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Comment by sjrd | Hacker News Reader

I develop the Scala-to-Wasm compiler, and also maintain the JVM backend of Scala. I can tell you that Wasm is very different from JVM bytecode.

The fundamental difference is that the JVM bytecode has an object model. When they talk to each other, Java, Scala and Kotlin do so at the abstraction level of the JVM object model. You can directly call methods between them because virtual dispatch of methods is a concept with semantics in the bytecode.

There's no such thing in Wasm, even with the GC extension. You get structs and arrays, but nothing like methods. If you want virtual dispatch, you encode it yourself using your own design of virtual method tables. That means Java, Scala and Kotlin, despite all having their Wasm GC backend at this point, cannot call each other's methods in Wasm.

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Yes, in retrospect I should have mentioned "Components" in my comment and just compared it to "Java" in general instead. I felt that was implied due to the thread and topic of discussion.

From a pragmatic end-user point-of-view explanation, I would still stand by saying WASM Component Model may be similar to working within the JVM multi-language ecosystem. One can work with code compiled from multiple different languages, but the generated code may be different because of the different compilers.

I am confused. You are referring to wasm modules here? And Component Model / WASI / WIT will give us polyglot interface-based programming then, right? Call each other's methods through the WIT interface between components.

Even the component model has nothing to say about the concept of methods. All you have are top-level functions with immutable arguments and immutable results. You can't hold on to an instance of an object created by the other language. You could hold an integer handle, but then you don't get garbage collection across the two languages.

So no, we're still a long way from the abstractions of a JVM, even taking the component model into account.

It's a good step in the direction of better interoperability between languages, though, don't get me wrong.

For the audience that would be looking to use the WASM Component Model, and not be an infrastructure implementer of it, whether or not they meet some definition of a method, the component model does define things called resources [1] that have "methods". You'll hold a "handle" to it like you would in your own programming language with the expected drop and/or GC semantics (once implemented [2]) because code is generated to access it like any other FFI like C/C++.

With that in mind, the other confusing thing one may come across is composition vs linking within your WASM runtime that supports the Component Model. When you hear "composition" think of compile-time merging of libraries such that the bundle may have less unresolved dependencies of WASM code/implemented component interfaces. Anything unresolved needs to be linked at runtime with your WASM runtime of choice, like wasmtime [3]. Pretty interesting reading/potential after reading if you ask me -- sounds like you could implement something like a custom Java classloader hierarchy [4].

But I'd agree with a statement saying it is still a long way for general usage.

[1] https://github.com/WebAssembly/component-model/blob/5a34794d...

[2] https://github.com/WebAssembly/gc

[3] https://github.com/bytecodealliance/wasmtime/blob/ba8131c6bf...

[4] https://www.digitalocean.com/community/tutorials/java-classl...

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