Elm Phoenix with ports

elm-package install eco-pro/elm-phoenix-ports 1.0.0



An Elm client for Phoenix Channels using ports.

This can be used to connect to Phoenix Channels using a very similar API to that of

It is compatible with Elm 0.19 as it does not use an Effects Manager and uses ports to communicate with the phoenix.js library.

Getting started

Migrating from saschatimme/elm-phoenix

In addition to the installation steps below please see Migration Guide


Install the package in the normal way:

elm install chazsconi/elm-phoenix-ports

As ports are required for this and Elm does not permit publishing port modules in packages, you also need to install the Elm and JS ports code. You can do this via npm (or similar) direct from github.

N.B. Ensure the version number usedin the package.json reference matches the version of the installed Elm package.

In package.json add the dependency from github:

"dependencies": {
  "elm-phoenix-ports": "github:chazsconi/elm-phoenix-ports#1.1.1",

In elm.json add the source directory for the ports module.

"source-directories": [

Connect your JS Elm launcher

You need to initialise the Elm Phoenix Ports JS code by passing your elm app object in your launcher. For example

import * as elmPhoenix from "elm-phoenix-ports"

var app = elmXingbox.Elm.MyElmApp.init({flags:{}});

To enable debug logging you can pass a 2nd parameter to the init function:

elmPhoenix.init(app, {debug: true});

Wiring up Elm

As there is no effects manager in Elm 0.19, the library has its own messages and model that you need to delegate in the update function.

Declare a new message with your existing messages and store Phoenix's model in your own model

import Phoenix

type Msg
  = NewMsg Value
  | PhoenixMsg (Phoenix.Msg Msg)

type alias Model =
  { myModelField : String
  , phoenixModel : Phoenix.Model Msg ()

Declare a phoenixConfig with your PhoenixMsg, the socket you want to connect to and the channels you want to join, along with what to do when messages are received. The library code will open the socket connection and join the channels.

import Phoenix
import Phoenix.Socket as Socket
import Phoenix.Channel as Channel
import Phoenix.Config
import PhoenixPorts

phoenixConfig = PhoenixMsg PhoenixPorts.ports

-- N.B. Do not suffix the endpoint url with `/websocket` as this is automatically added by `phoenix.js`.
endpoint =

socket =
    Socket.init endpoint

-- channelModel here can be used to dynamically connect to channels.  See advanced usage.
channels channelModel =
  [ Channel.init "room:lobby"
        -- register an handler for messages with a "new_msg" event
        |> Channel.on "new_msg" NewMsg

Initialise the phoenixModel, delegate PhoenixMsg to Phoenix in your update, and set the subscriptions:

import Phoenix
import PhoenixPorts

init : Flags -> ( Model, Cmd Msg )
init flags =
    ( { myModelField = "Hello"
      , phoenixModel =
      }, Cmd.none )

update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of

        PhoenixMsg phoenixMsg ->
                ( phoenixModel, cmd ) =
                    Phoenix.update phoenixConfig socket channels () phoenixMsg model.phoenixModel
            ( { model | phoenixModel = phoenixModel }, cmd )

        NewMsg value ->

subscriptions model =
    Phoenix.connect phoenixConfig

In order to push a message to a channel you can do this:

import Phoenix
import Phoenix.Push as Push
import Json.Encode as JE

pushMessage =
  let push =
    Push.init "room:lobby" "set_status"
          |> Push.withPayload (JE.string "away")
  Phoenix.push phoenixConfig push

Advanced usage

If you need to join or leave channels dynamically (e.g. in the example of a chat app the user can choose the rooms to join), you need to define a ChannelModel which defines the parameters needed to build the list of channels. Your phoenixModel must be parameterised with this instead of () e.g.

type alias Room = String

type alias ChannelModel =
  { rooms: List Room }

type alias Model =
  { myModelField : String
  , rooms : List Room
  , phoenixModel : Phoenix.Model Msg ChannelModel

In the update create this channel model from your model and pass it to Phoenix.update:

update msg model =
    case msg of
        PhoenixMsg phoenixMsg ->
                channelModel = { rooms = model.rooms }
                ( phoenixModel, cmd ) =
                    Phoenix.update phoenixConfig socket channels channelModel phoenixMsg model.phoenixModel
            ( { model | phoenixModel = phoenixModel }, cmd )

This can be then used to dynamically join channels. e.g.

channels channelModel =
  [ Channel.init "room:lobby"
        |> Channel.on "new_msg" NewMsg
  ] ++ Channel.init channelModel.rooms

Warning - If you want to leave all channels (e.g. when showing an error page to the user) do not remove Phoenix.connect from your subscriptions function as this will have no effect. Instead, return an empty list of channels from the channels function.

Module docs

Please see

Example app

Please see the example chat room app in the example folder.


  • Not all features from the saschatimme/elm-phoenix are implemented.
  • Only one socket can be connected to at a time. If you need more than one you should be able to have multiple instances of the phoenixConfig and phoenixModel although this has not been tested.

Design decisions

Replacing Effects Manager

Because there is no Effects Manager available in Elm 0.19, a time subscription is used to check every few 100ms if the model that is used to construct the channels has changed. If it has, then the channels function is evaluated. Any new channels are joined, and any old ones are left by making calls to ports. The JS objects that represent the channels are stored in the model along with their state. When phoenix.js receives an event this is passed back to Elm via a port, which triggers events in the main app.

In this way, a similar API to saschatimme/elm-phoenix is retained, without consumers of the library having to explicitly join and leave channels.

Why have a Phoenix.Config

This encapsulates the parent Msg type and also the real ports module and is a convenience for passing as a parameter to Phoenix.push.

An alternative would have been to put this information in the Socket however often you will want to have your socket constructed dynamically from your model - e.g. adding a user token as a connection parameter. However the config should not need to be parameterised and thus is easier to use without having to pass around the socket or model.


Contributions are welcome!


Thanks a lot to Sascha Timme for the original version of this library which I used happily and extensively with Elm 0.18 for several years and has a great API design which I wanted to keep.


If you use the package in your project, it would be nice to know


  • Implement missing features (See TODO markers)