Strings & Lists

Strings and Lists

Now we know how to write a WebAssembly library and add two numbers, let's work with something slightly more interesting - strings and lists!

First, we need to create a new project to hold our code. We'll also remove the existing code, so we start from blank slate.

$ cargo new --lib tutorial-02
$ cd tutorial-02 && rm src/lib.rs

The WIT File

Just like last time, our first step is to define a WIT file for our interface.

This file has two functions, the first function will create a string that greets a person by name (i.e. "Hello, Michael!")...

// strings-and-lists.wai

/// Greet a person by name.
greet: func(name: string) -> string

... and the other function will take a list of people's names, greeting them all at the same time.

/// Say hello to multiple people.
greet-many: func(people: list<string>) -> string

Writing Some Rust

Now we've defined our strings-and-lists.wai file, let's implement the crate.

The first thing we need to do is add wai-bindgen as a dependency.

$ cargo add wai-bindgen-rust

We also need to tell wai-bindgen that we're implementing strings-and-lists.wai.

// src/lib.rs

Next, we need to define a StringsAndLists type and implement the strings_and_lists::StringsAndLists on it.

struct StringsAndLists;
impl strings_and_lists::StringsAndLists for StringsAndLists {
    fn greet(name: String) -> String {
        format!("Hello, {name}!")
    fn greet_many(people: Vec<String>) -> String {
        match people.as_slice() {
            [] => "Oh, nobody's there...".to_string(),
            [person] => format!("Hello, {person}!"),
            [people @ .., last] => {
                let people = people.join(", ");
                format!("Hello, {people}, and {last}!")

The implementation of these functions is fairly straightforward, so we don't need to go into too much detain about it other than pointing out greet_many()'s use of Slice Patterns (opens in a new tab).

A Note on Ownership

While our code wasn't overly complex, there is something that needs to be pointed out,

Both functions use owned values for their arguments and results

This may seem odd, because it's idiomatic in normal Rust to pass strings and lists around by reference (i.e. &str and &[String]) so the caller can maintain ownership of the original value and doesn't need to make unnecessary copies.

This comes back to one of the design decisions for WebAssembly, namely that a guest (our tutorial-02 crate) is completely sandboxed and unable to access memory on the host.

That means when the host calls our WebAssembly function, arguments will be passed in by

  1. Allocate some memory inside the guest's linear memory
  2. Copy the value into this newly allocated buffer
  3. Hand ownership of the buffer to the guest function (i.e. our greet() method)

Luckily wai-bindgen will generate all the code we need for this, but it's something to be aware of.

Another thing to keep in mind is that all return values must be owned, too.

WebAssembly doesn't have any concept of ownership and borrowing, so it'd be easy for the host to run into use-after-free issues and dangling pointers if we were allowed to return non-'static values.


Similar to last time, if we want to publish our package to WAPM, we'll need to update our Cargo.toml file.

# Cargo.toml
description = "Greet one or more people"
crate-type = ["cdylib", "rlib"]
namespace = "wasmer"
abi = "none"
bindings = { wai-bindgen = "0.1.0", exports = "strings-and-lists.wai" }

Now, we can publish it to WAPM.

$ cargo wapm
Successfully published package `wasmer/tutorial-02@0.1.0`

Using The Bindings From TypeScript

For a change, let's use our bindings from TypeScript. First, we need to create a basic package.json file.

$ yarn init --yes
success Saved package.json

We'll need to install TypeScript and ts-node.

$ yarn add --dev ts-node typescript @types/node

The TypeScript compiler will need a basic tsconfig.json file.

// tsconfig.json
  "compilerOptions": {
    "target": "es2016",
    "module": "ESNext",
    "moduleResolution": "node",
    "strict": true,
    "skipLibCheck": true

Now, we can use wapm install to add our tutorial-02 package as a dependency.

$ wapm install --yarn wasmer/tutorial-02

Finally, we're able to start writing some code.

// index.ts
import { bindings } from "@wasmer/tutorial-02";
async function main() {
  const strings = await bindings.strings_and_lists();
  console.log(strings.greetMany(["a", "b", "c"]));

If we run it using the ts-node loader, we'll see exactly the output we expect.

$ node --loader ts-node/esm index.ts
Hello, World!
Hello, a, b, and c!


Strings and lists are the building blocks of all meaningful applications, so it's important to know how to use them.

Our first foray into non-primitive types has also introduced us to the repercussions of running your code inside a fully sandboxed virtual machine - any data received from the outside world must be copied into linear memory.