Elm (programming language): Difference between revisions

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{{Short description|Functional programming language}}

{{primary sources|date=May 2019}}

{{Infobox programming language

| name = Elm

| logo = Elm logo.svg

| paradigm = [[Functional reactive programming|functional reactive]], [[functional programming|functional]]

| logo caption = The Elm tangram

| year = 2011

| influenced_by paradigm = [[HaskellFunctional (programming language)|Haskell]], [[Standard ML]], [[OCamlfunctional]]

| family = [[Haskell]]

| typing = [[static typing|static]], [[strong typing|strong]], [[type inference|inferred]]

| designer = Evan Czaplicki

| released = {{Start date and age|2012|03|30}}<ref name="initialrelease">{{cite web |url=https://www.reddit.com/r/haskell/comments/rkyoa/my_thesis_is_finally_complete_elm_concurrent_frp/|last1=Czaplicki|first1=Evan |title=My Thesis is Finally Complete! "Elm: Concurrent FRP for functional GUIs" |website=[[Reddit]]|date=30 March 2012}}</ref>

| website = [http://elm-lang.org/ elm-lang.org]

| latest release version = 0.19.1

| file_ext = .elm

| latest release date = {{Start date and age|2019|10|21}}<ref>{{Cite web |url=https://github.com/elm/compiler/releases |title=Releases: elm/Compiler |website=[[GitHub]]}}</ref>

| typing = [[Static typing|static]], [[Strong and weak typing|strong]], [[Type inference|inferred]]

| platform = [[x86-64]], [[AArch64|ARM]]

| operating system = [[macOS]], [[Microsoft Windows|Windows]], [[Linux]]

| license = [[Permissive license|Permissive]] ([[BSD license|Revised BSD]])<ref name=license>{{cite web |url=https://github.com/elm/compiler/blob/master/LICENSE |title=elm/compiler |website=GitHub|date=16 October 2021}}</ref>

| file ext = .elm

| website = {{Official URL}}

| influenced by = [[Haskell]], [[Standard ML]], [[OCaml]], [[F Sharp (programming language)|F#]]

| influenced = [[Redux (JavaScript library)|Redux]],<ref>{{cite web |url=https://redux.js.org/introduction/prior-art |title=Prior Art - Redux |website=redux.js.org|date=28 April 2024 }}</ref> [[Rust (programming language)|Rust]],<ref>{{Cite web |title=Shapes of errors to come|url=https://blog.rust-lang.org/2016/08/10/Shape-of-errors-to-come.html |access-date=2016-10-08 |website= Rust Blog |quote=Those of you familiar with the Elm style may recognize that the updated --explain messages draw heavy inspiration from the Elm approach.}}</ref> [[Vue.js#Official libraries|Vue]],<ref>{{cite web |url=https://vuejs.org/v2/guide/comparison.html#Scale |title=Comparison with Other Frameworks — Vue.js}}</ref> Roc,<ref>{{Cite web |title=roc/roc-for-elm-programmers.md at main · roc-lang/roc |url=https://github.com/roc-lang/roc/blob/b8040bf6a224cc82a0d146a960c3f8dfca20d58b/roc-for-elm-programmers.md |access-date=2024-02-17 |website=[[GitHub]] |language=en |quote=Roc is a direct descendant of the Elm programming language. The two languages are similar, but not the same!}}</ref> Derw,<ref>{{cite web |title=Why Derw: an Elm-like language that compiles to TypeScript? |date= 20 December 2021|url= https://derw.substack.com/p/why-derw-an-elm-like-language-that}}</ref> Gren<ref>{{cite web |title=Gren 0.1.0 is released |url= https://gren-lang.org/news/220530_first_release/}}</ref>

}}

'''Elm''' is a [[functionalDomain-specific programminglanguage|___domain-specific]] [[programming language]] for [[Declarative programming|declaratively]] creating [[web browser]] -based [[graphical user interface]]s. Elm is [[purely functional programming|purely functional]], and is developed with emphasis on [[usability]], performance, and [[Robustness (computer science)|robustness]]. It advertises "no [[Runtime (program lifecycle phase)|runtime]] [[Exception handling|exceptions]] in practice",<ref>{{cite web |url=https://elm-lang.org/ |title=Elm home page}}</ref> made possible by the Elm compiler's [[Type system#Static type checking|static type checking]].

