WAI is the Answer !!!


Ding Dong

Sir, we are here to move your package.

packers and movers

That’s right we at Wasmer move your packages for free

🗒️ Note: Package refers to a library/executable that you might have developed for a certain language

What are we going to do?

For this tutorial we will be publishing the sgp4 crate, a library used to predict the location of satellites in orbit after a certain amount of time.

You can find the dynamite-bud/sgp4 package on WAPM.

At the end we’ll be able to use sgp4 from JavaScript like this.

const resolveResult = ({ tag, val }) => (tag === 'err' ? new Error(val) : val);

const wasm = await bindings.sgp4();
let response = await fetch(
let elementsArr = (await response.json()).map((e) =>
  resolveResult(Elements.fromJson(wasm, JSON.stringify(e))),
for (let elements of elementsArr) {
  let constants = resolveResult(Constants.fromElements(wasm, elements));
  for (let hours of [12, 24]) {
    console.log(`    t = ${hours * 60} min`);
    let prediction = resolveResult(constants.propagate(parseFloat(hours * 60)));
    console.log(`        r = ${prediction.position} km`);
    console.log(`        ṙ = ${prediction.velocity} km.s⁻¹`);

Here we used the official introductory example from original sgp4.

Tutorial Outline:

  1. Installation and Environment Setup
  2. Create and use the WAI
  3. Library Implementation
  4. Publish to WAPM
  5. Using the package from JavaScript
  6. Conclusion


Environment Setup

You will need to install several CLI tools.

  $ curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
  • the wasm32-unknown-unknown target so Rust knows how to compile to WebAssembly
  $ rustup target add wasm32-unknown-unknown
  $ curl https://get.wasmer.io -sSfL | sh
  $ cargo install cargo-wapm

Once you've installed those tools, you'll want to create a new account on wapm.io so we have somewhere to publish our code to.

Running the wapm login command will let you authenticate your computer with WAPM.

Project Setup

We want to start with a blank library project named sgp4.

$ cargo new --lib sgp4
$ cd sgp4

Cargo TOML Setup

In order to publish to WAPM, we’ll need to populate the project’s Cargo.toml file with some extra metadata.

💡 Some required attributes by WAPM are name, description and version.

Another property to update is crate-type. This can take various arguments such as cdylib , rlib , etc. For more info on linkage.

🔗 crate-type here specifies the kind of linkage we want. We’ll be using cdylib for generating a C Dynamic Library which will be further used to generate a .wasm binary.

⚠️ We will also introduce a package rename in our dependencies, otherwise we’d get naming conflicts as our package is also named sgp4. Therefore, we refer to the original sgp4 rust crate as original.

# Cargo.toml

name = "sgp4"
version = "0.1.0"
description = "sgp4 for wasm, published on WAPM"

crate-type = ["cdylib"]

wai-bindgen-rust = "0.2.2"
original = { version = "0.8.2", package = "sgp4" }

Create and use WAI

What is WAI ?

WAI or WebAssembly Interfaces is an Interface Definition Language (IDL)” like Protobufs or WebIDL, for interaction with multiple languages.

Files for WAI use the *.wai extension.

The WAI project has a variety of code generators, however the one we’ll be using today is wai_bindgen_rust. This Rust crate uses macros to take a WAI file and generate all the glue code we need to implement a WebAssembly module in Rust.

Let’s Port! 🪄

Creating the WAI file

Now let’s create a sgp4.wai file for our library for defining what our generated library should have.

$ touch sgp4.wai

The original sgp4 crate has multiple members in structs, enums and constants and functions.

Let’s look at a couple examples from the sgp4 crate’s public API and go through the thought process for representing them in WAI.


In the original crate, Orbit is defined as a struct that has only one constructor and no associated behaviour. The easiest way to represent this in WAI is using a record and a top-level function returning a new instance of that record.

record orbit {
    inclination: float64,
    right-ascension: float64,
    eccentricity: float64,
    argument-of-perigee: float64,
    mean-anomaly: float64,
    mean-motion: float64,

/// Create a new Brouwer orbit representation from Kozai elements.
orbit-from-kozai-elements: func(
    geopotential: geopotential,
    inclination: float64,
    right-ascension: float64,
    eccentricity: float64,
    argument-of-perigee: float64,
    mean-anomaly: float64,
    kozai-mean-motion: float64,
) -> expected<orbit, error>

The ResonanceState type is treated like an “object” with internal state and associated behaviour.

resource resonance-state {
    t: func() -> float64


Note: Enums in wai are compiled directly and don’t require an implementation. They also don’t have a specified type with them.

enum classification {


WAI doesn’t allow WebAssembly modules to expose constants directly, so we introduce a function the WGS72 constant.

wgs72: func() -> geopotential


iau-epoch-to-sidereal-time: func(epoch: float64) -> float64

💡 For the full code check this repository

Using the WAI file

We want to tell the wai_bindgen_rust crate that this crate exports sgp4.wai so that it can generate glue code for this wai file.

