Nominal Types in WebAssembly

https://wingolog.org/archives/2026/03/10/nominal-types-in-webassembly

by ingve • 4 months ago

39 16 4 months ago

field access is a bit odd; unlike structs which have struct.get, nominal types receive all their values via a catch handler.

I know this is meant to be silly, and I am no expert, but I kinda do like this syntax. Its like shaking the struct and seeing what falls out.

tlively3• 4 months ago

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...

flohofwoe4• 4 months ago

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?

tlively3• 4 months ago

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.

lioeters• 4 months ago

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.

kjksf• 4 months ago

Good lord. WebAssembly was sold as "portable assembly for the web". It's in the fricking name. Web. Assembly. Assembly for the web.

It was supposed to solve the problem of: some computers run x86, some arm, we need something that is equivalent, but portable across different cpus

What business is it for WebAssembly to know about complex types? What x86 instructions is there for `(type $t (struct i32))` ? Or doing garbage collection.

We would be better off standardizing on a subset of x86 and writing translators to arm etc. Or standardize on arm and translate to x86.

We know it can work. Apple did it with rosetta. Microsoft did it with Prism. I don't think WebAssembly implementation generate faster code than rosetta or prism.

QEMU did it simply (albeit slowly).

WebAssembly is becoming another JVM. It's not simple. It's not fast. It's not easy to use.

But now we're stuck with it and the only path is to add and add and add.

flohofwoe4• 4 months ago

I guess you didn't read to the end?

Friends, as I am sure is abundantly clear, this is a troll post :)

zffr• 4 months ago

It's not fast.

Not disagreeing with you, but here’s an article from Akamai about how using WASM can minimize cold startup time for serverless functions.

https://www.akamai.com/blog/developers/build-serverless-func...

flohofwoe4• 4 months ago

It's not fast.

My emulators here have roughly the same performance as the same code compiled as native executable (e.g. within around 5%) - this is mostly integer bit twiddling code. Unless you hand-optimize your code beyond what portable C provides (like manually tuned SIMD intrinsics), WASM code pretty much runs at native speed these days:

https://floooh.github.io/tiny8bit/

mavdol04• 4 months ago

I mean standardizing on an x86 subset would replace wasm's native portability with a kind of 'emulated' compatibility, and this is one of wasm's strengths. If we do that, non-x86 hardware(mobile etc.) will pay the translation tax. So, keeping Wasm agnostic makes more sense anyway.

14113• 4 months ago

This post is actually a joke, but it does bring about an important point: For an interpreter, having more information results in faster execution. WASM is much closer to Java bytecode than you might think, and SpiderMonkey/V8 are basically the JVM. WASM also undergoes multiple different stages and kinds of JIT compilation in most browsers, and detailed type and usage information helps that produce faster execution.

Also, don't forget that WASM is designed to replace JavaScript, thus it must interoperate with it to smooth the transition. Rosetta and Prism also work to smooth the transition from x86 -> ARM, and much of the difficult work that they do actually involves translating between the calling conventions of the different architectures, and making them work across binaries compiled both for and not for ARM, not with the bytecode translation. WebAssembly is designed to not have that limitation: it's much more closely aligned to JS. That's why it wouldn't make sense to use a subset of x86 or similar, as it would simply produce more work trying to get it to interface with JavaScript.

treyd• 4 months ago

We would be better off standardizing on a subset of x86 and writing translators to arm etc. Or standardize on arm and translate to x86.

This is basically what Native Client (NaCl) was, and it was really hard to work with! We don't use it anymore and developed WASM instead.

Rusky• 4 months ago

The types are there for garbage collection, which is there for integration with the Web APIs which are all defined in terms of garbage collected objects.

rusakov-field• 4 months ago

Not familiar with WebAssembly, but from the namesake was expecting the syntax to kind of resemble assembly.

He is showing S-expressions ? That is its' syntax ? I am intrigued now.

flohofwoe4• 4 months ago

Real-world WAT (WASM text format) looks more like this (e.g. it looks like a 'structured assembly' type of thing):

    i32.const 27512
    i32.load
    local.tee $var1
    if
      i32.const 27404
      i32.load
      local.get $var1
      call_indirect (param i32)
    end

S-expressions are only used outside such instruction blocks for the 'program-structure' (e.g. see: https://developer.mozilla.org/en-US/docs/WebAssembly/Referen...). IIRC early pre-release-versions of WASM were entirely built from S-expressions and as a 'pure stack machine' (I may remember wrong though).

To see what a complete WASM blob looks like in WAT format you can go here: https://floooh.github.io/sokol-html5/clear-sapp.html, open the browser devtools, go to the 'Sources' tab and click the `clear-sapp.wasm` file).

tlively3• 4 months ago

This is partially true, but the standard text format also allows the instructions to be nested as S-expressions, for example:

  (i32.add
    (i32.const 0)
    (i32.const 1))

Many projects, including the official spec test suite and the Binaryen test suite, primarily use this format.

IIRC early pre-release-versions of WASM were entirely built from S-expressions and as a 'pure stack machine' (I may remember wrong though).

Yes, the S-expressions predate WebAssembly even being a stack machine. Originally the design was that it encoded an AST, so the folded S-expression format was the only option.

There was a lot of discussion back in the day (before my time) about creating a better text format, but no one could agree on what it should be, so they just defaulted to the S-expression idea and focused on getting WebAssembly out the door.

Nominal Types in WebAssembly

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