Introducing elm-pages v3 - full-stack Elm and scripts!

Dillon Kearns

I'm excited to announce the release of elm-pages v3! This has been a real labor of love that I've been working on for over a year. I am truly excited to see what the Elm community builds with it. I believe the new features in v3 open up a lot more use cases, and I hope that it makes it delightful to build full-stack Elm applications!

If you're new to elm-pages, it is a framework that gives you file-based routing to create an Elm single-page application (SPA). Besides the file-based routing, the heart of elm-pages is a rich API for declaratively gathering data: BackendTask (previously called DataSource in v2). Route Modules in elm-pages have an extension to the traditional life-cycle in apps using The Elm Architecture (TEA): a data function that lets you resolve data from a BackendTask before the initial page render (no loading spinners because the data is resolved before your view is rendered). elm-pages serves pages with fully rendered HTML (not just a skeleton that waits for the JavaScript to render, but a fully rendered view with access to your Route Module's data).

While elm-pages v2 was focused on static site generation, elm-pages v3 is a hybrid framework, giving you all the same static site generation features from v2, but with a whole new set of use cases opened up with server-rendered routes.

Before I dive into the details of the new features in v3, here is a preview of some of the exciting new use cases that are now possible with elm-pages v3:

  • Write a full-stack, server-rendered Elm app with cookie-based authentication. With the elm-pages v3 Form API, you get full-stack Elm forms that re-use the same validation logic on the client and server. And you can build an entire Form submission workflow with realtime validations without touching your Model, including deriving pending UI state from the in-flight submissions that elm-pages manages for you. Check out this live demo of TodoMVC with database persistence and magic link authentication (demo) (source code).
  • Write an elm-pages Script (an Elm module that exposes a run function that executes a BackendTask) that reads files, logs, fetches HTTP data, and uses custom BackendTasks that run async NodeJS functions. Execute it with a single command (elm-pages run script/src/MyScript.elm).
  • Scaffold new Route Modules, and customize your template with the full power of BackendTasks! elm-pages v3 has a much more extensible scaffolding script, powered by elm-pages Scripts and elm-codegen. You can even scaffold new Route Modules with a full-stack Form submission workflow with a single command (elm-pages run script/src/AddRoute.elm Signup first email subscribe:checkbox).

Let's dive into some of the new features that make these workflows possible!

Server-rendered routes (full-stack Elm!)#

elm-pages v3 is focused on making it easy to build full-stack Elm apps. This means that you can use elm-pages to build a full-stack Elm app that can render pages on the server, and then hydrate them on the client. Because your data is resolved on the backend before it's sent to the client, you have your fully resolved data before your view is rendered, which means no intermediary Maybe loading or error states, and no loading spinners. You can resolve data on the server in a low-latency environment close to your data, and then ship the dense, processed data to the client for a rich initial render.

type alias Data =
data :
-> Request
-> BackendTask (Response Data ErrorPage)
data routeParams request =
findPost routeParams.slug
(\maybePost ->
case maybePost of
Just post ->
Response.render post
Nothing ->
Response.errorPage ErrorPage.notFound
type alias Post =
{ slug : String
, title : String
, body : List Markdown.Block.Block
, likes : Int
, views : Int
findPost : String -> BackendTask FatalError (Maybe Post)
findPost slug = "findPost"
(Encode.string slug)
view :
App Data ActionData RouteParams
-> Shared.Model
-> Model
-> View (PagesMsg Msg)
view app shared model =
{ title = "My Page"
, body =
[ -- we have access to the `Post`, no `Maybe`, no loading spinners!
postView : Post -> Html msg
postDecoder : Json.Decode.Decoder Post

Using, we can directly make a database request to find the post with a database query. This will run an async NodeJS function from our definitions in a file called custom-backend-task.ts. For example, we might use the NPM package Prisma to make a database query.

