Exposing Resources

So far we've covered basic functions, numbers, strings, lists, records, and variants, but there's one key aspect in programming we haven't touched on yet - objects with behaviour and internal state!

In WIT, we use a resource to give the host access to an "object" that has behaviour without exposing how that object is implemented (or even which language it is implemented in).

To explore this concept, we'll create a basic Calculator resource which lets the caller do various operations like addition and multiplication.

Like always, let's start off with a new Rust project.

$ cargo new --lib tutorial-05

The WIT File

The key thing to understand about a resource is that it only defines methods.

// resources.wai

resource calculator {
    static new: func(initial-value: float32) -> calculator
    current-value: func() -> float32
    add: func(value: float32)
    multiply: func(value: float32)

Prefixing a function with the static keyword will turn it into a static method (opens in a new tab). This is useful for defining constructors or factory functions.

Resource methods also allow the async modifier (i.e. add: async func(value: float32)), however that will require your runtime to support async functions.

Writing Some Rust

As always, we need to add wai-bindgen as a dependency.

$ cargo add wai-bindgen-rust

We also need to ask wai-bindgen to generate exports for our resources.wai interface.

// src/lib.rs

If we run cargo check, we'll see that - besides the missing Resources type we expected - it complains about not being able to find Calculator.

$ cargo check
error[E0412]: cannot find type `Calculator` in module `super`
error[E0412]: cannot find type `Resources` in module `super`

We can create the Resources struct and implement the resources::Resources trait for it. This module won't have any top-level functions, so the trait implementation can stay empty.

pub struct Resources;
impl resources::Resources for Resources {}

The way the resources::Calculator trait was designed adds certain constraints to our Calculator struct, though.

pub struct Calculator(Mutex<f32>);
impl resources::Calculator for Calculator {
    fn new(initial_value: f32) -> Handle<Calculator> {
    fn current_value(&self) -> f32 {
    fn add(&self, value: f32) {
        *self.0.lock().unwrap() += value;
    fn multiply(&self, value: f32) {
        *self.0.lock().unwrap() *= value;

A Note On Mutability

You'll notice that all methods in the resources::Calculator trait take an immutable &self. This means we'll need to use interior mutability (opens in a new tab) if we want to update internal state.

While this might feel a bit awkward, there is a very good reason for requiring all state in a resource to synchronise its mutations - WebAssembly makes no guarantees that the caller will respect the borrow checker.


We need to publish this package to WAPM, so let's update Cargo.toml with the relevant metadata.

# Cargo.toml
description = "A basic calculator"
crate-type = ["cdylib", "rlib"]
namespace = "wasmer"
abi = "none"
bindings = { wai-bindgen = "0.1.0", exports = "resources.wai" }

Now, we can publish it to WAPM.

$ cargo wapm
Successfully published package `wasmer/tutorial-05@0.1.0`

Using The Bindings From Python

Once the package has been published, create a new Python virtual environment and import the package into Python.

$ python -m venv env
$ source env/bin/activate
$ wapm install --pip wasmer/tutorial-05

Next, let's create our test script.

# main.py
from tutorial_05 import bindings
from tutorial_05.bindings.resources import Calculator
resources = bindings.resources()
calculator = Calculator.new(resources, 3)
print("(3 + 5)*2 =", calculator.current_value())

Running it from the command-line should show us that (3+5)*2 = 16.

$ python main.py
(3+5)*2 = 16


The resource is a useful tool for exposing stateful objects that have behaviour, and should be familiar to

With the addition of resources, we've introduced most of the fundamental constructs in WIT.

Exercises for the reader:

  • Expand the calculator resource to be a fully fledged calculator
  • Try to create your own Regex package using the regex crate