#wasm #gui #reactive

macro maple-core-macro

A VDOM-less web library with fine grained reactivity

9 unstable releases (3 breaking)

0.4.3 Apr 1, 2021
0.4.2 Apr 1, 2021
0.4.1 Mar 31, 2021
0.3.1 Mar 16, 2021
0.1.1 Mar 7, 2021

#11 in #sycamore

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Used in maple-core

MIT license

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Crates.io docs.rs GitHub contributors Discord

A VDOM-less web library with fine-grained reactivity.

Getting started

The recommended build tool is Trunk. Start by adding maple-core to your Cargo.toml:

maple-core = "0.4.3"

Add the following to your src/main.rs file:

use maple_core::prelude::*;

fn main() {
    let root = template! {
        p {
            "Hello World!"

    render(|| root);

That's it! There's your hello world program using maple. To run the app, simply run trunk serve --open and see the result in your web browser.

The template! macro

maple uses the template! macro as an ergonomic way to create complex user interfaces.

// You can create nested elements.
template! {
    div {
        p {
            span { "Hello " }
            strong { "World!" }

// Attributes (including classes and ids) can also be specified.
template! {
    p(class="my-class", id="my-paragraph", aria-label="My paragraph")

template! {
    button(disabled="true") {
        "My button"

// Events are attached using the `on:*` directive.
template! {
    button(on:click=|_| { /* do something */ }) {
        "Click me"


Instead of relying on a Virtual DOM (VDOM), maple uses fine-grained reactivity to keep the DOM and state in sync. In fact, the reactivity part of maple can be used on its own without the DOM rendering part.

Reactivity is based on reactive primitives. Here is an example:

use maple_core::prelude::*;
let state = Signal::new(0); // create an atom with an initial value of 0

If you are familiar with React hooks, this will immediately seem familiar to you.

Now, to access the state, we call the .get() method on state like this:

println!("The state is: {}", state.get()); // prints "The state is: 0"

To update the state, we call the .set(...) method on state:

println!("The state is: {}", state.get()); // should now print "The state is: 1"

Why would this be useful? It's useful because it provides a way to easily be notified of any state changes. For example, say we wanted to print out every state change. This can easily be accomplished like so:

let state = Signal::new(0);

create_effect(cloned!((state) => move || {
    println!("The state changed. New value: {}", state.get());
}));  // prints "The state changed. New value: 0" (note that the effect is always executed at least 1 regardless of state changes)

state.set(1); // prints "The state changed. New value: 1"
state.set(2); // prints "The state changed. New value: 2"
state.set(3); // prints "The state changed. New value: 3"

How does the create_effect(...) function know to execute the closure every time the state changes? Calling create_effect creates a new "reactivity scope" and calling state.get() inside this scope adds itself as a dependency. Now, when state.set(...) is called, it automatically calls all its dependents, in this case, state as it was called inside the closure.

What's that cloned! macro doing?

The cloned! macro is an utility macro for cloning the variables into the following expression. The previous create_effect function call could very well have been written as:

    let state = state.clone();
    move || {
        println!("The state changed. New value: {}", state.get());

This is ultimately just a workaround until something happens in Rust RFC #2407.

We can also easily create a derived state using create_memo(...) which is really just an ergonomic wrapper around create_effect:

let state = Signal::new(0);
let double = create_memo(cloned!((state) => move || *state.get() * 2));

assert_eq!(*double.get(), 0);

assert_eq!(*double.get(), 2);

create_memo(...) automatically recomputes the derived value when any of its dependencies change.

Now that you understand maple's reactivity system, we can look at how to use this to update the DOM.

Using reactivity with DOM updates

Reactivity is automatically built-in into the template! macro. Say we have the following code:

use maple_core::prelude::*;

let state = Signal::new(0);

let root = template! {
    p {

This will expand to something approximately like:

use maple_core::prelude::*;
use maple_core::internal;

let state = Signal::new(0);

let root = {
    let element = internal::element(p);
    let text = internal::text(String::new() /* placeholder */);
    create_effect(move || {
        // update text when state changes

    internal::append(&element, &text);


If we call state.set(...) somewhere else in our code, the text content will automatically be updated!


Components in maple are simply functions that return HtmlElement. They receive their props through function arguments.

For components to automatically react to prop changes, they should accept a prop with type StateHandle<T> and call the function in the template! to subscribe to the state.

Getting a StateHandle<T> for a Signal<T> is easy. Just call the .handle() method.

Here is an example of a simple component:

// This is temporary and will later be removed.
// Currently, the template! macro assumes that all components start with an uppercase character.

use maple_core::prelude::*;

fn Component(value: StateHandle<i32>) -> TemplateResult {
    template! {
        div(class="my-component") {
            "Value: " (value.get())

// ...
let state = Signal::new(0);

template! {

state.set(1); // automatically updates value in Component


Issue reports and PRs are welcome! Get familiar with the project structure with ARCHITECTURE.md.


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