// custom-backend-task.ts
import { PrismaClient } from "@prisma/client";
const prisma = new PrismaClient();
// this runs when our Elm code uses ` "findPost"`
export async function findPost(slug) {
return await{
where: { slug },

BackendTasks also let us run any markdown parsing and other expensive processing on the backend instead of the user's browser. By doing this work on the server, we can resolve the core data for the page in a single pass, and avoid sending unprocessed data or doing multiple round trips to the server from the client. As a bonus, because we are only running our markdown parsing from our data function (which is only executed on our Backend), this is dead-code eliminated from our client bundle! That means that not only does the execution of running markdown parsing not bog down the user's browser, but it doesn't even need to download the markdown parser code!

Plus, we get the initial page load with a rich initial render, no loading spinners or flashes of blank content. If we architect our app effectively with our data center co-located with our elm-pages Backend server, and well-tuned database queries, we can get a very compelling performance story.

Notice also that we have the ability to dynamically render an error page if the post is not found (learn more in the ErrorPage docs). This opens up new use cases because we can decide whether to render a 404 page based on the data we get back from our database at request-time (rather than pre-rendering a finite set of pages at build-time). With this workflow, we could even publish a post to our database and have it show up without running a build. elm-pages v3 provides error handling abstractions to render routes with your happy path data clean and free of error states and loading spinners, while still letting you bail out of the happy path to present an error page when needed.

elm-pages v3 still fully supports static site generation with RouteBuilder.preRender (in fact, this blog is an example of that!). But you can choose the right architecture, or even transition to more flexibility when you need it with the new suite of hybrid features in v3.

Server-rendered API Routes#

You can also define API routes that are rendered on the server. That means in addition to generating static files like RSS feeds, you can serve dynamic APIs like a dynamic RSS feed that pulls in on-demand data from a database, or even a JSON API.


If you're wondering how the server-side part of an elm-pages app is hosted, take a look at the adapter docs page. There is a built-in adapter for Netlify serverless functions, and there are some community adapters being developed for frameworks like Express. You can define your own for your deployment target of choice.

And if you're wondering how the magic of full-stack routes in elm-pages works overall, check out the docs page on The elm-pages Architecture.

DataSource renamed to BackendTask#

To reflect the broader use cases that are supported now with this abstraction in v3 with full-stack server-rendered routes and scripts, we've renamed DataSource to BackendTask.

In v2, this was a mechanism for pulling in static data to your Routes. In v3, you might want to perform a side-effect using this tool. For example, you could delete an item when a form is submitted.

This means that the semantics have changed. The v3 BackendTask.Http API provides some caching options to explicitly manage cases where you want to perform HTTP GET requests with a local cache. However, if you perform a non-GET request, since it may represent a side-effect (like deleting an entry), the v3 semantics do not cache non-GET requests.

Explicit errors with BackendTask FatalError data#

In addition to the new name to better reflect the broader use cases that are supported in v3, BackendTask also has an explicit type variable for errors now, just like the elm/core Task.

In v2, a DataSource could cause a build failure without that being reflected in its type. This was reasonable for static sites because an unexpected build failure is more manageable than an unexpected error at run-time. For example, if an HTTP request fails because an API is down, you probably want the build to fail and don't need to do any sophisticated error handling. With server-rendered routes in v3, I wanted it to be possible to see whether or not a BackendTask can fail just by looking at its type, and also to be able to gracefully handle possible error cases.

If you have a BackendTask Never String, for example, you know that it will never result in an error.

