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Elm at Rakuten

In our team at Rakuten, we have been using Elm1 in production for almost two years now. This post is about our story, the lessons we learned, and our likes and dislikes.

This post is quite long so if you prefer to see an overview, feel free to jump to the index.

Everything started in the Berlin branch of Rakuten during the summer of 2017. We were maintaining a medium-size single-page application written in Vanilla JavaScript when things started going out of control.

“Our JavaScript application had global variables everywhere, and debugging was a nightmare.”

Fixing something in one place would break the code in several other locations. We had global variables everywhere, and debugging was a nightmare.

We decided to impose some discipline and start rewriting functions in a pure style2 to regain some control over our application. The code was getting better, more self-contained, and simpler to understand.

We were thinking: “if we only could find a tool to enforce these rules so we wouldn’t have to rely on our self-discipline...” and then we came across the post "Introduction to The Elm Architecture and How to Build our First Application"3 published in the website It was love at first sight.

"A delightful language with no runtime exceptions."

-- The Elm website 4

Elm was promising that all of our functions would be pure by design and that there would be no errors at runtime5 anymore.

Moreover, The Elm Architecture, included in the Elm language, seemed an excellent way to structure an application. It ended up being one of the most influential Elm ideas in the front-end world, more about this later.

The Elm Architecture
The Elm Architecture, illustration by Kolja Wilcke, licensed under CC BY 4.0.

So, we started learning Elm by building some prototypes, and the first results were promising. But the technology stack was still heavily server-side based, mostly PHP, and the adoption was proceeding slowly.

🕒 One year later

Fast forward one year (and move 9,000 kilometers to the right), and in the headquarters of Rakuten, in Tokyo, there was an environment where Elm would have taken root successfully.

Several engineers were already pushing for a more functional-style way of writing code, and in a department heavily based on back-end APIs, there was a strong need for a decoupled way to write user interfaces.

🕒 Two years later

Fast forward another couple of years, and here we are, with several applications in production built with Elm for a total of ~100k lines of code.6

These are some of our public projects made in Elm: A highly-customizable authentication and registration system used across different Rakuten services (for example Rakuten Taiwan and Rakuten Sport) with different user journey requirements, a UI library to build stuff like this, an HTTP library, the Rakuten Open Source website (source code), a simple — but infinitely cute! — 404 error page, an informative page about security (in Japanese).

Credit Card Demo
Example of a credit card form made using the R10 library.


What we like about Elm

In no particular order.

  1. Guarantees
  2. Controlled state
  3. Principle of least astonishment
  4. “Making impossible states impossible”
  5. One way of doing things
  6. Stability
  7. Functional programming
  8. Enforced discipline
  9. Learnability
  10. Compiler as assistant
  11. Elm as an influencer
  12. The Elm Architecture
  13. The Elm debugger
  14. Elm-UI, the alternative to CSS/HTML
  15. Readability and Elm syntax
  16. Refactoring
  17. Hiring
  18. Fast performance and small assets
  19. Content-driven static websites

What we don't like about Elm

In no particular order.

  1. Not mainstream
  2. Lack of googleable resources
  3. Reinventing the wheel
  4. Mindset shift
  5. Some JavaScript and CSS are still necessary

What we like about Elm

1. Guarantees

These are probably the most objective and important guarantees that Elm provides and that are difficult (impossible?) to find in other frameworks.

  • ⛔ No runtime exceptions.
  • 🗿 100% immutable data.
  • 💧 100% pure functions, also in all dependencies.
  • ♻️ 100% type inference.

There are tradeoffs when choices, like these above, are made. For example, is not possible to call a JavaScript function directly from Elm. If that is of paramount importance for you, Elm is not the right choice.

If instead, you think that Elm guarantees are more important, then Elm is the right choice.

In the section The Limits of Elm/JS Interop of the Elm guide, Evan Czaplicki elaborates more on this concept.7

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2. Controlled state

JavaScript lets us do what we want with the state of a program. This can be useful for a quick prototype, but it is the precursor of bugs because it is difficult to track and understand changes in global variables.

“A large fraction of the flaws in software development are due to programmers not fully understanding all the possible states their code may execute in”

-- John Carmack 8

All functions in Elm must be pure, so they cannot hold any state, and all data must be immutable. The entire state of your application needs to be stored in one place, by design, making your application simpler to comprehend and easier to debug.

The place where the state is, in Elm, is the Model and it is managed by the Elm Runtime system9 so that 100% of the code that we write can be pure (these concepts will be explained in more detail later).

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3. Principle of least astonishment

One of the ideas of Elm is that the outcome of the code should be predictable, without surprises.10 For example:

  • The Elm static type system, discussed in detail below, removes a whole category of surprises related to dynamic typing.
  • The Elm Package Manager supports Enforced Semantic Versioning.11 There are no surprises in PATCH releases because version numbers are enforced by a script that scans libraries detecting, for example, removal or renaming of exposed functions.
  • Elm-UI, a library to render views that translate developers' intents into layouts in a clear manner, discussed in detail later.

Languages with automatic type conversion (also called implicit type casting), like JavaScript, can lead to surprises.12 What happens if we add a number and a string together? For example, 1 and "2"? Do we get 3? Do we get "12"? Do we get an error? Do we get something else?