// lib.rs

💡 Note: sgp4.wai is relative to the crate's root - the folder containing your Cargo.toml file

Now, As we included this sgp4.wai in our lib.rs. We can do a cargo expand as a smoke test to see if the glue code gets generated.

$  cargo expand
unsafe extern "C" fn __wai_bindgen_sgp4_constants_initial_state(arg0: i32) -> i32 {
        let result = <super::Constants as Constants>::initial_state(
        let ptr0 = SGP4_RET_AREA.0.as_mut_ptr() as i32;

Library Implementation

Let’s run cargo check and use the error messages to see what we need to do next.

$ cargo check

error[E0412]: cannot find type `ResonanceState` in module `super`
  --> src/lib.rs:12:1
12 | wai_bindgen_rust::export!("sgp4.wai");
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not found in `super`
   = note: consider importing one of these items:
   = note: this error originates in the macro `wai_bindgen_rust::export` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0412]: cannot find type `ResonanceState` in module `super`
  --> src/lib.rs:12:1
12 | wai_bindgen_rust::export!("sgp4.wai");
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not found in `super`
   = note: consider importing one of these items:
   = note: this error originates in the macro `wai_bindgen_rust::export` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0412]: cannot find type `Sgp4` in module `super`
  --> src/lib.rs:12:1
12 | wai_bindgen_rust::export!("sgp4.wai");
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not found in `super`

So it looks like we need to create Sgp4,Constants, Elements and ResonanceState.

💡 Note: The type Sgp4 is a special type as it contains the top level functions we defined and it can't have a static modifier for initialisation, only member functions.

  • Sgp4

    // lib.rs
    struct Sgp4;
    impl sgp4::Sgp4 for Sgp4 {
        fn orbit_from_kozai_elements(
            geopotential: Geopotential,
            inclination: f64,
            right_ascension: f64,
            eccentricity: f64,
            argument_of_perigee: f64,
            mean_anomaly: f64,
            kozai_mean_motion: f64,
        ) -> Result<Orbit, SgpError> {
        fn wgs84() -> Geopotential {
        fn iau_epoch_to_sidereal_time(epoch: f64) -> f64 {
  • ResonanceState We gave resonance state a type of Mutex<original::ResonanceState> here as we want to hold a the original::Resonance state in a Mutex because later we see that ResonanceState is used as a mutable reference in an argument in a Constants struct’s function propagate_from_state. For more information you can follow this read.

    struct ResonanceState(Mutex<original::ResonanceState>);
    impl ResonanceState {
        fn new(state: original::ResonanceState) -> Self {
    impl sgp4::ResonanceState for ResonanceState {
        fn t(&self) -> f64 {
            self.0.lock().expect("The mutex was poisioned").t()

All of the types we’ve implemented are just wrappers around the corresponding struct from the original sgp4 crate. We can implement the From trait to make converting back and forth easier.

🔗 The From implementations can be found here

Publishing to WAPM

Now we’ve ported sgp4 we can publish it to WAPM.

For publishing to wapm.io we will use the cargo-wapm subcommand.

The cargo wapm sub-command can do a lot of the heavy lifting (compiling to WebAssembly, constructing a WAPM package, publishing, etc.), but it needs some extra metadata to do its job:

# Cargo.toml

namespace = "dynamite-bud"
abi = "none"
bindings = { wai-version = "0.2.0", exports = "sgp4.wai" }

We need three things here:

  • namespace; you need to replace it with your username on WAPM
  • abi; The application binary interface tells hosts how the library was compiled. We compiled to wasm32-unknown-unknown so we set the ABI to none.
  • bindings; It specifies the version of WAI we’re using and the path to our sgp4.wai file

Let’s try our configuration 📦:

$ cargo wapm --dry-run
2023-02-07T10:31:34.216323Z  INFO publish: cargo_wapm: Publishing dry_run=true pkg=&quot;sgp4&quot;
Successfully published package `dynamite-bud/sgp4@0.1.0`
[INFO] Publish succeeded, but package was not published because it was run in dry-run mode
2023-02-07T10:31:34.395731Z  INFO publish: cargo_wapm: Published! pkg=&quot;sgp4&quot;

Let’s Publish 🚀

Now the package is ready, let’s publish it.