Here are two different ways you could handle HTTP errors in v3. handled will give Nothing on failure, and will never result in a BackendTask error. unhandled yields a FatalError if anything goes wrong. The data function in your elm-pages Route Modules has type BackendTask FatalError Data, and the framework will handle the FatalError by printing the error message and failing the build for static pre-rendered routes, or by rendering a 500 error page for server-rendered routes.

handled : BackendTask.BackendTask Never (Maybe Int)
handled =
(Decode.field "stargazers_count"
|> Just
|> BackendTask.onError (\_ -> BackendTask.succeed Nothing)
unhandled : BackendTask FatalError Int
unhandled =
(Decode.field "stargazers_count"
|> BackendTask.allowFatal

elm-pages Scripts#

With the more general-purpose BackendTask API, it made sense to provide a way to just run a BackendTask directly from the command-line (no HTML view or routing, just execute a task as a script). One of the major goals for elm-pages Scripts was to make it as frictionless as possible to execute headless Elm code, and I think we've achieved that. The script is part of an Elm project (a folder with an elm.json listing out its dependencies). You can run a script from any directory by passing in the file path, for example elm-pages run scripts/src/HelloWorld.elm. It will find the closest elm.json based on the file path you pass in.

Here's the HelloWorld.elm script:

module HelloWorld exposing (run)
import Pages.Script as Script exposing (Script)
run : Script
run =
(Script.log "Hello, World!")

I think this is one of the best ways to try out elm-pages to get a feel for the BackendTask API and what you can do with it, as well as how error handling works in elm-pages. Check out the quick start and intro to elm-pages scripts in the docs. We also have a deep dive on elm-pages scripts on an episode of the Elm Radio podcast.

elm-pages Scripts also has a Script.withCliOptions that lets you parse command-line options using dillonkearns/elm-cli-options-parser, so you can build full-fledged CLI utilities in pure Elm.

There is also an elm-pages bundle-script command for bundling into a single executable JavaScript file (including any NodeJS dependencies).

Customizable scaffolding scripts with elm-codegen#

elm-pages v3 introduces a new approach to scaffolding Route Modules that is more customizable. You may have guessed already - the scaffolding commands are actually just elm-pages Scripts!

The scaffolding uses the excellent tool mdgriffith/elm-codegen to generate the Route Modules. elm-codegen provides a high-level way to write Elm code that generates Elm code. Sounds scary, but it's a lot of fun to use! elm-pages abstracts out the boilerplate around Routes and Forms, so you can focus on customizing your template within the confines of generating a valid Route Module.

With elm-pages Scripts-based scaffolding, you have full programmatic control over your scaffolding, and all in pure Elm. You can run arbitrary BackendTasks, and customize your scaffolding with command-line options. Here's an example of the scaffolding script. The elm-pages-starter repo and the elm-pages init project skeleton both come with a script/src/AddRoute.elm script that you can customize to your needs.

Built-in Vite Integration#

In v2, there was a philosophy of "bring your own bundler". The web ecosystem had so many different approaches to post-processing - Webpack, Parcel, Rollup, Snowpack. Or just the TypeScript compiler, PostCSS CLI, and other standalone post-processing tools. There wasn't a clear winner, and often tools that choose a bundler like Webpack ended up exposing dangerous configuration options that could interfere with the way the framework bundled the core app.

That all changed when Vite came out with a refreshingly sane approach to bundling. It is conventions-based (no configuration needed for many common tools, for example it will find your TypeScript config file and use that automatically). Plus it is extremely fast, and has a rich ecosystem, and a simple plugin API. So elm-pages v3 comes with a built-in Vite integration.

elm-pages still does its own processing of the core Elm code in your app so you can't break your app by mistake, and you get your Elm code bundled and optimized for production with zero configuration (it even runs elm-optimize-level-2 on the production build!). And you get seamless hot data reloading as well - try defining a Route Module with Data that pulls in content from a file (BackendTask.File) or uses a Glob pattern to list matching files (BackendTask.Glob), and you'll see the page hot reload with the latest data as you touch files that your Route Module depends on.

For all of your non-Elm bundling needs, you get the simplicity and power of Vite. You can customize your Vite configuration in the elm-pages.config.mjs file (here's this docs site's Vite configuration).

