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WebAssembly will soon be able to use the same high-level types in Python, Rust, and Node says Lin Clark, a Principal Research Engineer at Mozilla, with the help of a new proposal: WebAssembly Interface Types. This proposal aims to add a new set of interface types that will describe high-level values like strings, sequences, records, and variants in WebAssembly.

Why WebAssembly Interface Type matters

Mozilla and many other companies have been putting their efforts into bringing WebAssembly outside the browser with projects like WASI and Fastly’s Lucet. Developers also want to run WebAssembly from different source languages like Python, Ruby, and Rust. Clark believes there are three reasons why developers want to do that. First, this will allow them to easily use native modules and deliver better speed to their application users. Second, they can use WebAssembly to sandbox native code for better security. Third, they can save time and maintenance cost by sharing native code across platforms.

However, currently, this “cross-language integration” is very complicated. The problem is that WebAssembly currently only supports numbers, so it becomes difficult in cases like passing a string between JS and WebAssembly. You will first have to convert the string into an array of numbers and then convert them back into a string. “This means the two languages can call each other’s functions. But if a function takes or returns anything besides numbers, things get complicated,” Clark explains. So, to get past this hurdle you either need to write “a really hard-to-use API that only speaks in numbers” or “add glue code for every single environment you want this module to run in.”


This is why Clark and her team have come up with WebAssembly Interface Types. It will allow WebAssembly modules to interoperate with modules running in their own native runtimes and other WebAssembly modules written in different source languages. It will also be able to talk directly with the host systems. It will achieve all of this using rich APIs and complex types.

Source: Mozilla

WebAssembly Interface Types are different from the types we have in WebAssembly today. Also, there will not be any new operations added to WebAssembly because of them. All the operations will be performed on the concrete types on both communicating sides. Explaining how this will work, Clark wrote, “There’s one key point that makes this possible: with interface types, the two sides aren’t trying to share a representation. Instead, the default is to copy values between one side and the other.

What WebAssembly developers think about this proposal

The news sparked a discussion on Hacker News. A user commented that this could in the future prevent a lot of rewrites and duplication, “I’m very happy to see the WebIDL proposal replaced with something generalized. 

The article brings up an interesting point: WebAssembly really could enable seamless cross-language integration in the future. Writing a project in Rust, but really want to use that popular face detector written in Python? And maybe the niche language tokenizer written in PHP? And sprinkle ffmpeg on top, without the hassle of target-compatible compilation and worrying about use after free vulnerabilities?

No problem use one of the many WASM runtimes popping up and combine all those libraries by using their pre-compiled WASM packages distributed on a package repo like WAPM, with auto-generated bindings that provide a decent API from your host language.

Another user added, ”Of course, cross-language interfaces will always have tradeoffs. But we see Interface Types extending the space where the tradeoffs are worthwhile, especially in combination with wasm’s sandboxing.

Some users are also unsure that this will actually work in practice. Here’s what a Reddit user said, “I wonder how well this will work in practice. effectively this is attempting to be universal language interop. that is a bold goal. I suspect this will never work for complicated object graphs. maybe this is for numbers and strings only.

I wonder if something like protobuf wouldn’t actually be better. it looked from the graphics that memory is still copied anyway (which makes sense, eg going from a cstring to a java string), but this is still marshalling. maybe you can skip this in some cases, but is that important enough to hinge the design there?

To get a deeper understanding of WebAssembly Interface Types, watch this explainer video by Mozilla:

Also, check out Lin Clark’s article, WebAssembly Interface Types: Interoperate with All the Things.

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