Elm, in contrast, is strongly and statically typed so cases like the one mentioned above are not possible.13 It is not even necessary to add types annotations14 because types are inferred by the Elm compiler. The type inference covers 100% of the code, including all external libraries.

Dynamic Typing vs Static Typing
Dynamic Typing vs Static Typing, illustration by Kolja Wilcke, licensed under CC BY 4.0.

If you need to create a quick proof of concept, dynamic typing may be faster and the puzzle does resemble a giraffe even if it contains mistakes. But for robust applications and correct puzzle solutions, static typing is the right way to go.

TypeScript, which adds optional static typing to JavaScript and is probably one of the best things to happen to JavaScript, can partially mitigate the issues with the JavaScript dynamic type system. But being a superset of JavaScript it needs to compromise on elegance and simplicity. It also has several "blind spots".15 For example, type declarations are optional (any as escape hatch), inference doesn’t cover all the code, it requires type guards,16 JSON data is not type-checked, and not all JavaScript libraries have type annotations.

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4. “Making impossible states impossible”

Penrose Triangle
Penrose Triangle.17

The depth of the Elm type system allows us to model scenarios precisely, to make impossible states impossible.18 This is more a coding pattern19 rather than a language feature, and it requires a fully-fledged type system to work.

To clarify, let's make an example modeling the HTTP state with a type alias:20

type alias HttpState = 
    { loading : Bool
    , error : Maybe String
    , success : Maybe String
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The cardinality (number of possible states) for this structure is 2 x 2 x 2 = 8 because both Bool and Maybe21 have cardinality = 2.

But the possible states of the HTTP request are only three: Loading, Error, and Success. To make these extra five impossible states impossible, we can rewrite the code using a custom type:22

type HttpState
    = Loading
    | Error String
    | Success String
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Custom types are also called Sum types in the sense that the cardinality is now a sum: 1 + 1 + 1 = 3. The right number of possible states.

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5. One way of doing things

One application of this principle23 is about finding the best solution to a problem and then enforcing it in the language.

For example:

  • The adoption of The Elm Architecture as the standard way to structure web applications
  • The linter Elm-Format is not configurable. Therefore, all Elm code is formatted using the same style. The end of the tab vs. space war.

The principle guarantees consistency across codebases, even when they belong to different teams and organizations.

Other languages and frameworks follow different principles. For example, JavaScript follows the “One JavaScript” principle.24 It means that JavaScript is not versioned and is back compatible. Back-compatibility is the precursor of "several ways of doing things."

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6. Stability

A lot of work has been done to improve the Elm compiler, but the language per se has not undergone any major updates in more than four years.25 Moreover, no foreseeable updates are coming soon.26 The latest versions were mainly about improvements of the compiler’s performances and removal of features that were considered unnecessary or even detrimental, like the infix operator.27

And this is great because we can concentrate on building great products instead of spending time updating our code to the latest release.

"It seems that perfection is attained not when there is nothing more to add, but when there is nothing more to remove"

-- Antoine de Saint Exupéry

The core modules are also very stable. Most of the action nowadays is happening in non-core modules and toolings.28

We started writing Elm in version 0.18, and the transition to version 0.1929 was very smooth. We had more issues with the update of the HTTP library30 when, for lack of internal communication, one of our in-house dependencies was suddenly updated to HTTP 2.0, forcing us to refresh all the rest of the code in a short time.

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7. Functional programming

Functional programming31 is on the rise again! Maybe we are already in the middle of a third paradigm shift.32

"No matter what language you work in, programming in a functional style provides benefits. You should do it whenever it is convenient, and you should think hard about the decision when it isn't convenient."

-- John Carmack 33

Functional programming is good at handling complexity using function composition, splitting difficult problems into manageable problems. Then the functions that solve these manageable problems are composed together to solve the original difficult problem.

Functions obtained with this process tend to be small, increasing their reusability, maintainability, and readability.

An interesting fact is that Elm is acting, for many developers, as a gateway to Functional Programming, as a pedagogical tool, because it is simpler to learn than other functional languages.

Recently several new functional programming languages have appeared: Gleam, Unison, Roc, Koka, Formality.

It is an exciting moment for functional programming.


Currying is a characteristic that is found in many functional languages. In Elm, all functions are curried by default. Currying means converting a function that takes multiple arguments into a sequence of functions that take a single argument:34

add a b = a + b -- <function> : number -> number -> number
add 1           -- <function> : number -> number
add 1 2         -- 3 : number
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The main advantage of currying is the increased flexibility in combining functions, like a puzzle game done with type signatures.35 For example, if you need to add 10 to the items of a list, you can write, using the add function defined above: (add 10) [1, 2, 3] -- Gives [11,12,13]
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8. Enforced discipline

Purely functional languages motivate programmers to think better about the programs they are building. Although the initial development time can increase with such restrictions, the increased maintainability compensates for the effort.

Elm enforces discipline on developers rather than letting developers be disciplined on their own. This fact, in conjunction with other characteristics, makes Elm a good fit for large front-end teams.

“Everything that is syntactically legal will eventually end up in your codebase”

-- John Carmack 36

Another example of enforced discipline is that it is not possible to include JavaScript code in Elm libraries.37 This fact means that the Elm guarantees, like no runtime errors, are also valid for your dependencies.