Session API#

One of the core foundational principles of this v3 release is "Use the platform". elm-pages leverages Web standards. That brings a lot of conveniences for server-rendered routes, like using cookie-based sessions through the Session API. The elm-pages Session automatically manages serializing your session data as key-value pairs in a signed cookie. The cookie is signed using the secrets you pass in. That means while you can read the key-value data from the cookie if you have access to it, if you modify its contents, the signature will no longer match and the cookie will be rejected. This allows you to use the cookie to store session data like a a user session ID since you can trust that the server set the value. HTTP-only cookies are used by default, giving you the extra layer of security that the cookie cannot be accessed from JavaScript.

You can rotate your signing secrets. The first secret will be used to sign new values, but unsigning will go through each of the secrets until it finds one that works. This high-level abstraction allows you to use powerful primitives that the web platform provides, while making it easy to use common patterns simply and securely.

Check out a full examples of using the Session API to manage dark mode, and to manage a user session using magic link authentication (try the live demo here).

Forms and Pending UI#

One of my favorite features in v3 is the Form API. As Web developers, one of the core things we do is provide a way for users to input data and send it to a server (ideally with nice client-side validations). This can involve a lot of boilerplate in an Elm app, and a lot of opportunities to forget wiring. Managing Pending UI state tends to be fairly imperative.

elm-pages v3 introduces a Form API that manages wiring for you, and manages pending form submissions, allowing you to derive Pending UI state declaratively. elm-pages v3 also introduces the concept of an action. action is the same as a Route Module's data function, except instead of running a BackendTask for the initial page load, action runs in resopnse to a non-GET request (such as a form submission using the default form submission method in elm-pages: POST). Check out the derived pending state in the TodoMVC demo, as well as the action function in the TodoMVC demo.

My thinking on progressively enhancing forms and building a framework around Web platform primitives has been heavily influenced by RemixJS. A big thank you to the Remix team for their amazing vision and for sharing their ideas and paving a path for a more standards-based approach to full-stack Web development.

Goodbye OptimizedDecoders!#

This is one of those wonderful cases where it's all upside. In v3, you no longer need to use OptimizedDecoders to ensure that your page loads pull in only the essential data they depend on. elm-pages v2 introduced OptimizedDecoders which would strip out any JSON data that wasn't consumed by the decoders you used in your DataSources. This helped optimize the size of the data that was used to hydrate the Elm application on the client with the same data that was used to render the page on the server. However, it was an extra concept to understand, and it came with some footguns and inconveniences. You could accidentally end up with sensitive data in your page data, and you couldn't re-use existing JSON decoders or JSON decoding utilities since you had to use the OptimizedDecoder type instead of Json.Decode.Decoder. There was a Secrets API that let you define DataSources using sensitive data to perform the request, but scrubbing the sensitive data while still preserving the key-value data associated with those requests. You can read about some of the old caveats of data serialization from v2 for comparison in the docs archive.

If that's a lot to take in, don't worry, with v3 you no longer need to think about any of these details! Instead, elm-pages automatically serializes your Route data, only serializing exactly the final data you end up with in your data function. Not only that, but the data is serialized in a more compact binary format, giving even better performance. That means instead of using the Json.Decode drop-in replacement for your v2 data with import OptimizedDecoder, you can just use vanilla JSON Decoders and you will end up with compact data.

The data that is sent to the client is exactly the type you define in your Route Module's Data type. That means you don't need to worry about whether any sensitive intermediary data ended up in the page data. What you see is what you get.

Try It Out#

If you're interested in trying out elm-pages v3, you can get started with the starter repo, or by running npx elm-pages@latest init. The starter templates come with a script/src/AddRoute.elm module that is ready to customize for your app, and come with the Netlify adapter configured so you can deploy a full-stack Elm app right from the starter template.

Be sure to join the #elm-pages channel in the Elm Slack to get help and share your feedback! I'd love to hear what you build with elm-pages v3.