Use Elm as a Reducer in Redux

28 July 2018 ·

If you’ve ever used Elm, you might already know that it makes your code free (almost) of runtime errors, which are quite common in JavaScript applications.

If you haven’t, then go check it out, it’s awesome and there is no way back. Once you felt this taste of functional programming language that’s tailored to fix all issues we’ve ever had with JavaScript, you won’t be able to unsee it.


After creating a bunch of applications with Elm, you start missing some incredibly awesome features like pattern matching, strict type checking, somewhat complete functional programming experience.

Some principles from The Elm Architecture were taken as a basis for one of the most popular libraries for JavaScript, an awesome state managing library, Redux. And when you have any Elm experience, you start missing error messages that you haven’t implemented some message in an update function, or that you forgot to update a handful of functions after some refactoring.

When I started working on a new project a year ago, I made a decision to use Elm in combination with React, React Native and Redux. The main reason was that React components were easily reused for web and mobile, and Elm could protect the application from runtime exceptions and from potential painful refactorings, that are inevitable in every startup.

So the basic idea is that you can write Redux reducer in Elm and use all nifty libraries and components available for React and React Native.

When I was looking for my options I found a library called redux-elm-middleware. The original repo was abandoned and after some time I just forked it, fixed a bunch of bugs, added a couple of nice features and a year later would like to give you a brief intro to how to make your life awesome.

How It Works

Elm supports basic interoperability with JavaScript using ports. The basic idea behind ports is that you can define senders and listeners that will create a “bridge” between JS and Elm. These ports are available through Elm.ports object in JavaScript.

Senders, ports that send data from Elm, have subscribe method in JavaScript, and listeners, ports that receive data in Elm, have send method in JavaScript.

redux-elm-middleware looks for available ports and depending on which port is triggered, or which Redux action is dispatched, passes it further to other reducers, or to Elm Reducer, accordingly.


Setting up redux-elm-middleware is fairly easy procedure:

// createStore.js
// or anywhere where you define your store
import { createStore, applyMiddleware, combineReducers } from 'redux';

import createElmMiddleware, { reducer as elmReducer } from '@cureous/redux-elm-middleware';
import Elm from './elm/Reducer.elm';

const elmWorker = Elm.Reducer.worker();
const { run as runElmWorker, elmMiddleware } = createElmMiddleware(elmWorker);

const reducer = combineReducers({
    elm: elmReducer,

const store = createStore(


Then you need to create ./elm/Reducer.elm file with the Redux.program as an entry point:

-- ./elm/Reducer.elm
port module Reducer exposing (..)

import Json.Encode as Json
import Redux

-- This is a listener port
port increment : ({} -> msg) -> Sub msg

subscriptions : State -> Sub Msg
subscriptions _ =
        [ increment <| always Increment

-- State definition
type alias State =
    { counter : Int }

-- Initial state
initState : State
initState =
    { counter = 0 }

-- This encoder will be used to send your state to Redux
encode : State -> Json.Value
encode state =
        [ ( "counter", state.counter )

-- Messages that we need to handle in update function
type Msg
    = Increment

-- That is essentially a reducer
update : Msg -> State -> ( State, Cmd Msg )
update action state =
    case action of
        Increment ->
            ( { state | counter = state.counter + 1 }
            , Cmd.none

main =
        { init = ( initState, Cmd.none )
        , update = update
        , encode = encode
        , subscriptions = subscriptions

The latest version, v6.0.0, allows to send data back from Elm to Redux, what makes it easy to write simple middlewares that can wire together different libraries with Elm, like react-router. I will include an example of using this middleware and react-router in the upcoming post.


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