Nondeterministic Palindrome Machine

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    Elm task but with ReScript and React

    It's ${currentYear} and there're sufficient reasons for not using Elm, yet there isn't a real alternative(except Elmish, but it has its own problem too). Not until the release of React hook, in which useReducer alone is powerful enough for mimicking TEA. However there is one big thing missing: side effect.

    This package is intended to port said system to ReScript so you can write Elm without really having to write Elm, or more shortly, Elm as a hook. You could also use it in plain JavaScript or TypeScript.

    Effect Manager:

    • [x] Task composition
    • [x] Time
    • [ ] Ajax
    • [x] WebSocket
    • [x] Dom Event

    Quick start

    Install @wicke/retask use your favourite package manager.

    And get the following example run on your favourite build system.

    open ReTask;
    let make = () => {
      let (state, dispatch) = useReducerT({
        init: (0, Time.delay(1000, _ => 1)),
        update: (state, action) => (state + action, Cmd.none),
        sub: state => DomEvent.onDocument("click", ev => state),
      <div> {state->} </div>

    or in JavaScript

    import React from 'react'
    import { useReducerT, delay, noCmd, onDocument } from '@wicke/retask'
    export function FooComp() {
        const [state, dispatch] = useReducerT({
            init: [0, delay(1000, () => 1)]
            update: (state, action) => [state + action, noCmd],
            sub: state => onDocument('click', ev => state)
        return <div>{state}</div>

    You would see a 0 on screen and then a 1 one second later, and every time you click on the screen, that number will double.

    Intro to ReTask

    There are three elements in ReTask: Cmd, Task and Sub.

    A Cmd is for one shot job like setTimeout. You may use Cmd to send message to your component or simply for performing side effects. For the former its constructor will take a tagger to map payload to your intended message. Don't worry if your jobs hasn't finished when your component is unmounting, they will be cancelled.

    And what do you use when you want to compose two Cmds? It doesn't make sense to provide each of them a tagger, so you should use Task. Task is like Cmd without tagger. Using andThen/thenTask you could make two tasks runing one by one. And then by attempt/attemptTask you could turn it into a Cmd. If the Task won't fail, perform/performTask is more simple.

    Sub, on the other hand, is for long running job like setInterval. Every time it see fit, a Sub would send message to your component. And if such message cause a state change, your sub function will be recalled with updated state and calcuate a new Sub. If it's of different type or be constructed with different parameter, your old Sub will be cancelled. Beware that although ReScript is not a pure language unlike Elm, here parameter changes are still detected with ===

    How to write your own effect manager

    Currently there are some effect managers missing and things will occur that maybe you have some specific task to run. So here's a guide on how to write your own Cmd | Task | Sub.

    A Task is merely a function who takes a callback as an argument and return a function for cancelling ongoing task.

    function voidTask(cb) {
        return () => {}

    And a Cmd is not far from it. In the function body, you call register so your task will be registered to your component. The returned value is for cancellation.

    import { register } from '@wicke/retask/lib/src/es6/cmd'
    function voidCmd(tagger) {
        register(send => {
            return () => {}

    TODO: Sub

    API Doc



    npm i @wicke/retask

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    • austaras