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0.10.0 • Public • Published


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Relude-based utilities for ReasonReact


ReludeReact.Reducer.useReducer hook

The ReludeReact.Reducer.useReducer hook was inspired by the original/pre-hooks ReasonReact record API, and the hooks-based reason-react-update libray by Matthias Le Brun (bloodyowl).

ReludeReact.Reducer.useReducer is similar to the React useReducer hook with the key difference that the React useReducer only allows you to change the state, whereas the ReludeReact.Reducer.useReducer allows you to both change the state, and to safely emit side effects or Relude.IO-based actions, which can result in the emission of further actions.

To use the ReludeReact.Reducer.useReducer hook, you must provide a reducer function of the following type:

type reducer('action, 'state) = ('state, 'action) => update('action, 'state);

A function that accepts the current 'state and an 'action, and returns a value of type update('action, 'state)

The update value is a variant with the following type:

type update('action, 'state) =
  | NoUpdate
  | Update('state)
  | UpdateWithSideEffect('state, SideEffect.Uncancelable.t('action, 'state))
  | SideEffect(SideEffect.Uncancelable.t('action, 'state))
  | UpdateWithCancelableSideEffect(
      SideEffect.Cancelable.t('action, 'state),
  | CancelableSideEffect(SideEffect.Cancelable.t('action, 'state))
  | UpdateWithIO('state, SideEffect.Uncancelable.IO.t('action, 'state))
  | IO(SideEffect.Uncancelable.IO.t('action, 'state));

Basically, this means that in your ReludeReact.useReducer component, for any action that occurs, you can respond to the action by doing any of the following things:


Don't change the state, and don't perform an side effects or IO-based effects. Basically a no-op - useful as a placeholder or to stub out actions prior to providing the implementations.


Update the component state to the given value, but don't perform any side effects or IO-based effects.

UpdateWithSideEffect(state, {state, send} => unit)

Update the component state to the given value, and perform the given side effect (basically a function that is given a context record of state and send and is allowed to perform any type of sync or async side effect, and emit additional actions via send, which is a function of type action => unit, and ultimately return unit (). In this case, the side effect is Uncancelable, which means, there is no way to cancel it later.

These types of side effects are useful for doing things like pushing a history state to navigate to a different URL, doing one-off DOM manipulations, or other types of things you don't want or need to manage or control.

SideEffect({state, send} => unit)

Same as UpdateWithSideEffect, but with no state update.

UpdateWithCancelableSideEffect(state, {state, send} => (unit => unit))

Same as UpdateWithSideEffect, but the side effect can be cancelled via a returned canceler function.

CancelableSideEffect({state, send} => (unit, unit))

Same as UpdateWithCancelableSideEffect, but with no state update.

UpdateWithIO(state, Relude.IO.t(action, action))

Similar to UpdateWithSideEffect, but instead of a function that accepts the side effect context and returns (), you return a Relude.IO.t('action, 'action). An IO is a data type which can perform any type of synchronous or asynchronous side effect - see below. Relude.IO is a bi-functior which has a typed error channel, and a typed "success" channel. In this case the success and error channels are both constrained to the type 'action, which means that your IO, when executed, must produce an 'action to dispatch on either success or failure.

A common pattern with component actions is to perform some async action (which typically can fail, e.g. an AJAX/fetch call), and then send a new 'action when the async invocation either succeeds or fails. This patterns is exactly what's captured by the Relude.IO.t('action, 'action) type. See below for a more illustrative example.

IO(Relude.IO.t(action, action))

Similar to UpdateWithIO, but with no initial state update.

Relude.IO Aside

Relude.IO is a data type that can be used to execute side effects in a purely functional way. For those coming from the JavaScript world, you can think of IO as something similar to a lazy promise, but with lots of extra capabilities, brought to you by the power of math and functional programming.

Using IO rather than ad-hoc side effect functions gives you all sorts of useful functions for mapping/flatMapping results and errors, catching and transforming errors, combining multiple async results, and so much more.

See Relude IO documentation for more information.

ReludeReact.Effect hooks


ReludeReact.useOnMount is a simple shortcut which allows you to register a simple unit => unit function to run when a component is first mounted. This is typically used to send an initial 'action into your reducer for initializing the component (e.g. fetch any initial data).

useIOOnMount hook

ReludeReact.Effect.useIOOnMount (and it's variations) allows you to trigger a Relude.IO-based action when the component is mounted, and handle the final resulting value (either success or failure) using a side-effect callback.

This could be useful if you need to dispatch a fetch request on mount, and then dispatch some reducer actions on success and/or failure, or if you need to store the result of the fetch request in localStorage, etc.

Variations of this function exist which allow different types of result callbacks - i.e. a callback from Belt.Result.t('a, 'e) => unit, separate 'a => unit and 'e => unit callbacks, etc.


These effect hooks are very similar to their React.useEffectN counterparts, except that you provide your own equality function along with any values the hook depends upon for re-running.