== History ==

Elm uses the [[Functional reactive programming|Functional Reactive Programming]] style and [[purely functional]] graphical layout to build user interface without any destructive updates.

Elm was initially designed by Evan Czaplicki as his thesis in 2012.<ref>{{cite web |url=https://elm-lang.org/assets/papers/concurrent-frp.pdf |title=Elm: Concurrent FRP for Functional GUIs}}</ref> The first release of Elm came with many examples and an online editor that made it easy to try out in a [[web browser]].<ref>{{cite web |url=https://elm-lang.org/try |title=Try Elm |website=elm-lang.org |access-date=2025-04-26 }}</ref> Czaplicki joined [[Prezi]] in 2013 to work on Elm,<ref>{{cite web |url=https://elm-lang.org/news/elm-and-prezi |title=elm and prezi |website=elm-lang.org}}</ref> and in 2016 moved to [[NoRedInk]] as an Open Source Engineer, also starting the Elm Software Foundation.<ref>{{cite web |url=https://elm-lang.org/news/new-adventures-for-elm |title=new adventures for elm |website=elm-lang.org}}</ref>

The initial implementation of the Elm compiler targets HyperText Markup Language ([[HTML]]), [[Cascading Style Sheets]] (CSS), and [[JavaScript]].<ref>{{cite web |url=https://github.com/elm/compiler |title=elm/compiler |website=GitHub|date=16 October 2021}}</ref> The set of core tools has continued to expand, now including a [[read–eval–print loop]] (REPL),<ref>{{cite web |url=https://elm-lang.org/news/repl |title=repl |website=elm-lang.org}}</ref> [[Package management system|package manager]],<ref>{{cite web |url=https://elm-lang.org/news/package-manager |title=package manager |website=elm-lang.org}}</ref> time-travelling debugger,<ref>{{cite web |url=https://elm-lang.org/news/time-travel-made-easy |title=Home |website=elm-lang.org}}</ref> and installers for [[macOS]] and [[Microsoft Windows|Windows]].<ref>{{cite web |url=https://guide.elm-lang.org/install.html |title=Install |website=guide.elm-lang.org}}</ref> Elm also has an ecosystem of community created [[Library (computing)|libraries]],<ref>{{Cite web |url=https://package.elm-lang.org/ |title=Elm packages |website=Elm-lang.org}}</ref> and Ellie, an advanced online editor that allows saved work and including community libraries.<ref>{{Cite web |url=https://ellie-app.com/ |title=Ellie |website=Ellie-app.com}}</ref>

== Description ==

== Features ==

The primary implementation of Elm compiles to HTML, CSS, and JavaScript.

Elm has a small set of language constructs, including traditional if-expressions, let-expressions for storing local values, and case-expressions for [[pattern matching]].<ref name="syntax">{{cite web |url=https://elm-lang.org/docs/syntax |title=syntax |website=elm-lang.org |access-date=2025-04-26 }}</ref> As a functional language, it supports [[anonymous function]]s, functions as arguments, and functions can return functions, the latter often by partial application of [[currying|curried]] functions. Functions are called by value. Its semantics include immutable values, [[pure function|stateless functions]], and static typing with type inference. Elm programs render HTML through a virtual DOM, and may interoperate with other code by using "JavaScript as a service".

=== Immutability ===

In Elm, Functional Reactive Programming takes the place of event handlers and callbacks; it also manages all screen updates automatically. Purely functional graphical layout takes the place of working with the DOM. Elm also allows [[Markdown]] to be embedded directly.

All values in Elm are [[Immutable object|immutable]], meaning that a value cannot be modified after it is created. Elm uses [[persistent data structure]]s to implement its arrays, sets, and dictionaries in the standard library.<ref>{{cite web |url=https://package.elm-lang.org/packages/elm/core/latest/ |title=elm/core |website=package.elm-lang.org}}</ref>

=== SyntaxStatic and Semanticstypes ===

Elm is statically typed. Type annotations are optional (due to type inference) but strongly encouraged. Annotations exist on the line above the definition (unlike C-family languages where types and names are interspersed). Elm uses a single colon to mean "has type".

