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Foreign Function Interface(FFI)

You can use foreign function in MoonBit through FFI


Declare Foreign Function

You can declare a foreign function like this:

fn get_pi() -> Double = "math" "get_pi"

It's similar to a normal function definition except that the function body is replaced with two strings.

These two strings are used to identify the specific function from a Wasm import object, the first string is the module name, and the second string is the function name.

After declaration, you can use foreign functions like regular functions.

Use compiled Wasm

To use the compiled Wasm, you must provide All declared foreign functions in Wasm import object.

For example, to use wasm compiled from above code snippet in javascript:

WebAssembly.instantiateStreaming(fetch("xxx.wasm"), {
math: {
get_pi: () => Math.PI,

Full example

Let's walk through a full example to draw a simle face using convas API in MoonBit.

type Canvas_ctx

fn begin_path(self: Canvas_ctx) = "canvas" "begin_path"
fn arc(self: Canvas_ctx, x: Int, y: Int, radius: Int, start_angle: Double, end_angle: Double, counterclockwise: Bool) = "canvas" "arc"
fn move_to(self: Canvas_ctx, x: Int, y: Int) = "canvas" "move_to"
fn stroke(self: Canvas_ctx) = "canvas" "stroke"
fn get_pi() -> Double = "canvas" "get_pi"

let pi: Double = get_pi()

pub fn draw(self: Canvas_ctx) {
self.arc(75, 75, 50, 0.0, pi * 2.0, true); // Outer circle
self.move_to(110, 75);
self.arc(75, 75, 35, 0.0, pi, false); // Mouth (clockwise)
self.move_to(65, 65);
self.arc(60, 65, 5, 0.0, pi * 2.0, true); // Left eye
self.move_to(95, 65);
self.arc(90, 65, 5, 0.0, pi * 2.0, true); // Right eye

Compile the file using moonc to get draw.mbt.wasm.

moonc compile draw.mbt
wat2wasm draw.mbt.wat

Use it from javascript:

<html lang="en">
<canvas id="canvas" width="150" height="150"></canvas>
const spectest = {
canvas: {
stroke_rect: (ctx, x, y, width, height) =>
ctx.strokeRect(x, y, width, height),
begin_path: (ctx) => ctx.beginPath(),
arc: (ctx, x, y, radius, startAngle, endAngle, counterclockwise) =>
ctx.arc(x, y, radius, startAngle, endAngle, counterclockwise),
move_to: (ctx, x, y) => ctx.moveTo(x, y),
stroke: (ctx) => ctx.stroke(),
get_pi: () => Math.PI,
spectest: {
print_i32: (x) => console.log(String(x)),
print_f64: (x) => console.log(String(x)),
print_char: (x) => console.log(String.fromCharCode(x)),

const canvas = document.getElementById("canvas");
if (canvas.getContext) {
const ctx = canvas.getContext("2d");

WebAssembly.instantiateStreaming(fetch("draw.wasm"), spectest).then(
(obj) => {

Make sure that draw.mbt.wasm and index.html are in the same folder, then start a http server at this folder. For example, using python:

python3 -m http.server 8080

Goto http://localhost:8080 in your browser, there should be a smile face like this: