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#1197 in WebAssembly

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Used in 5 crates (2 directly)



WebAssembly Interfaces

A language bindings generator for wai

A Wasmer project building on wai

build status supported rustc stable


Note: Unfortunately, the maintainers behind wit-bindgen didn’t want to add support for Wasmer upstream, so we had to do a hard-fork in order to make things work with Wasmer.

This project is a bindings generator framework for WebAssembly programs and embeddings of WebAssembly. This works with *.wai files which describe the interface of a module, either imported or exported. For example this project can be used in cases such as:

  • Your language (say, Rust) is compiled to WebAssembly and you'd like to import WASI. This project will generate Rust bindings to import WASI APIs that are described with *.wai.

  • Your runtime (say, Wasmer) wants to then provide WASI functionality to guest programs. This project will generate a Rust trait for you to implement for the WASI interface.

  • You're consuming a WebAssembly module (say, in a browser) and you don't want to deal with funky ABI details. You'd use this project to generate JS bindings which give you a TypeScript interface dealing with native JS types for the WebAssembly module described by *.wai.

This project is based on the interface types proposal. This repository will be following upstream changes. The purpose of wai is to provide a forwards-compatible toolchain and story for interface types and a canonical ABI. Generated language bindings all use the canonical ABI for communication, enabling WebAssembly modules to be written in any language with support and for WebAssembly modules to be consumed in any environment with language support.


View generated bindings online!

If you're curious to poke around and see what generated bindings look like for a given input *.wai, you can explore the generated code online to get an idea of what's being generated and what the glue code looks like.


At this time a CLI tool is provided mostly for debugging and exploratory purposes. It can be used easily with the wasmer CLI.

// browser.wai

record person {
  name: string,
  age: u32,

// Say hello to either the specified person or the current user
hello: func(who: option<person>) -> string
$ wasmer run wasmer/wai-bindgen-cli --dir=. -- js --import browser.wai
Generating "browser.d.ts"
Generating "browser.js"
Generating "intrinsics.js"

This tool is not necessarily intended to be integrated into toolchains. For example usage in Rust would more likely be done through procedural macros and Cargo dependencies. Usage in a Web application would probably use a version of wai-bindgen compiled to WebAssembly and published to NPM.

For now, though, you can explore what bindings look like in each language through the CLI. Again if you'd like to depend on this if you wouldn't mind please reach out on Slack so we can figure out a better story than relying on the CLI tool for your use case.

Supported Languages

First here's a list of supported languages for generating a WebAssembly binary which uses interface types. This means that these languages support *.wai-defined imports and exports.

  • rust-wasm - this is for Rust compiled to WebAssembly, typically using either the wasm32-wasi or wasm32-unknown-unknown targets depending on your use case. In this mode you'd probably depend on the wai-bindgen-rust crate (located at crates/rust-wasm) and use the import! and export! macros to generate code.

  • c - this is for C compiled to WebAssembly, using either of the targets above for Rust as well. With C the wai-bindgen CLI tool will emit a *.h and a *.c file to be compiled into the wasm module.

This repository also supports a number of host languages/runtimes which can be used to consume WebAssembly modules that use interface types. These modules need to follow the canonical ABI for their exports/imports:

  • wasmer - this is for Rust users using the wasmer crate. This generator is used through the wai-bindgen-wasmer crate (located at crates/wasmer) and, like the compiled-to-wasm Rust support, has an import! and an export! macro for generating code.

  • js - this is for JavaScript users executing WebAssembly modules. This could be in a browser, Node.js, or Deno. In theory this covers browser use cases like web workers and such as well. In this mode the wai-bindgen CLI tool will emit a *.js and a *.d.ts file describing the interface and providing necessary runtime support in JS to implement the canonical ABI. Note that the intended long-term integration of this language is to compile wai-bindgen itself to WebAssembly and publish NPM packages for popular JS build systems to integrate wai-bindgen into JS build processes.

  • wasmer-py - this is for Python users using the wasmer PyPI package. This uses Wasmer under the hood but you get to write Python in providing imports to WebAssembly modules or consume modules using interface types. This generates a *.py file which is annotated with types for usage in mypy or other type-checkers.

All generators support the --import and --export flags in the wai-bindgen CLI tool:

$ wasmer run wasmer/wai-bindgen-cli --dir=. -- js --import browser.wai
$ wasmer run wasmer/wai-bindgen-cli --dir=. -- rust-wasm --export my-interface.wai
$ wasmer run wasmer/wai-bindgen-cli --dir=. -- wasmer --import host-functions.wai

Here "import" means "I want to import and call the functions in this interface" and "export" means "I want to define the functions in this interface for others to call".

Finally in a sort of "miscellaneous" category the wai-bindgen CLI also supports:

  • markdown - generates a *.md and a *.html file with readable documentation rendered from the comments in the source *.wai file.

Note that the list of supported languages here is a snapshot in time and is not final. The purpose of the interface-types proposal is to be language agnostic both in how WebAssembly modules are written as well as how they are consumed. If you have a runtime that isn't listed here or you're compiling to WebAssembly and your language isn't listed here, it doesn't mean that it will never be supported! A language binding generator is intended to be not the hardest thing in the world (but unfortunately also not the easiest) to write, and the crates and support in this repository mostly exist to make writing generators as easy as possible.

Some other languages and runtimes, for example, that don't have support in wai-bindgen today but are possible in the future (and may get written here too) are:

  • wasmer-go - same as for wasmer-py but for Go. Basically for Go users using the wasmer-go package who want to work with interface types rather than raw pointers/memories/etc.

  • wasmer-ruby - same as for wasmer-py but for Ruby. Basically for Go users using the wasmer-ruby package who want to work with interface types rather than raw pointers/memories/etc.

Note that this is not an exclusive list, only intended to give you an idea of what other bindings could look like. There's a plethora of runtimes and languages that compile to WebAssembly, and interface types should be able to work with all of them and it's theoretically just some work-hours away from having support in wai-bindgen.


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