Types include primitives like integers and strings, and basic data structures such as lists, tuples, and records. Functions have types written with arrows, for example <code>round : Float -> Int</code>. [[tagged union|Custom types]] allow the programmer to create custom types to represent data in a way that matches the problem ___domain.<ref>{{cite web |url=https://guide.elm-lang.org/types/custom_types.html |title=Model The Problem |website=Elm |access-date=4 May 2016 |ref=tagged-unions}}</ref>

Elm's adopts a [[Haskell (programming language)|Haskell]] styled syntax, with influences from [[OCaml]] and [[FSharp]]. For example, "has type" is written with a single colon <code>(:)</code>, and forward and backward function application use the <code>(<|)</code> and <code>(|>)</code> operators<ref name="syntax">[http://elm-lang.org/learn/Syntax.elm The Syntax of Elm]</ref> where <code>(f x)</code> equals <code>(f |> x)</code> equals <code>(x <| f)</code>.

Types can refer to other types, for example a <code>List Int</code>. Types are always capitalized; lowercase names are type variables. For example, a <code>List a</code> is a list of values of unknown type. It is the type of the empty list and of the argument to <code>List.length</code>, which is agnostic to the list's elements. There are a few special types that programmers create to interact with the Elm runtime. For example, <code>Html Msg</code> represents a (virtual) DOM tree whose event handlers all produce messages of type <code>Msg</code>.

Elm has extensible records <ref>[http://elm-lang.org/learn/Records.elm Elm - Extensible Records]</ref> which safely provide much of the flexibility of [[Javascript]]'s object model.

Rather than allow any value to be implicitly nullable (such as JavaScript's <code>undefined</code> or a [[null pointer]]), Elm's standard library defines a <code>Maybe a</code> type. Code that produces or handles an optional value does so explicitly using this type, and all other code is guaranteed a value of the claimed type is actually present.

The type system supports primitive types like integers and floats, structured data like tuples and records, and custom [[Algebraic data type|ADT's]].<ref>[http://elm-lang.org/learn/Getting-started-with-Types.elm Elm - Getting started with Types]</ref>

Elm provides a limited number of built-in [[type class|type classes]]: <code>number</code> which includes <code>Int</code> and <code>Float</code> to facilitate the use of numeric operators such as <code>(+)</code> or <code>(*)</code>, <code>comparable</code> which includes numbers, characters, strings, lists of comparable things, and tuples of comparable things to facilitate the use of comparison operators, and <code>appendable</code> which includes strings and lists to facilitate concatenation with <code>(++)</code>. Elm does not provide a mechanism to include custom types into these type classes or create new type classes (see [[#Limits|Limits]]).

Elm's version of [[Functional reactive programming|Functional Reactive Programming]] is event-driven, meaning that updates are only performed as necessary. It shares the closest resemblance to Event-Driven FRP <ref>{{cite journal

| last1 = Wan | first1 = Zhanyong

| last2 = Taha | first2 = Walid

| last3 = Hudak | first3 = Paul

| author3-link = Paul Hudak

| title = Event-Driven FRP

| journal = Proceedings of the 4th International Symposium on Practical Aspects of Declarative Languages

| pages = 155–172

| year = 2002 }}</ref><ref>[http://elm-lang.org/learn/What-is-FRP.elm Elm - What is Functional Reactive Programming?]</ref> and Arrowized FRP.<ref>{{cite doi|10.1145/581690.581695}}</ref><ref>[http://elm-lang.org/blog/announce/version-0.5.0.elm Elm - The Libraries You Need] ''Automaton'' as an arrow implementation</ref>

=== Module system ===

The following program displays the position of the mouse as it moves around the screen, automatically updating the screen in real-time.

Elm has a [[Modular programming|module system]] that allows users to break their code into smaller parts called modules. Modules can hide implementation details such as helper functions, and group related code together. Modules serve as a namespace for imported code, such as <code>Bitwise.and</code>. Third party libraries (or packages) consist of one or more modules, and are available from the [https://package.elm-lang.org/ Elm Public Library]. All libraries are versioned according to [[Software versioning#Degree of compatibility|semver]], which is enforced by the compiler and other tools. That is, removing a function or changing its type can only be done in a major release.

=== Interoperability with HTML, CSS, and JavaScript ===

<source lang="haskell">

Elm uses an abstraction called ports to communicate with [[JavaScript]].<ref>{{cite web |url=https://guide.elm-lang.org/interop/ |title=JavaScript interop |website=elm-lang.org}}</ref> It allows values to flow in and out of Elm programs, making it possible to communicate between Elm and JavaScript.

main = lift asText Mouse.position

</source>

Elm has a library called elm/html that a programmer can use to write HTML and CSS within Elm.<ref>{{cite web |url=https://package.elm-lang.org/packages/elm/html/latest/ |title=elm/html |website=package.elm-lang.org}}</ref> It uses a virtual [[Document Object Model|DOM]] approach to make updates efficient.<ref>{{cite web |url=https://elm-lang.org/news/blazing-fast-html |title=Blazing Fast HTML |website=elm-lang.org}}</ref>

The value of main is displayed on screen. Function asText turns any value into a displayable textual representation. Mouse.position is a value that changes over time, called a signal. Function lift ensures that asText is applied to Mouse.position every time the mouse moves.

=== Backend ===

Elm has a small but expressive set of language constructs, including if-expressions, let-expressions, case-expressions, anonymous functions, and list interpolation.<ref name="syntax"/><ref>[http://elm-lang.org/About.elm About Elm]Elm features</ref>

Elm does not officially support server-side development. Czaplicki does consider it a primary goal at this point, but public progress on this front has been slow. Nevertheless, there are several independent projects which attempt to explore Elm on the backend.

Elm also has both a module system and a foreign function interface for JavaScript.<ref>[http://elm-lang.org/learn/JavaScript-Integration.elm Elm - JavaScript Integration]</ref>

The primary production-ready full-stack Elm platform is Lamdera, an open-core "unfork" of Elm.<ref>{{Cite AV media |url=https://www.youtube.com/watch?v=nSrucNcwlA8 |title=Mario Rogic - Elm as a Service |date=2019-11-28 |last=Elm Europe |access-date=2025-03-27 |via=YouTube}}</ref><ref>{{Cite AV media |url=https://www.youtube.com/watch?v=3Nn5meBieh4 |title=Building a Meetup clone on Lamdera - Martin Stewart |date=2021-07-23 |last=Elm Online Meetup |access-date=2025-03-27 |via=YouTube}}</ref><ref>{{Cite web |title=Episode 38: Lamdera |url=https://elm-radio.com/episode/lamdera/ |access-date=2025-03-27 |website=Elm Radio Podcast |language=en}}</ref> Czaplicki has also teased Elm Studio, a potential alternative to Lamdera, but it isn't available to the public yet.<ref>{{Cite web |title=Elm Studio |url=https://www.elm.studio/ |access-date=2025-03-27 |website=www.elm.studio}}</ref> Current speculation is that Elm Studio will use a future version of Elm that targets C, uses Emscripten to compile to WASM, and supports type-safe [[Postgres]] table generation.<ref>{{Cite web |date=2021-11-03 |title=Status Update - 3 Nov 2021 |url=https://discourse.elm-lang.org/t/status-update-3-nov-2021/7870/3 |access-date=2025-03-27 |website=Elm |language=en}}</ref><ref>{{Cite web |last=Cesarini |first=Francesco |date=22 May 2023 |title=@evancz tempting the demo gods… |url=https://xcancel.com/FrancescoC/status/1660591792780111872 |access-date=26 March 2025 |website=[[Twitter]]}}</ref>

The predefined functions are in the ''Prelude'' module.<ref>[http://elm-lang.org/docs/Prelude.elm Elm's Prelude]</ref>

For full-stack frameworks, as opposed to [[Backend as a service|BaaS]] products, elm-pages is perhaps the most popular fully open-source option.<ref>{{Cite web |title=elm-pages - pull in typed elm data to your pages |url=https://elm-pages.com/ |access-date=2025-03-27 |website=elm-pages |language=en}}</ref> It does not extend the Elm language, but just runs the compiled JS on [[Node.js]]. It also supports scripting. There is also Pine, an Elm to .NET compiler, which allows safe interop with C#, F#, and other [[Common Language Runtime|CLR]] languages.<ref>{{Cite web |title=Pine — Run Elm Everywhere |url=https://pine-vm.org/ |access-date=2025-03-27 |website=pine-vm.org}}</ref>

== Examples ==

* [[UNIX]] installation with [[Haskell (programming language)|Haskell]]'s cabal-dev

