Elmtastic Updates

Getting your head around Elm 17 updates


6 minute read

With the release of Elm 0.17 there were some fundamental changes to the Elm language. This post is my attempt to help those that may be struggling with these changes

I’ve played with lots of new languages over the last year or so namely Elixir, Rust, and Elm. Elm and Elixir have been my favorites and I hope to cover those much more in this and future blog posts.

So whats new in Elm 0.17 why do I need to upgrade anything?

I’ll summarise here:

  • Signals have been removed (Hence some upgrade is required if you had any code using signals.)
  • Faster HTML renderer
  • Libraries for geolocation, page visibility, and web sockets
  • Generated JS is smaller and works with Google’s Closure Compiler
  • Generated JS works with RequireJS and CommonJS
  • Features in place for services like GraphQL and Elixir Phoenix
  • Improved documentation at guide.elm-lang.org
  • Helpful messages when decoding JSON fails

The big things are that are going to throw a spanner in the works are Signals have been removed and the following packages have moved around:

evancz/elm-html    -> elm-lang/html
evancz/elm-svg     -> elm-lang/svg
evancz/virtual-dom -> elm-lang/virtual-dom
evancz/start-app   -> elm-lang/html
evancz/elm-effects -> elm-lang/core

Be sure to read the official post on the subject for the full information.

How Do I upgrade? Are there any resources to help?

There are a few guides already that do help, the official upgrade plan is really useful as is migrating from elm 0.16 to 0.17 and I would advise that you read the official plan before this or any other guides. Hopefully my whistle stop tour of upgrading an existing package may be of help.

Example Upgrade

Lets take the elm-sprite package as an example, its fairly simple with only a few dependencies.


Lets look at the elm.package.json file:

    "version": "1.0.0",
    "summary": "Simple sprite rendering for elm-html",
    "repository": "https://github.com/Fresheyeball/elm-sprite.git",
    "license": "MIT",
    "source-directories": [
    "exposed-modules": [
    "dependencies": {
        "elm-lang/core": "3.0.0 <= v < 4.0.0"
    "elm-version": "0.16.0 <= v < 0.17.0"

We need to update the dependencies for elm-lang/core to 4.0.1 <= v < 5.0.0 and elm-version to 0.17.0 <= v < 0.18.0. Pretty easy in terms of dependencies.


This ones pretty easy, the only thing to change is the module definition which is using an obsolete syntax:

module Sprite (..) where

Now becomes

module Sprite exposing (..)

Actually the Elm compiler does a fantastic job here by actually telling us what the problem is:

-- SYNTAX PROBLEM --------------------------------------------------- Sprite.elm

I ran into something unexpected when parsing your code!

1| module Sprite (..) where
I am looking for one of the following things:

    something like `exposing (..)` which replaced `where` in 0.17

So that was pretty painless, lets have a look at the example file: One.elm


This ones a bit more tricky as there are signals involved and packages that have moved about.
Lets have a look at the changes needed.

First of all lets address the obsolete where syntax:

module One exposing (..)

We need to remove the Signal, Html.Events and Effects imports packages, let’s remove these:

import Signal exposing (message, Address)
import Html.Events exposing (on, targetValue)
import Effects exposing (Effects, none)

Now we need to address the changes in the Time package as fps is no longer available, we can use milliseconds instead.

import Time exposing (Time, millisecond)

Now we need to adjust the StartpApp import and use Html.App instead.

import Html.App as Html

So all in all the imports section will look like this:

module One exposing (..)

import Html exposing (..)
import Html.App as Html
import Time exposing (Time, millisecond)
import Html.Attributes as A
import Sprite exposing (..)
import Array


The first thing we will tackle is the Action which flows though this application. It now looks like this:

type Action
    = Tick Time

As Action has now been replaced with Msg so we need to change this to the following:

type Msg
  = Tick Time


Next lets look at the init function its signature is slightly different now. Html.Program now starts the application

init : (model, Cmd msg)

So rather than:

init : Sprite {}
init =
   { sheet = "https://10firstgames.files.wordpress.com/2012/02/actionstashhd.png"
     , rows = 16
     , columns = 16
     , size = ( 2048, 2048 )
     , frame = 0
     , dope = idle

It will now become:

init : (Sprite {}, Cmd Msg)
init = (
  { sheet = "https://10firstgames.files.wordpress.com/2012/02/actionstashhd.png"
    , rows = 16
    , columns = 16
    , size = ( 2048, 2048 )
    , frame = 0
    , dope = idle
    } , Cmd.none)


This is what view looks like:

view : Address Action -> Sprite {} -> Html
view address s =
        onInput address contentToValue =
                (message address << contentToValue)
            [ node
                [ A.style (sprite s) ]

At first glance this looks a bit more complex but when you look at the code a little more you come to realise that the onInput function is not even used anymore this is just dead code. So no all that remains is to change view to match the new Elm 0.17 architecture, so instead of Address Action -> Sprite {} -> Html it will now be Sprite {} -> Html Msg as Address and Action are now no longer needed.

view : Sprite {} -> Html Msg
view s =
          [ A.style (sprite s)]


Ok, now for update lets have a look at that:

update : Action -> Sprite {} -> ( Sprite {}, Effects Action )
update action s =
        s' =
            case action of
                Tick _ ->
                    advance s
        ( s', none )

update now has a signature of msg -> model -> (model, Cmd msg) so all we really have to do is replace Action with Msg, and Effects Action with Cmd Msg. Finally I change the return to: Cmd.none which was previously Effects.none.

update : Msg -> Sprite {} -> (Sprite {}, Cmd Msg)
update action s =
        s' =
            case action of
                Tick _ ->
                    advance s
        ( s', Cmd.none )


This part is new, with the old 0.16 based version there was a signal which was mapping time to a sprite update:[ Signal.map Tick (fps 30) ]. Now that we will be using subscriptions this is a simple function like this:

subs : Sprite {} -> Sub Msg
subs model =
  Time.every (millisecond * 33) Tick

So every 33 milliseconds (30 frames per second) we are sending a Tick command to the update function.

application start

The last part is the application start, heres what it looks like in 0.16:

app : StartApp.App (Sprite {})
app =
        { view = view
        , update = update
        , init = ( init, none )
        , inputs = [ Signal.map Tick (fps 30) ]

main : Signal Html
main =

You can see the signal I talked about above. This whole section has now become a lot simpler in Elm 0.17.

main : Program Never
main =
    { view = view
    , update = update
    , init = init
    , subscriptions = subs

StartApp has now become Html.App which we aliased to Html at the beginning import Html.App as Html and we use the program function to feed in all the functions we just declared.

Ok, we’re all done, hopefully someone found this useful!

One tip I can give is to update the function signatures for each function first and it should make things a little clearer on what you need to do.

Until next time!

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