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9. Learnability

Elm is beginner-friendly. It doesn't mean that Elm is not sophisticated. It means that it is well designed. There are both simple constructs for beginners and complex constructs for masters. Complexity gets introduced gradually.38 This concept is sometimes called "gradual learning" or "progressive disclosure of complexity".39

Moreover, during its evolution, features that created confusion and were not important have been removed or modified, transforming it into a lean language that is easy to learn.40

To write a web application in Elm you don't need to be an expert in JavaScript, CSS, or HTML.

Setting up a development environment is also simple because all tools usually needed in a “modern web” setup, like bundlers, linters, and web frameworks, in Elm are either embedded or unnecessary.41

A beginner Elm developer, in our experience, can be productive in a couple of weeks and can master the language in a couple of months.

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10. Compiler as assistant

The Elm compiler can statically analyze the code for inconsistencies and provide precise feedback to the programmer.42

This feature is so crucial that it inspired a new coding style: Compiler Driven Development. In short: change parts of the code and then let the compiler errors guide you in the rest of the task. Then rinse and repeat.43

When Compiler Driven development44 involves defining type signatures first, we enter the realm of Type Driven Development.45

“If it compiles, it works” 46

Best practices should be automated as much as possible and the Elm compiler is playing an important role in this direction. Elm makes “best practices” the default.47

The compiler guarantees that all edge cases are covered, something difficult to achieve with hand-made unit tests. Another advantage of the compiler static analysis is that it is extremely fast and can provide the exact location of errors.48

John Carmack on Elm Errors
John Carmack's comment about the Elm error messages.

The Elm compiler produces state-of-the-art error messages and its high standard of quality is now an inspiration for other designers of languages too.49

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11. Elm as an influencer

Most technologies get influenced by existing ideas in one way or another. Elm, for example, was influenced by Haskell, Standard ML, OCaml, and F#.

On the other side, Elm is influencing the front-end community and the programming industry in general, thanks to its innovative ideas.

For example:

We now use Elm because we believe it is one of the best options for web development, and the number of tools inspired by it is a testament to its qualities. At the same time, we are keeping an eye on what is happening in the field, and we are not concerned about switching if something better becomes available.

Considering how influential Elm is and the general trend toward Functional Programming, it seems that this “something better” will be something similar to Elm. So the transition, if any, should be smooth.52

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12. The Elm Architecture

The Elm Architecture is probably the most relevant and influential innovation of Elm.53 It is a unidirectional data flow54 that helps to keep your application well organized. Also, it helps you to quickly understand applications built by other developers as this is the standard way to build applications in Elm.

The Elm Architecture, animated
A simple representation of the unidirectional data flows in The Elm Architecture. (Source: The Elm Guide) .55 Licensed under CC BY-NC-ND 4.0 license.

There are three building blocks in The Elm Architecture:

  • Model --- the state of your application, the only thing that can mutate
  • view --- a way to turn your state into HTML
  • update --- a way to update your state based on the Model and the messages

If we zoom in on the Elm block in the diagram above, this is what we would see inside:

The Elm Architecture, animated
How the Elm runtime system56 orchestrates the infinite loop57 of an Elm application using The Elm Architecture.

The Elm runtime system:

  • Waits for something to happen, for example, "a button is pressed"
  • Converts the event into the appropriate message Msg
  • Sends Msg and Model to update that will return an updated Model and optional commands Cmd, for example, an HTTP request
  • Sends Cmd, if any, to the effects engine
  • Sends the updated Model to view that will return new HTML
  • Updates the DOM using the new HTML
  • GOTO start

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13. The Elm debugger

The built-in Elm debugger58 is a useful tool to debug Elm applications. It shows the state of the application and keeps track of all the messages fired during the application's life. It also gives you the ability to go back in time, creating an immediate connection to what we are coding.59

The Elm Debugger
The Elm debugger. From the left to the right: the application; the history of messages; the current message and the model. 60

This is similar to what Bret Victor showed in his famous talk "Inventing on Principle."61

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14. Elm-UI, the alternative to CSS/HTML

Elm-UI is a new language for layout and interface.62 It is a complete alternative to HTML and CSS. It is the most used non-core Elm library, and we use it in almost all of our projects.63

It applies the Principle of least astonishment seen earlier to the design of a web page. Your intents are translated clearly in the design, a rarity using CSS, making the design process fun and quick.

For example, let's suppose we have a blue box and we want to center (horizontally and vertically) an element of unknown width and height containing the text "I'm centered! 🎉" in it:


A possible solution in HTML/CSS using Flexbox is:64

    .parent {
        height: 100%;
        display: flex;
        justify-content: center;
        align-items: center;

<div class="parent">
    <div>I'm centered! 🎉</div>
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To obtain the same result in Elm-UI you would write:65

el [centerX, centerY] <| text "I'm centered! 🎉"
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Note how this solution is less verbose and our intents, centerX, and centerY are clearly stated, and directly applied to the element containing the text, and not to its parent.

How does it work? Let’s briefly see what Elm-UI is doing behind the scenes.