<source lang="bash">

mkdir elm-compiler && cd elm-compiler

cabal-dev install elm

There were also some attempts in Elm versions prior to 0.19.0 to use the [[BEAM (Erlang virtual machine)]] to run Elm, but they are stuck due to the removal of native code in 0.19.0 and changes to the package manager. One of the projects executed Elm directly on the environment,<ref>{{Cite web |url=https://github.com/hkgumbs/elm-beam |title=Kofigumbs/Elm-beam |website=[[GitHub]] |date=24 September 2021}}</ref> while another one compiled it to Elixir.<ref>{{Cite web |url=https://github.com/wende/elchemy |title=What is it? |website=[[GitHub]] |date=24 September 2021}}</ref>

export PATH=$PWD/cabal-dev/bin:$PATH

</source>

Finally, the Gren programming language started out a fork of Elm primarily focused on backend support, although its goals have since shifted.

=== Polymorphism on Record types ===

== The Elm Architecture (TEA pattern) ==

<source lang="haskell">

The Elm Architecture is a [[software design pattern]] and as a [[Three-letter acronym|TLA]] called TEA pattern for building interactive web applications. Elm applications are naturally constructed in that way, but other projects may find the concept useful.

An Elm program is always split into three parts:

module MyModule where

* Model - the state of the application

* View - a function that turns the model into HTML

* Update - a function that updates the model based on messages

Those are the core of the Elm Architecture.

import Color

For example, imagine an application that displays a number and a button that increments the number when pressed.<ref>{{Cite web |title=Buttons · An Introduction to Elm |url=https://guide.elm-lang.org/architecture/buttons.html |access-date=2020-10-15 |website=guide.elm-lang.org}}</ref> In this case, all we need to store is one number, so our model can be as simple as <code>type alias Model = Int</code>. The <code>view</code> function would be defined with the <code>Html</code> library and display the number and button. For the number to be updated, we need to be able to send a message to the <code>update</code> function, which is done through a custom type such as <code>type Msg = Increase</code>. The <code>Increase</code> value is attached to the button defined in the <code>view</code> function such that when the button is clicked by a user, <code>Increase</code> is passed on to the <code>update</code> function, which can update the model by increasing the number.

type Named a = { a | name : String } -- records with a ''name'' field

In the Elm Architecture, sending messages to <code>update</code> is the only way to change the state. In more sophisticated applications, messages may come from various sources: user interaction, initialization of the model, internal calls from <code>update</code>, subscriptions to external events (window resize, system clock, JavaScript interop...) and URL changes and requests.

getName : Named a -> String

getName {name} = name

== Limits ==

dude = {name="John Doe", age=20}

Elm does not support [[Type class#Higher-kinded polymorphism|higher-kinded polymorphism]],<ref>{{cite web |title=Higher-Kinded types Not Expressible? #396 |url=https://github.com/elm-lang/elm-compiler/issues/396 |website=github.com/elm-lang/elm-compiler |access-date=6 March 2015}}</ref> which related languages [[Haskell]], [[Scala (programming language)|Scala]] and [[PureScript]] offer, nor does Elm support the creation of [[type class]]es.

lady = {name="Jane Doe", eyesColor = Color.blue}

This means that, for example, Elm does not have a generic <code>map</code> function which works across multiple data structures such as <code>List</code> and <code>Set</code>. In Elm, such functions are typically invoked qualified by their module name, for example calling <code>List.map</code> and <code>Set.map</code>. In Haskell or PureScript, there would be only one function <code>map</code>. This is a known feature request that is on Czaplicki's rough roadmap since at least 2015.<ref>{{cite web |title=Higher-Kinded types Not Expressible #396 |url=https://github.com/elm/compiler/issues/396#issuecomment-128190898 |website=github.com/elm-lang/elm-compiler |access-date=19 November 2019}}</ref> On the other hand, implementations of TEA pattern in advanced languages like [[Scala (programming language)|Scala]] does not suffer from such limitations and can benefit from [[Scala (programming language)|Scala]]'s type classes, [[Type-level programming|type-level]] and [[Kind (type theory)|kind-level]] programming constructs.<ref>{{Cite web |title=The Elm Architecture |url=https://tyrian.indigoengine.io/documentation/architecture/the-elm-architecture.html |access-date=2024-09-07 |website=tyrian.indigoengine.io}}</ref>

names : [String]

names = [ getName dude, getName lady]

