roc that run in the browser using wasm. It is pretty simple, but it shows a full js platform with wasm app. That being said, there is a decent chance the roc is outdated and needs to be update some if you were to directly copy from it.

theduke (Sep 24 2022 at 17:31):

Uh, I just saw wasm3 is a dependency.
Is that somehow for the repl?

theduke (Sep 24 2022 at 17:32):

Or maybe just for tests?

Folkert de Vries (Sep 24 2022 at 17:32):

just for tests right now I believe

Folkert de Vries (Sep 24 2022 at 17:32):

repl is planned

Last updated: Jul 26 2025 at 12:14 UTC