First of all, Elm doesn't have a separate templating language, the templating language for Elm is Elm.66

For example, this block of Elm code that uses the standard Elm HTML library (not Elm-UI):67

div [] [ text "I'm centered! 🎉" ]
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generates this HTML

<div>I'm centered! 🎉</div>
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There is a one-to-one mapping between these two representations. So nothing special here.

Now, using Elm-UI, when we write:

el [centerX, centerY] <| text "I'm centered! 🎉"
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Elm-UI programmatically generate the following HTML (plus a bunch of CSS, omitted for brevity):68

<div class="hc ah cx av cy s e wc">
    <div class="s t wf hf">
        I'm centered! 🎉
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Elm-UI does all the heavy lifting for us, adding styling and elements to ensure that the page looks exactly the way we meant.69

With Elm-UI, the one-to-one mapping is not between the Elm code and the HTML/CSS anymore, but between the Elm code and the layout, making the outcome predictable.

Elm-UI treats CSS/HTML as bytecode the same way that Elm treats Javascript as bytecode.70

It feels like a breath of fresh air after years spent learning all sorts of CSS tricks.71

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15. Readability and Elm syntax

Functional languages, being declarative, allow us to concentrate on writing what and not how. Hiding the how details make the code easier to read and understand, the “intentions” of the code became transparent.

In the Elm community, writing readable code is considered a high priority. This fact is important because, as developers, we spend more time reading code than writing code.

Elm has an ML-style syntax, in contrast to the C-style syntax of Java, JavaScript, and other popular languages. It was a choice that traded familiarity with convenience and fitness,72 as sometimes familiarity hides complexity.73

What we like about this syntax is its simplicity. Compared to C-style syntax, most parentheses, keywords, and punctuations are not necessary.

For example, this is how we define add, a function that adds two numbers together:

add a b = a + b
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Pipeline operator

The pipeline operator, which is also present in other languages like Elixir, F#, and (maybe) JavaScript,74 can help to deal with multiple parentheses or with data flows. Let's consider this snippet that calls four nested functions:

f ( g ( h ( i 7 ) ) )
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It can be re-written with the pipeline operator as:

f <| g <| h <| i 7
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The benefit of this style is that we don't need the closing parentheses anymore.

With a reversed pipeline operator, we can re-write it in a second style to make the data flow explicit:

    |> i
    |> h
    |> g
    |> f
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Pattern matching

An example of pattern matching is the case .. of that allows us to branch based on the custom type variant. For example:

type TrafficLight = Green | Yellow | Red -- Or Blue | Yellow | Red in Japan 🚦

hexColor trafficLight = 
    case trafficLight of
        Green  -> "00ff00"
        Yellow -> "ffff00"
        Red    -> "ff0000"        
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If we wanted to add a fourth variant to the color type, the compiler would force us to add that case to this construct – this is very useful.

Separator-leading lists

Elm-Format, the formatter of Elm, formats lists putting the separator (a comma) at the beginning of the line and not at the end. For example:

trafficLights =
    [ Green
    , Yellow
    , Red
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This formatting style has several benefits, for example, the code looks more organized (all commas are aligned), and there are fewer merge conflicts.75

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16. Refactoring

The idea that the Elm compiler is like an assistant and the fact that once the code compiles, it usually works makes refactoring a pleasant experience.

An additional factor that makes refactoring easy is that being a purely functional language, the order in which we write the code doesn’t matter.76

For example, in Elm, we can write:77

b = a + 2
a = 1
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Even if the two lines seem in the wrong order, it works well in Elm, but the same code will throw the error “b is not defined” in imperative languages.78

Refactoring became simpler because we can shuffle pieces of code around without warring.

In our largest project, we are in the middle of a third major refactoring iteration, and we have pieces of code that are still at the first iteration and some code in the second iteration. And all of our code works well together. We are now incrementally moving all our code toward the third iteration. In Elm, you don't need to get things right from the beginning.

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17. Hiring

Hiring for a language that is not mainstream has some downsides. For example, not many developers know it fluently.

However, learning Elm is a fast process. As already stated, our experience is that it takes a couple of weeks to be productive and a couple of months to master it.

As a result, rather than asking “How many applicants know X?” we should better ask: “What does, knowing X, tell us about an applicant?” focusing on engineers that have the passion and are capable of adapting and learning new concepts.

Moreover, hiring for a minor technology can boost your brand as an innovative company, making you the coolest kid on the block.

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18. Fast performance and small assets

The Elm compiler can apply several optimizations specific to the fact that Elm is a purely functional language. This leads to several benefits, including:

  • The performances of Elm applications are among the fastest. Internally Elm uses the concept of a virtual DOM, similar to React. The speed of the Elm virtual DOM is comparable to Svelte, which uses a different mechanism to update the DOM.79
  • The Elm compiler produces smaller assets compared to other frameworks. Among the various optimizations to achieve this result, there is the dead code elimination with granularity to the single function that works across the entire ecosystem. If you import a large package and use only one of the functions contained, the compiler will ensure that only that function ends up in your generated code.80

The Elm compiler per se is also fast. Our bigger codebase contains ~66,500 lines of Elm code, and it compiles incrementally in 0.3 seconds and from scratch in 2.5 seconds.81

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19. Content-driven static websites

Elm is not a good fit to build static websites that are mostly content-driven. In these cases, an old server-side rendered website can be a better option.