React's useEffect dependencies are simply checked by (===), which is fast but may lead to false positives when deciding if a hook dependencies have changed (particularly with complex types like records and lists). In cases where running an effect may be expensive, useEffectNWithEq allows much more control over whether that effect should run.

ReludeReact.Render utilities

ReludeReact.Render contains a variety of useful functions for rendering different data types, to avoid extra boilerplate/noise in your components. The purpose of these functions is to try to streamline conditional rendering, so you don't have to write lots of _ => React.null cases when rendering conditional values, variants like Relude.AsyncResult.t('a, 'e), etc.

// And many more!


See the demo app in examples/demo

> npm run demo

Below is a somewhat contrived/simple example of what a ReludeReact component might look like.

open Relude.Globals;
// The state of this component
// We're using a Relude.AsyncResult to represent the state of animals, which are loaded asynchronously and can fail.
type state = {
  title: string,
  animalsResult: AsyncResult.t(list(Animal.t), Error.t),
// The initial state for the component (used in the reducer initialization below)
let initialState = {title: "Animals", animalsResult: AsyncResult.init};
// The actions that our component emits and handles in the reducer
type action =
  | FetchAnimals
  | FetchAnimalsSuccess(list(Animal.t))
  | FetchAnimalsError(Error.t)
  | ViewCreateForm
  | ViewAnimal(Animal.t)
  | DeleteAnimal(Animal.t)
  | NoOp;
// The reducer function which accepts and action and the current state, and emits
// an "update" which can do things like updating the state, running raw or IO-based effects
let reducer =
    (state: state, action: action): ReludeReact.Reducer.update(action, state) =>
  switch (action) {
  | FetchAnimals =>
      {...state, animalsResult: state.animalsResult |> AsyncResult.toBusy},
      |> IO.bimap(a => FetchAnimalsSuccess(a), e => FetchAnimalsError(e)),
  | FetchAnimalsSuccess(animals) =>
    Update({...state, animalsResult: AsyncResult.completeOk(animals)})
  | FetchAnimalsError(error) =>
    Update({...state, animalsResult: AsyncResult.completeError(error)})
  | ViewCreateForm => SideEffect(_ => ReasonReactRouter.push("/create"))
  | ViewAnimal(_animal) => NoUpdate
  | DeleteAnimal(_animal) => NoUpdate
  | NoOp => NoUpdate
// Various inline components
module AnimalsLoading = {
  let make = () => {
    <div> {React.string("Loading animals...")} </div>;
module AnimalsTable = {
  let make = (~animals: list(Animal.t), ~send: action => unit) => {
    let _ = send; // TODO
      {React.string("Animals: " ++ string_of_int(List.length(animals)))}
module AnimalsError = {
  let make = (~error: Error.t) =>
    <div> {React.string(} </div>;
module AnimalsResult = {
  let make = (~result: AsyncResult.t(list(Animal.t), Error.t), ~send) =>
    |> ReludeReact.Render.asyncResultByValueLazy(
         _ => <AnimalsLoading />,
         animals => <AnimalsTable animals send />,
         error => <AnimalsError error />,
// The main view - accepts the state and send values we get from the reducer
module Main = {
  let make = (~state, ~send) => {
      <h1> {React.string(state.title)} </h1>
          onClick={e => {
          onClick={e => {
          {React.string("No-Op Action")}
      <AnimalsResult send result={state.animalsResult} />
// The main component definition
// Here, we invoke our hooks and render the main view
let make = () => {
  // Initialize the ReludeReact reducer
  let (state, send) = ReludeReact.Reducer.useReducer(reducer, initialState);
  // Trigger an initialization action on mount
  // This is just using the send function from our reducer to send an action,
  // which is handled by the reducer
  ReludeReact.Effect.useOnMount(() => send(FetchAnimals));
  // This is just demonstrating triggering an IO action on mount, and handling
  // the result via side-effecting functions
  // In reality, the IO would probably be making a fetch request, or doing some
  // other async action and then storing or dispatching the results.
    IO.suspend(() => {
      Js.log("Suspend 42");
    intValue => Js.log("Got suspended value: " ++ string_of_int(intValue)),
    _error => Js.log("Suspend 42 failed"),
  // This just demonstrates that with our special effect hooks, you can provide
  // a custom EQ function that will prevent a hook from running even if React's
  // basic (===) check thinks the value has changed
    () => Js.log("Running effect because some array has changed!"),
    (a, b) => List.String.(eq(sort(a), sort(b))),
    List.shuffle(["a", "b", "c", "d"]),
  // Render our main view, passing the state and dispatcher function down
  <Main state send />;

Developer info

Project setup

> git clone
> cd relude-reason-react
> npm install
> npm run server:demo


> npm run clean
> npm run build
> npm run cleanbuild
> npm run test
> npm run cleantest
> npm run watch
> npm run demo

Publishing to npm

> npm version major|minor|patch
> git push origin --follow-tags
> git push upstream --follow-tags
> npm publish


If you have trouble building/installing the Bucklescript/Reason tools try this:

> nix-shell
%nix%> npm install


npm i relude-reason-react

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