Inter-Component Communication in 0.17 as discussed https://groups.google.com/forum/?fromgroups#!topic/elm-discuss/i99LBvYSkpY

ChildOne.elm

module ChildOne exposing (..)

import Ports exposing (..)
import Html exposing (Html, button, text)
import Html.Events exposing (onClick)


type alias Model = String


type Msg
    = Click

subscriptions : Model -> Sub Msg
subscriptions model =
    Sub.none

update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        Click ->
            ( model, dispatchSomeMessage "Hello World!" )


view : Model -> Html Msg
view model =
    button [ onClick Click ] [text "Click Me"]

ChildTwo.elm

module ChildTwo exposing (..)

import Ports exposing (..)
import Html exposing (Html, text)

type alias Model = 
        { message : String }

type Msg
    = ChangeMessage String

subscriptions : Model -> Sub Msg
subscriptions model =
    receiveSomeMessage ChangeMessage


update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        ChangeMessage msg ->
            ({ model | message = msg }, Cmd.none )


view : Model -> Html Msg
view model =
    text model.message

index.js

var app = Elm.Main.fullscreen();

app.ports.dispatchSomeMessage.subscribe(function(msg) {
    app.ports.receiveSomeMessage.send(msg);
});

Main.elm

module Main exposing (..)

import ChildOne
import ChildTwo

type alias Model =
    { childOneModel : ChildOne.model
    , childTwoModel : ChildTwo.model
    }

-- ..init func. nothing special here. 

type Msg
    = ChildOneMsg ChildOne.Msg
    | ChildTwoMsg ChildTwo.Msg


subscriptions : Model -> Sub Msg
subscriptions model =
    Sub.batch
        [ Sub.map ChildOneMsg (ChildOne.subscriptions model. childOneModel)
        , Sub.map ChildTwoMsg (ChildTwo.subscriptions model. childTwoModel)
        ]

-- .. update function. nothing special here. 

main : Program Never
main =
    Html.App.program
        { init = init
        , update = update
        , view = view
        , subscriptions = subscriptions
        }

Ports.elm

port module Ports exposing (..)

-- dispatchSomeMessage will create a command with a
-- string payload representing an important message
-- to broadcast to listening components.

port dispatchSomeMessage : String -> Cmd msg


-- receiveSomeMessage is the port which our components 
-- subscribe to receive the dispatched message

port receiveSomeMessage : (String -> msg) -> Sub msg