On the other side, if you get to like Elm, it is hard to go back to plain JavaScript/HTML/CSS, so we experimented with Elm for static websites. There are several tools for static site generation. We used Elm-Starter,82 a library that transforms an Elm website into a server-side rendered PWA that is also installable, works off-line and works without JavaScript.

These characteristics help to achieve good Lighthouse scores and good search engine ranking (SEO).

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What we don't like about Elm

1. Not mainstream

If many people believe something, does it make it true? 83

There are common issues with all technologies that are not mainstream. When they are not mainstream, it is hard to promote them, especially if decision-makers use the fallacious “appeal to popularity” argument. Something that goes along the lines of "The technology X is better than Y because it has more stars in GitHub".84

We believe that arguments should be considered case by case. Sometimes not being mainstream does have related implications (see Reinventing the wheel); other times is more nuanced than it seems (see Hiring); most of the time, not being mainstream is unrelated to good qualities.85

If you still need to be reassured, consider that, regardless of the popularity of Elm, many companies are using it, including several large companies like Microsoft, IBM, eBay, Ford, Amazon, Zalando, and Thoughtbot.86

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2. Lack of Googleable resources

Asking an Elm question in Google does not always yield good results.

Most of the Elm public conversation happens in the Elm Slack channel87 that is not visible to the Google bots.

Also, another consequence is that the quantity of materials on the Stack Overflow website is limited. This fact is not always as bad as it seems.

Stack Overflow sometimes suffers from having information that is not updated or still "old school," making it useless and occasionally even harmful.

In the Elm Slack channel, information is always fresh, and the community is very supportive. It is just not visible, so it requires extra effort to join the Slack channel.

Other times, resources are scattered and are not published with good SEO. Take, for example, this valuable list of hints that seldom appear in Google results.88

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3. Reinventing the wheel

Because Elm is not a mainstream language, it is sometimes necessary to reinvent something that could otherwise have been acquired by adopting a different technology. For example, we wrote a library inspired by react-jsonschema-form to create HTML forms.89

This issue was more significant in the past because nowadays the number of Elm packages covers a wide spectrum of subjects.

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4. Mindset shift

Purely functional programming could be mind-bending and intimidating for a developer that has programmed only in an object-oriented style.

“Faced with the choice between changing one’s mind and proving that there is no need to do so, almost everyone gets busy on the proof.”

- John Kenneth Galbraith

Some consider this to be a benefit as it brings you out of your comfort zone and makes you think differently about programming.

But for someone else is a burden and could discourage a team from adopting Elm.

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5. Some JavaScript and CSS are still necessary

Ideally, we could build an application by just writing in the Elm language. But if you need to use a third-party library not converted to Elm, we still need to use JavaScript. Doing so means entering again into the realm of possible runtime errors.

Elm provides three ways to interact with external libraries: Flags, Ports, and Custom Elements.90 All of them require you to write some JavaScript.

In our case, for example, we must use a JavaScript library for handling payments.

The required CSS, while using the library Elm-UI, is limited. In our applications, we have small snippets of CSS that are mainly tricks to support IE11.

Related to this, Elm is probably not a good fit for short projects that require lots of integration with third-party JavaScript libraries.

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We mentioned some of the benefits of coding with Elm, or with a purely functional language for that matter. We also talked about the main issues.

For us the benefits are overwhelming, compared to the issues, and this is why we are happy with the choice we made.

“Then I came to Functional Programming … and programming was fun again!"

- Rúnar Bjarnason91

A consequence of these technical benefits is the great feeling of relaxation, not being left alone, and not being afraid of breaking things.

Compared to the pre-Elm experience, coding is now more enjoyable, more productive, and without runtime errors! 🎉

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Other testimonies

It is always good to hear a different opinion on every argument. Here you can find other testimonies of Elm being adopted in different companies:

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  1. Elm is a compiled, immutable, strongly statically typed, and purely functional programming language that compiles to JavaScript. JavaScript is a just-in-time compiled, weakly dynamically typed, multi-paradigm programming language. To know more about Elm, a good start is the official guide. If you are familiar with JavaScript you can check From JavaScript? which is a short comparison between the syntax of the two languages. The Elm language, including the compiler and the core libraries, is designed and developed by Evan Czaplicki, with the support of a small core team of developers. Evan retains the final say in disputes or arguments. This setup, a common practice for the initial few years of many languages, guarantees a coherent vision and well-designed APIs. 

  2. Pure functions are those functions where the output only depends on the input and don’t have any side effects. 

  3. The Introduction to The Elm Architecture and How to Build our First Application by James Kolce was the second part of a three-part series published on the CSS-Tricks website in 2017. 

  4. The Elm website is the main source of information and documentation to start with Elm. A quote about runtime errors from the site: "Elm uses type inference to detect corner cases and give friendly hints. NoRedInk switched to Elm about two years ago, and 250k+ lines later, they still have not had to scramble to fix a confusing runtime exception in production."  

  5. In the front-end, errors at runtime are errors that happen in the browser. These errors can completely halt the functionality of the website and you, as the creator, may not even know about them because they are happening on other peoples' devices. Some tools allow you to get notified when these errors happen. These errors are usually JavaScript errors, for example, trying to access values that are null or undefined.  