$ cargo wapm
2023-02-07T10:33:22.597922Z  INFO publish: cargo_wapm: Publishing dry_run=false pkg=&quot;sgp4&quot;
[1/2] ⬆️   Uploading...
[2/2] 📦  Publishing...
Successfully published package `dynamite-bud/sgp4@0.1.0`
2023-02-07T10:33:26.705101Z  INFO publish: cargo_wapm: Published! pkg=&quot;sgp4&quot;

🎉 Yaaay! We’ve published our package to WAPM!

As you see my package is published at wapm.io. You’ll also see that the Python and JavaScript bindings were automatically generated.

WAPM package Upload

Using the Package from JavaScript 🌏

Let’s initialise an empty directory and install the JavaScript package for sgp4.

$ mkdir sgp4-js
$ wapm install dynamite-bud/sgp4@0.1.0 --npm
up to date, audited 2 packages in 3s
found 0 vulnerabilities

Our package.json would be updated to this

  "dependencies": {
    "@dynamite-bud/sgp4": "https://registry-cdn.wapm.io/bindings/generator-0.6.0/npm/dynamite-bud/sgp4/sgp4-0.1.0.tar.gz"

Let’s try a sample test case from original crate.

// main.js

const { bindings } = require('@dynamite-bud/sgp4');
const {
} = require('@dynamite-bud/sgp4/src/bindings/sgp4/sgp4.js');

const TEST_CASE = {
    '1 11801U          80230.29629788  .01431103  00000-0  14311-1 0    13',
    '2 11801  46.7916 230.4354 7318036  47.4722  10.4117  2.28537848    13',
  states: [
      time: 0,
      position: [7473.37102491, 428.94748312, 5828.74846783],
      velocity: [5.107155391, 6.444680305, -0.186133297],
      time: 360,
      position: [-3305.22148694, 32410.84323331, -24697.16974954],
      velocity: [-1.301137319, -1.1513156, -0.283335823],
      date: '1980-08-17T13:06:40.136Z',
      time: 720,
      position: [14271.29083858, 24110.44309009, -4725.76320143],
      velocity: [-0.320504528, 2.679841539, -2.084054355],
      date: '1980-08-17T19:06:40.136Z',
      time: 1080,
      position: [-9990.05800009, 22717.34212448, -23616.88515553],
      velocity: [-1.016674392, -2.290267981, 0.728923337],
      date: '1980-08-18T01:06:40.136Z',
      time: 1440,
      position: [9787.87836256, 33753.32249667, -15030.79874625],
      velocity: [-1.094251553, 0.923589906, -1.522311008],
      date: '1980-08-18T07:06:40.136Z',

const resolveResult = ({ tag, val }) => {
  if (tag === 'err') {
    throw val;
  return val;

const POSITION_PRECISION = Math.pow(10, -6);
const VELOCITY_PRECISION = Math.pow(10, -9);

(async () => {
  const wasm = await bindings.sgp4();
  try {
    let elements = resolveResult(
      Elements.fromTle(wasm, null, TEST_CASE.line1, TEST_CASE.line2),

    let constants = resolveResult(
      Constants.fromElementsAfspcCompatibilityMode(wasm, elements),

    let result = TEST_CASE.states.reduce((acc, state) => {
      let { time, position, velocity } = state;
      let prediction = resolveResult(

      return (
          (acc, val, i) =>
            acc & (Math.abs(val - prediction.position[i]) < POSITION_PRECISION),
        ) &
          (acc, val, i) =>
            acc & (Math.abs(val - prediction.velocity[i]) < VELOCITY_PRECISION),
    }, true);

    console.log(result ? 'Test passed' : 'Test failed');
  } catch (e) {

Running the JavaScript code:

$ node main.js
Test passed


In this article, we learned many things about the Wasmer ecosystem such as:

  • Using a *.wai to write an IDL file for your favourite package
  • Writing a library implementation for a WAI file in Rust
  • How to configure your package for WAPM
  • Ported your library to JavaScript and Python

And congratulations, with the gracious powers of WebAssembly and Wasmer; Now you have published not one but three packages.

🏋🏼 Exercise Time

Here, is a WAI file for you to experiment with and make a library for yourself.

// complex-number.wai

record complex {
  re: float32,
  im: float32,

resource calculator {
  static new: func() -> calculator
  add: func(value: complex)
  sub: func(value: complex)
  mul: func(value: complex)
  current-value: func() -> complex
  history: func() -> list<operation>

variant operation {

add: func(a: complex, b: complex) -> complex
mul: func(a: complex, b: complex) -> complex
sub: func(a: complex, b: complex) -> complex

Using this WAI file you can make a complex number calculator that can add, subtract and multiply while preserving the history of all the operations performed.

💡 Don’t forget to publish to WAPM 🚀