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Elm is also available on Linux and Arm Tags: Mobile edit Mobile web edit |
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{{Infobox programming language
| name = Elm
| logo =
| logo caption = The Elm tangram
| paradigm = [[Functional programming|
| released = {{Start date and age|2012|3|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>▼
| family = [[Haskell]]
| latest release version = 0.19.1▼
| 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>▼
| influenced_by = [[Haskell (programming language)|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}}</ref> [[Rust (programming language)|Rust]],<ref>{{Cite web |title=Uniqueness Types |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> Derw,<ref>{{cite web| title= Why Derw: an Elm-like language that compiles to TypeScript? |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>▼
| typing = [[static typing|Static]], [[strong typing|Strong]], [[type inference|Inferred]]▼
| designer = Evan Czaplicki
▲| released = {{Start date and age|2012|
▲| latest release version = 0.19.1
▲| latest release date = {{Start date and age|2019|10|21}}<ref>{{Cite web |url=https://github.com/elm/compiler/releases |title
▲| typing = [[
| 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>▼
| website = {{Official URL}}
▲|
▲| 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>
▲| 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=
▲| file_ext = .elm
}}
'''Elm''' is a [[Domain-specific language|___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 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=
The initial implementation of the Elm compiler
== Features ==
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=
=== Immutability ===
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>
=== Static types ===
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
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>.
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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.
Elm provides a limited number of built-in [[type class|type classes]]
=== Module system ===
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=== Interoperability with HTML, CSS, and JavaScript ===
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.
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>
=== Backend ===
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.
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 Elm Architecture ==▼
The Elm Architecture is a pattern for building interactive web applications. Elm applications are naturally constructed in that way, but other projects may find the concept useful.▼
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>
An Elm program is always split into three parts:▼
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>
Finally, the Gren programming language started out a fork of Elm primarily focused on backend support, although its goals have since shifted.
▲== The Elm Architecture (TEA pattern) ==
▲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:
* Model - the state of the application
* View - a function that turns the model into HTML
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Those are the core of the Elm Architecture.
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.
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.
==
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)|
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>
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
== Example code ==
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"Hello World!"
hello : String
hello =
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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
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if number < 0 then negate number else number
book : { title : String, author : String, pages : Int }
book =
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depth tree =
case tree of
Empty ->
Node _ left right ->
1 + max (depth left) (depth right)
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== See also ==
{{Portal|Free and open-source software}}
* [[PureScript]]
* [[Reason (programming language)|Reason]]
== References ==
{{Reflist
== External links ==
* {{Official website}}
* {{GitHub|elm|Elm}}
{{Haskell programming}}
{{Programming languages}}
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[[Category:Domain-specific programming languages]]
[[Category:Functional languages]]
[[Category:Haskell programming language family]]
[[Category:Pattern matching programming languages]]
[[Category:Programming languages created in 2012]]
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[[Category:Statically typed programming languages]]
[[Category:Web frameworks]]
[[Category:Free and open-source software]]
[[Category:Software using the MIT license]]
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