  6. Even if it is almost two years old now, some more details about the use of Elm at Rakuten can be found in my talk Elm at large (companies) given at the Oslo Elm Day 2019 conference. 

  7. In this section of the Elm guide, Evan Czaplicki explains what are the tradeoffs of Elm guarantees. 

  8. A quote from an in-depth piece by John Carmack that looks at the value of using functional-style programming with C++. John Carmack is an independent AI researcher, consultant CTO at Oculus VR, and founder of Armadillo Aerospace and Id Software, where he was the lead programmer of the games Commander Keen, Wolfenstein 3D, Doom, and Quake

  9. The Elm runtime system is the part of the code in charge of directing the application. For example, it figures out how to render HTML, how to send an HTTP request, redirect users' clicks back into the Elm code, etc.  

  10. The Principle of Least Astonishment states that the result of performing some operation should be obvious, consistent, and predictable, based upon the name of the operation and other clues. 

  11. The Elm Package Manager allows you to check differences (functions added/removed/modified) in any published Elm package simply by running the command elm diff like in elm diff elm/json 1.0.0 1.1.2. Evan Czaplicki gives a live example of this feature in the video Convergent Evolution

  12. There are several funny videos about this topic on YouTube. One of the most popular is probably What the... JavaScript? by Kyle Simpsons 

  13. Note that Elm requires explicit conversion between integers and floating numbers. Someone find this cumbersome. This explicit conversion is necessary to make the Elm compiler faster. You can read more about this in the Implicit Casts article. 

  14. Type annotations in Elm are not required but are considered good practice to add them. It helps the compiler to provide more precise errors and also to detect bugs, like in the case of Infinite Types

  15. The article TypeScript's Blind Spots by Dillon Kearns illustrates several weak points of TypeScript. 

  16. Type guards are TypeScript expressions that perform a runtime check to discriminate between custom types. For example:

    After that is possible to write code like:

    In Elm type guards are not needed, it is possible to just directly use a case .. of construct

    The case .. of construct also guarantees that we are considering all possible custom types. 
  17. The Penrose Triangle is a triangular impossible object. Illustration made with Elm and Elm-Playground, here the source code

  18. The concept of making impossible states impossible is well explained by Richard Feldman in his homonymous talk at Elm-conf 2016

  19. These coding patterns in Elm are considered good practice. Making impossible states impossible is one of them. 

  20. A type alias is a shorter name for a type. 

  21. The type Maybe is how Elm handles missing values because null or undefined don't exist. Maybe is defined as

    The Elm compiler will refuse to compile until you handle all the cases where a value may be missing in your code. 
  22. As Evan Czaplicki put it, “Custom types are the most important feature in Elm”. 

  23. This principle is also mentioned in the Zen of Python: "There should be one — and preferably only one — obvious way to do it." 

  24. The One JavaScript principle is about removing versioning and being always back-compatible. This fact, combined with ten days of design and 25 years of back compatibility, inevitably accumulated a large number of different ways of doing things. For example, defining a function can be done in several different ways.  

  25. The latest large change was the Farewell to Functional Reactive Programming in May 2016. Here a high-level picture of the updates

  26. Evan Czaplicki's ideas about the future of Elm.  

  27. The infix operator has been removed because it was leading to unreadable code due to the creation of fancy non-standard operators. This is a longer explanation of why the infix operators were removed

  28. The feed that shows all the updates done on Elm packages is a good way to measure the activity that is happening on Elm libraries. Again, this activity should not be confused with the adoption of the language. Stable libraries tend not to be updated often. You will seldom see a core library in the feed. Sometimes there are complaints about lack of transparency about the work done on the compiler and core modules. Elm does not follow the standard "open source" culture in certain ways and most of the core work is done on a private repository. If you are interested in supporting the language, the best way is by getting involved in its community and contributing to the ecosystem

  29. The update from version 0.18 to version 0.19 was mainly about the optimization of the compiler. 

  30. The update of the HTTP library is probably the latest large update within the core modules. 

  31. Functional programming is a programming paradigm where programs are constructed by applying and composing functions. It is a declarative programming paradigm based on a sequence of functions that only depend on each other in terms of arguments and return values. It looks something like this:

    By contrast, the procedural paradigm is based on a sequence of imperative commands that may implicitly alter the shared state. It looks something like this:
  32. From Object Orient to Functional Programming, talk by Richard Feldman about paradigm shifts. 

  33. In-depth: Functional programming in C++ by John Carmack. 

  34. Out of curiosity, the closest equivalent in JavaScript of the Elm function

    that also supports currying, is
    That is mostly syntactic sugar for
    To learn more about how currying and how functions are defined in Elm, I recommend reading Functions in Elm
  35. Passing a smaller number of arguments to a function, like in add 10, is called partial application and it opens the door to interesting coding techniques. If you squint, it is like dependency injection where the 10 is the injected part. Scott Wlaschin explains this and other patterns in his talk Functional Design Patterns.  

  36. John Carmack during his Keynote at QuakeCon 2013  

  37. A loophole that allowed using JavaScript in libraries was closed in version 0.19

  38. For example sandbox, element, document, and application in Elm-Browser; get, post, and request in Elm-HTTP; picture, animation, and game in Elm-Playground; etc. 

