How to represent callbacks in WASI?

[npetrangelo]

I'm looking to create a Signal engine like how SolidJS works, with a WASI interface. The way Solid handles dependency tracking is the provided callback is called once during declaration and the getters add the callback as a dependency of their signals, using a global variable to store the callback for the getters to reference. To implement this kind of functionality, I need to implement callbacks. In other words, the engine will provide a createEffect() function that a wasm component will use by passing in the callback to invoke whenever the signals update. Since WASI doesn't seem to support functions as parameters directly, what's the best way to do this?

[npetrangelo]

Someone elsewhere suggested I use a resource. This makes sense to me. Following this, createEffect() will be a constructor, and the resulting object will have get() and set() methods.

That being said, the constructor must still accept a callback. I think I would like to implement this by having the constructor take the function's name as a parameter, so it can look up the function and save it for later use. WASM can export any function, so at that level this should be possible. However, although the function exists for a concrete component implementation, it does not exist in the interface. So now my question is, does the current tooling make it easy for a component to export a function outside of the WIT's definition?

[lukewagner]

FWIW, this is definitely a known expressivity gap with WIT (other examples discussed and linked to CM/#223 and, as discussed near the end of that issue, it's something I think we should fix by adding a proper callback type to WIT.