Andy jests, but I would actually like to add nominal types to Wasm (along with type imports to make them usable). No proposal yet, but maybe later this year.
This blog post mentions that you can kind of emulate nominal types by putting all your types in one rec group, but then it brushes that off as inferior to using exceptions. (Which is hilarious! Good work, Andy.) What it doesn’t make clear is that people actually use this rec group trick in practice. There are two ways to do it: you can put literally all your types in one rec group, or you can emit minimal rec groups with additional “brand types” that serve no purpose but to ensure the groups have different structures. The former solution is better for code size when the entire application is one module, but the latter solution is better if there are multiple modules involved. You don’t want to repeat every type definition in every module, and using smaller rec groups lets you define only the types that are (transitively) used in each module.
The Binaryen optimizer has to ensure that it does not accidentally give distinct types the same structural identity because that would generally be observable by casts. Most of its type optimizations therefore put all the types in one rec group. However, it does have a type merging optimization that takes the casts into account[0]. That optimization is fun because it reuses the DFA minimization code from the original equirecursive type system we were experimenting with for Wasm GC. We also have a rec group minimization optimization[1] that creates minimal rec groups (by finding strongly connected components of the type definition graph), then ensures the types remain distinct first by using different permutations of the types within a rec group and then only as necessary by adding brand types.
[0]: https://github.com/WebAssembly/binaryen/blob/main/src/passes...
[1]: https://github.com/WebAssembly/binaryen/blob/main/src/passes...
I'm using WASM via Emscripten almost since the beginning but have never encountered 'rec' or 'struct' (or generally types beyond integers and floats). Why would WASM even need to know how structs are composed internally, instead of 'dissolving' them at compile time into offsets? Was this stuff coming in via the GC feature?
I've been learning to use WebAssembly directly, by hand-writing the Lisp-like assembly text, and also compiling C to Wasm without Emscripten or LLVM. It's given me a deeper appreciation for the original specification, version 1. Its technical design is a solid foundation, a lot of good thought went into it: Briging the Web Up to Speed With WebAssembly - https://github.com/WebAssembly/spec/raw/refs/tags/wg-1.0/pap...
In a way it's a complete instruction set and bytecode format, that could have been frozen in time and still be a useful addition to x86, ARM, RISC-V. It's great that the Wasm v1 specs is small enough for various implementations to arise. There are Wasm interpreters written in C, Zig, Go, Rust, as well as Wasm to C compiler, disassembler, little languages that compile to Wasm.. I see great value in that simplicity and smallness, it contributes to easier cross-platform and cross-language compatibility.
That's why newer features in the specs after v1, like garbage collection, components, interfaces, feel like they're higher-level abstractions that are not so relevant for my use case. Some feel like they could be developed outside of Wasm specs, like you said, dissolved at compile time to existing primitives. I'm guessing much of the benefit is for integrating with Rust ecosystem, and perhaps other languages gradually.
Yes, this is all part of Wasm GC. WebAssembly needs to know the structures of heap objects so that a GC can trace them and also to preserve type safety when accessing them. Treating the heap objects as uninterpreted bags of bytes wouldn't have worked because so many of their fields are references, which must remain opaque in Wasm.