bbenne10/melange-restorative

Restorative is a simple ReasonML state management library. Comes with React hooks.

While you can get very far with core ReasonML and React functionality, you may find yourself with global state to manage. Restorative may be your solution. Features a reducer store with performant subscriptions and selectors.

Similar projects in JavaScript include Redux and zustand.

Installation

npm install --save restorative

Add to bsconfig.json

"bs-dependencies": [
  "restorative"
]

Create store

type state = int;
type action =
  | Increment
  | Decrement;

let api =
  Restorative.createStore(0, (state, action) =>
    switch (action) {
    | Increment => state + 1
    | Decrement => state - 1
    }
  );

Basic subscription

let {dispatch, subscribe, getState} = api;
let unsubscribe = subscribe(state => Js.log(state));
dispatch(Increment); // calls subscriptions
getState(); // 1
unsubscribe();

React hook

let {useStore, dispatch} = api;

[@react.component]
let make = () => {
  let state = useStore();
  <button onClick={_ => dispatch(Increment)}>
    {React.string(string_of_int(state))}
  </button>;
};

Selector

type state = {
  a: int,
  b: int,
};
type action =
  | IncrementA
  | IncrementB;

let {subscribeWithSelector, dispatch} =
  createStore({a: 0, b: 0}, (state, action) =>
    switch (action) {
    | IncrementA => {...state, a: state.a + 1}
    | IncrementB => {...state, b: state.b + 1}
    }
  );

subscribeWithSelector(state => state.a, a => Js.log(a), ());
dispatch(IncrementA); // calls listener
dispatch(IncrementB); // does not call listener

useStoreWithSelector

[@react.component]
let make = () => {
  let a = useStoreWithSelector(state => state.a, ());
  // Only updates when a changes
  ...
};

Equality

Restorative will not call listeners if the selected state has not "changed" (entire state if no selector). By default, uses Object.is for equality checking. All subscribe and useStore functions take an optional ~areEqual: ('state, 'state) => bool.

useStoreWithSelector(
  state => [|state.a, state.b|],
  ~areEqual=(a, b) => a == b,
  ()
);

Comparison with JavaScript libraries

We get all the benefits of Reason's great type system. Instead of plain JavaScript objects, we use variants to model actions. All operations have sound types and some work is moved to compile time (e.g. action creators).

Comparison with React Context

Restorative maintains a list of subscriptions for each store. In contrast, React Context iterates through all children Fiber nodes to find context consumers when the context value changes. React context is not well suited for fast-changing data. Subscriptions, on the other hand, allow for more precise operations at the cost of more complexity (maintaining list of subscribers).

Comparison with Redux

Redux applications typically use a single global store and dispatcher. With Restorative, you can create multiple stores, each with its own dispatcher. This allows better separation of state logic.

Quick Start

npm install

# In separate terminals:
npm run build:watch
npm run serve

React

React support is provided by reason/react. The entry point of the sample React app is src/ReactApp.re.

Commands

• npm install - installs the npm/JavaScript dependencies, including the esy package manager (see below)
• npm run build:watch - uses esy to:
  • Install OCaml-based dependencies (e.g. OCaml and Melange)
  • Symlink Melange's JavaScript runtime into node_modules/melange, so that JavaScript bundlers like Webpack can find the files when they're imported
  • Compile the project's ReasonML/OCaml/ReScript source files to JavaScript
  • Rebuild files when changed (via Melange's built-in watch mode)
• npm run serve - starts a dev server to serve the frontend app

JavaScript output

Since Melange just compiles source files into JavaScript files, it can be used for projects on any JavaScript platform - not just the browser.

All ReasonML/OCaml/ReScript source files under src/ will be compiled to JavaScript and written to _build/default/src/* (along with some other build artifacts).

For example, src/Hello.ml (using OCaml syntax) and src/Main.re (using ReasonML syntax) can each be run with node:

node _build/default/src/node/src/Hello.js
node _build/default/src/node/src/Main.js