  39. Chris Krycho talks about it in the article Progressive Disclosure of Complexity and Typed FP Languages while Evan Czaplicki explains this concept in the talk Let's be mainstream

  40. Other features are simply not added as the design of Elm languages was focused on users. For example type classes. As Evan Czaplicki explains in Let's be mainstream! User-focused design in Elm, "If you are going to give away simplicity, you better be doing it for a very good reason". 

  41. You can start experimenting with Elm using elm reactor, a web server built-in the Elm compiler that automatically re-compiles your code every time you refresh the browser. For more advanced coding there is Elm-Live, a web server that supports custom HTML and hot reload. Elm-Live is what we use for our team. You can also use the usual suspects, like Webpack or Parcel. 

  42. The idea of the Compiler as Assistant was born together with Elm itself. In this article, Evan Czaplicki explains the further improvement in this direction done for version 0.16. 

  43. Moving Faster with Tiny Steps in Elm by Dillon Kearns 

  44. Kevin Yank explains what is Compiler Driven Development in this video. Louis Pilfold explains how a compiler can be an assistant during software development, referring to BEAM, a language inspired by Elm. And this is yet another sample of Compiler Driven Development during one of the talks of Evan Czaplicki. 

  45. Idris is probably the fittest language for Type Drive Development. Idris has the feature of holes, while Elm can use Debug.todo

  46. This way of saying probably originated in Haskell but it applies to Elm too. 

  47. An example of Elm making “best practices” the default, is about variable shadowing (same variable name defined twice in an ambiguous way). While most linters produce warnings in case of variable shadowing, the Elm compiler generates an error and stops the compilation, until the variable shadowing issue is removed. More on this in the Variable Shadowing article. 

  48. To push the static analysis further, other tools can be used, like Elm-Review

  49. "This should be an inspiration for every error message", John Carmack commenting on Elm error messages 

  50. The Prior Art document of Redux explains the Elm influence in detail. 

  51. Joe Groff, one of the designers of the Swift language, mentions that SwiftUI was inspired by Elm and React

  52. ClojureScript, ReScript, and PureScript are three languages that have a similar concept to Elm. 

  53. Details about The Elm Architecture can be found in the official Elm Guide. The Elm Architecture is the predominant way to build applications in Elm. Different variants are also possible. Elm-spa by Ryan Haskell-Glatz is a tool that helps to create single-page applications and create extra abstraction above The Elm Architecture. Rémi Lefèvre built the RealWorld example app using the Effect pattern

  54. The Elm Architecture is based on unidirectional data flow (a.k.a. one-way data binding) like React, in contrast to the bidirectional data flow (a.k.a. two-way data binding) of frameworks like Angular, Vue, and Svelte (in Svelte two-way binding can be disabled). There have been issues with two-way data binding. For example, the many-to-many dependencies between the view and the model can create an infinite loop of cascading updates. Another issue is the lack of control of the change detection mechanism. It is an implicit behavior that is not easy to control. Unidirectional data flow tends to be more predictable. 

  55. The illustration A simple representation of the Elm Architecture is from the Elm Guide

  56. When we write Elm code, 100% of our code is pure so there are no side effects. But without side effects, our application would just be a boring silent empty screen. The Elm runtime system is the part of the code that is in charge of the side effects. In our code, we just request these side effects to be done and we wait for the outcomes. Examples of side effects are HTTP requests or DOM modifications. How do we do side effects while remaining pure? In Elm, there are two ways. For things like HTTP requests, for example, there are commands (Cmd), that are instructions, in the form of data, that we send as requests to the Elm runtime system. For changing the DOM, the way to do side effects is to take the entire state of the world as an argument and return a new version of it. So we can change the world (side effects) by remaining pure. The world in our case is the Model and the function that does that is the update function: update: Msg -> Model -> (Model, Cmd msg) (see The Elm Architecture for more details). The video What is IO monad? by Alexey Kutepov explains this concept in general terms. 

  57. If you are familiar with game development you can find similarities between The Elm Architecture and The Game Loop. The main difference is that usually games don't wait for something to happen, but the loop keeps running all the time. When we develop games in Elm, we do the same using onAnimationFrame so that the loop keeps running with a usual speed of 60 times per second. 

  58. More about the Elm debugger in The Perfect Bug Report 

  59. The Elm debugger is usually disabled for applications that are released in production, but you can find an example of it in where it has been kept active for educational purposes. 

  60. Demo and source code of the application used to demonstrate the Elm Debugger.  

  61. Bret Victor is an interface designer, computer scientist, and electrical engineer known for his talks on the future of technology. In his talk, Inventing on Principle, Victor showed his vision about fixing the fundamentally broken way we make software. The vision, in short, is that "Creators need an immediate connection to what they’re creating." More about this in The Coming Software Apocalypse by James Somers. 

  62. Elm-UI is developed by Matthew Griffith. More information about Elm-UI in the module documentation

  63. This enhanced mirror of the Elm Package Manager list packages in order of popularity. If we exclude the core library, the top 5 packages are Elm-UI | Elm-JSON-Decode-Pipeline | Elm-CSS | elm-color | Remotedata

  64. There are at least three different ways to center an element using CSS, probably more. You can use Pre-Flexbox style (example at Codepen), Flexbox (example at Codepen), or Grid (example at Codepen). The version using flexbox is probably simpler. 

