Stream: wasmtime

Topic: How do you run a function exported in an interface?

Xandaros (Mar 21 2024 at 22:27):

Currently playing around with components, trying to understand how everything fits together. I think I understand the gist of it, but I really struggle with running a function exported from an interface.

I'm using this WIT:

package cluster:main@0.1.0;

interface main {
    test: func(message: string);
}

interface runnable {
    run: func();
}

world cluster {
    import main;
    export runnable;
}

If, instead of export runnable;, I use export run: func();, I can use instance.get_typed_func::<(), ()>(&mut store, "run") on the instance of the component to get a function I can then execute.
However, how do I get this run function if the world is instead exporting this runnable interface?

Using the same invocation, get_typed_func returns an Err. Instead of "run", I have also tried "runnable/run", "cluster:main/run", and some other variations to no avail. The documentation is not particularly helpful here, the Instance::exports function does not appear very helpful either. The option to list everything that is being exported would be quite useful here.

Lann Martin (Mar 21 2024 at 23:04):

You can do this with the wasm-tools cli command e.g. wasm-tools dump <wasm> | grep Export

Lann Martin (Mar 21 2024 at 23:08):

Programmatically there is also a more recently added introspection mechanism that gives more detail. You can access that via https://docs.rs/wasmtime/latest/wasmtime/component/struct.Linker.html#method.substituted_component_type

Xandaros (Mar 21 2024 at 23:29):

Aha! Thank you very much!

I did try wasm-tools component wit, but that wasn't very helpful. With wasm-tools dump, I do find cluster:main/runnable@0.1.0#run, but that is not immediately runnable like that, either.
With the introspection, though, I noticed that it's a separate instance, and I can get the function with

instance
        .exports(&mut store)
        .instance("cluster:main/runnable@0.1.0")
        .unwrap()
        .typed_func::<(), ()>("run")
        .unwrap();

This does mean I need to know the exact version, but I suppose I can use the introspection API to match it more loosely.

I will say, though, substituted_component_type is not something I would have thought useful from the name, nor would I have expected to look for something in Linker. It makes sense in hindsight, though.

Alex Crichton (Mar 21 2024 at 23:38):

FWIW the main branch has Component::component_type which should be much more discoverable

Xandaros (Mar 21 2024 at 23:56):

Oh yeah, that is much better. Especially with that documentation - very nice

Last updated: Dec 06 2025 at 06:05 UTC