Another outcome is a large amount of [[boilerplate code]] in medium to large size projects as illustrated by the author of "Elm in Action," a former Elm core team member, in his single page application example<ref>{{cite web |title=Main.elm |url=https://github.com/rtfeldman/elm-spa-example/blob/23dee34dd7a8c26229a03bc8e9f0e034f6222f13/src/Main.elm |website=github.com/rtfeldman/elm-spa-example |access-date=30 June 2020}}</ref> with almost identical fragments being repeated in update, view, subscriptions, route parsing and building functions.

fullData = [show dude, show lady]

== Example code ==

main = flow down <| map plainText

["Names: " ++ show names

, show fullData

]

</source>

* embedding it in a ''div'' element

<sourcesyntaxhighlight lang="html4strictelm">

-- This is a single line comment.

<!DOCTYPE HTML>

<!-- MyModule.html -->

<html>

<head><meta charset="UTF-8">

<title>MyModule</title>

<script type="text/javascript" src="/your-install-directory/cabal-dev/share/Elm-0.8.0.3/elm-runtime.js"></script>

<script type="text/javascript" src="MyModule.js"></script>

</head>

<body>

<div id="myId" style="width:100;height:100;">&nbsp;</div>

<script type="text/javascript">Elm.byId('myId',Elm.MyModule)</script>

<noscript>Javascript is not enabled</noscript>

</body></html>

</source>

* compile and test

<source lang="bash">

$ elm --make -s --only-js MyModule.elm

$ browser MyModule.html

</source>

{-

==References==

This is a multi-line comment.

It is {- nestable. -}

-}

-- Here we define a value named `greeting`. The type is inferred as a `String`.

greeting =

"Hello World!"

-- It is best to add type annotations to top-level declarations.

hello : String

hello =

"Hi there."

-- Functions are declared the same way, with arguments following the function name.

add x y =

x + y

-- Again, it is best to add type annotations.

hypotenuse : Float -> Float -> Float

hypotenuse a b =

sqrt (a^2 + b^2)

-- We can create lambda functions with the `\[arg] -> [expression]` syntax.

hello : String -> String

hello = \s -> "Hi, " ++ s

-- Function declarations may have the anonymous parameter names denoted by `_`,

-- which are matched but not used in the body.

const : a -> b -> a

const k _ = k

-- Functions are also curried; here we've curried the multiplication

-- infix operator with a `2`

multiplyBy2 : number -> number

multiplyBy2 =

(*) 2

-- If-expressions are used to branch on `Bool` values

absoluteValue : number -> number

absoluteValue number =

if number < 0 then negate number else number

-- Records are used to hold values with named fields

book : { title : String, author : String, pages : Int }

book =

{ title = "Steppenwolf"

, author = "Hesse"

, pages = 237

}

-- Record access is done with `.`

title : String

title =

book.title

-- Record access `.` can also be used as a function

author : String

author =

.author book

-- We can create tagged unions with the `type` keyword.

-- The following value represents a binary tree.

type Tree a

= Empty

| Node a (Tree a) (Tree a)

-- It is possible to inspect these types with case-expressions.

depth : Tree a -> Int

depth tree =

case tree of

Empty -> 0

Node _ left right ->

1 + max (depth left) (depth right)

</syntaxhighlight>

== See also ==

{{Portal|Free and open-source software}}

* [[PureScript]] – A strongly-typed, purely-functional programming language that compiles to JavaScript

* [[Reason (programming language)|Reason]] – A syntax extension and toolchain for OCaml that can also transpile to JavaScript

== References ==

{{Reflist}}

== External links ==

* [http://elm-lang.org/ Elm official{{Official website]}}

* {{GitHub|elm|Elm}}

{{Haskell programming}}

{{Programming languages}}

[[Category:2012 software]]

[[Category:Domain-specific programming languages]]

[[Category:Functional languages]]

[[Category:Haskell programming language family| ]]

[[Category:Pattern matching programming languages]]

[[Category:Programming languages created in 2012]]

[[Category:Source-to-source compilers]]

[[Category:Statically typed programming languages]]

[[Category:Web frameworks]]

[[Category:Free and open-source software]]

[[Category:Software using the MIT license]]