  65. Code example at Ellie

  66. Evan Czaplicki mentions that the templating language for Elm is Elm in the video Convergent Evolution that compares Elm to React.  

  67. Note that div is a function that accepts two lists, one for attributes and one for children elements. text is also a function. It may help to see the type signature of these functions to understand better:

  68. You can check the entire outcome of Elm-UI in this live example

  69. The benefits of Elm-UI are more relevant in complex layouts than in this is a trivial example. Moreover, Elm-UI ensures that our HTML is valid and accessible. For example, forcing us to add a description to the image and blocking us from adding children to the HTML img element. The img function, with the standard HTML Elm library, is defined as

    The second list is the one that allows creating children elements, producing invalid HTML. Using Elm-UI, we cannot add children to the HTML element img due to the definition of the image function itself:
    The function image doesn't accept a list of children, but just an argument containing src and description
  70. CSS as Bytecode is also the title of one of the talks of Richard Feldman. 

  71. Before moving to Elm-UI we were avid consumers of, an excellent source of CSS tricks and information related to web development. Ironically it was also the place where we learned the existence of Elm that led us to use Elm-UI and eventually made way less relevant. 

  72. Evan Czaplicki explains the decision of using the ML-style syntax throughout the video Convergent Evolution

  73. Rich Hickey mentions the idea that familiarity hides complexity in his talk Are we there yet? (11th minute) where he advocated for the reexamination of the basic principles of OOP. 

  74. The pipeline proposal is currently at stage 1 of the TC39 proposal process

  75. Read more about the benefits of the comma-leading lists approach in The Case for Comma-Leading Lists. This approach applies to any list separator. For example, in CSS we could write:

  76. Elm also doesn't have hoisting, the JavaScript mechanism where variable and function declarations are put into memory during the compile phase giving the impression that they are moved to the top of their scope before code execution. 

  77. The Elm code example is available here. Another situation where the order matter is when mutability is in action, for example

    These constructs don't compile in Elm because all data is immutable. By the way, have you ever noted that with mutability, the = sign loses its mathematical meaning? a = a + 2 is an impossible construct in mathematics. With immutability, it is still holding because you can only write newA = a + 2. More about this in the Hints for Bad Recursion article. 
  78. Live example in JavaScript 

  79. Elm and Svelte performances are neck and neck as it can be verified from the JavaScript framework benchmark. This thread has an interesting conversation about web frameworks' performances. 

  80. The equivalent of dead code elimination, in JavaScript, is called tree shaking and it usually works at the granularity of modules instead of single functions. Other optimizations contribute to the small assets of Elm. Our largest application of ~66,500 lines of Elm code is 188kb zipped, including the SVG assets, the extra JavaScript, and translations in several languages. 

  81. These numbers are calculated using this method on a MacBook Pro 2.3GHz Quad-Core i7. 

  82. These are the most common tools to generate static sites in Elm: Elm-Pages | ElmStatic | Elm-Starter

  83. Appeal to popularity, or Argumentum ad populum, is a fallacious argument that concludes that something must be true because many or most people believe it. Illustration made with Elm and Elm-Playground, here the source code. On a fun note, Elm was recently featured in a New York Times crossword puzzle. Does this make Elm a mainstream programming language now? 

  84. Evan Czaplicki in the talk What is Success? discusses this topic. 

  85. For example, did the object-oriented paradigm become mainstream for its inherently good qualities and its ability to deal with complex problems? Was it by chance? (As Richard Feldman suggests in his video, Why Isn't Functional Programming the Norm?) Was it because it is an inferior paradigm (as Brian Will highlights in Object-Oriented Programming is Bad) but Microsoft and the industry that created, promoted it? 

  86. Some of these companies are mentioned by Richard Feldman in the video Building UIs in the Dark (aka Elm Programming) and the list Elm companies

  87. The Elm Slack channel counts around 20,000 members. Another platform used by the Elm community is There is also a channel on the Reddit website that tends to be unpleasant so not many Elm developers usually comment there. 

  88. There is no lack of good resources to learn Elm. A good aggregator for these resources is the Awesome-Elm.  

  89. The library that we wrote is now open source

  90. Elm provides several methodologies to communicate with JavaScript. Here is an introduction to JavaScript interoperability with some examples

  91. Rúnar Bjarnason is an advocate of functional programming. He is the co-author of the Scala “Red book” and the creator of the programming language Unison. Unison, “A friendly programming language from the future.”, has similarities to Elm as they are both inspired by Haskell, as explained in the video Introduction to the Unison programming language.  

Top comments (7)

storytellercz profile image
Jan Dvorak

Thank you for the exhaustive article! Definitely put Elm on my map.

practicalprogramming profile image
Christian Neumanns

Very well written and lots of useful information!
Thank you.

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Per Lundholm

Wow, what a ride that was. Great job. This is my new goto article when someone wants me to explain the real deal about Elm.

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James Robb

Excellent article! 🔥

brahayandev profile image

Great information to take decisions! So thanks for sharing helps us!

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Etienne Burdet

Wow, thanks for sharing this! Absolute must read!

thrashabaddon profile image

This is such a well written post. Thank you :)