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Elm (programming language)

The Elm tangram
Paradigm functional
Designed by Evan Czaplicki
First appeared 2012
Stable release
0.18 / November 14, 2016; 5 months ago (2016-11-14)
Typing discipline static, strong, inferred
License Permissive (Revised BSD) [1]
Filename extensions .elm
Influenced by
Haskell, Standard ML, OCaml, F#

Elm is a domain-specific programming language for declaratively creating web browser-based graphical user interfaces. Elm is purely functional, and is developed with emphasis on usability, performance, and robustness. It advertises "no runtime exceptions in practice,"[3] made possible by the Elm compiler's static type checking.



Elm was initially designed by Evan Czaplicki as his thesis in 2012.[4] The first release of Elm came with many examples and an online editor that made it easy to try out in a web browser.[5] Evan Czaplicki joined Prezi in 2013 to work on Elm,[6] and in 2016 moved to NoRedInk as an Open Source Engineer, also starting the Elm Software Foundation.[7]

The initial implementation of the Elm compiler targets HTML, CSS, and JavaScript.[8] The set of core tools has continued to expand, now including a REPL,[9] package manager,[10] time-traveling debugger,[11] and installers for Mac and Windows.[12] Elm also has an ecosystem of community created libraries.[13]


Elm has a small but expressive set of language constructs, including if-expressions, let-expressions, case-expressions, anonymous functions, and list interpolation.[14][15] From there the key features include immutability, static types, and interoperability with HTML, CSS, and JavaScript.


All values in Elm are immutable, meaning that a value cannot be modified after it is created. Elm uses persistent data structures to implement its Array, Dict, and Set libraries.[16]

Static typesEdit

Elm is statically typed. Every definition in Elm can be given a type annotation that describes the exact shape of the value. Types include:

  • primitive types such as integers and strings
  • basic data structures such as lists, tuples, and extensible records
  • custom types called tagged unions that let you build entirely new types[17]

Elm also supports full type inference, so the compiler can verify that a program is type-safe without any type annotations.

Module systemEdit

Elm has a module system that allows users to break their code into smaller parts called modules. Users can import and export values, making it possible to hide implementation details that other programmers do not need to think about. Modules form the basis of Elm's community library website, the Elm Public Library.

Interoperability with HTML, CSS, and JavaScriptEdit

Elm uses an abstraction called ports to communicate with JavaScript.[18] 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.[19] It uses a virtual DOM approach to make updates efficient.[20]


Unlike Haskell or PureScript, Elm has no support for higher-kinded types, and thus cannot provide generic abstractions for many common operations.[21] For example, there is no generic map, apply, fold, or filter function. Instead, such names are used prefixed by their module, such as and


Example codeEdit

-- This is a single line comment

{- This is a multi-line comment.
   It can span multiple lines.

{- It is possible to {- nest -} multi-line comments -}

-- 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)

-- 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 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 =

-- Record access `.` can also be used as a function
author : String
author =
    .author book

-- We can create entirely new types 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 ->

        Node value left right ->
            1 + max (depth left) (depth right)


External linksEdit