Elm (programming language)

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Paradigm functional reactive, functional
Designed by Evan Czaplicki
Appeared in 2011
0.14 / December 9, 2014; 20 days ago (2014-12-09)
static, strong, inferred
License Permissive (Revised BSD) [1]
Website elm-lang.org

Elm is a functional programming language for declaratively creating web browser based graphical user interfaces. Elm uses the Functional Reactive Programming style and purely functional graphical layout to build user interface without any destructive updates.


Elm was designed by Evan Czaplicki as his thesis in 2012.[2] The first release of Elm came with many examples and an online editor that made it easy to try out in a web browser.[3] Evan Czaplicki now works on Elm at Prezi.[4]

The initial implementation of the Elm compiler targets HTML, CSS, and JavaScript.[5] The set of core tools has continued to expand, now including a REPL,[6] package manager,[7] time-traveling debugger,[8] and installers for Mac and Windows.[9] Elm also has an ecosystem of community created libraries.[10]


Elm has a small but expressive set of language constructs, including if-expressions, let-expressions, case-expressions, anonymous functions, and list interpolation.[11][12] From there the key features include signals, immutability, static types, and interoperability with HTML, CSS, and JavaScript.


The key abstraction in Elm is called a Signal. It is a value that changes over time.[13] For example, the Mouse.position signal in the following code acts on "the current position of the mouse", so the programmer does not need to manually handle an event each time the mouse moves:[14]

import Mouse
main : Signal Element
main =
    lift asText Mouse.position

The Signal library allow users to model change over time without resorting to callbacks and shared mutable memory.[15] This leads to an architecture that centralizes state, making it much harder for parts of your model to get out of sync.[16]


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.[17]

Static Types[edit]

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 ADTs that let you build entirely new types[18]

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

Module System[edit]

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 JavaScript[edit]

Elm uses an abstraction called ports to communicate with JavaScript.[19] It allows values to flow in and out of Elm programs, making it possible to communicate between Elm and JavaScript.

Elm also has a library called elm-html which lets you use HTML within Elm and allows you to style it with CSS.[20] It uses a Virtual DOM approach to make updates efficient.[21]


Example Code[edit]

-- 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 will be 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)
-- If-expressions are used to branch on values
absoluteValue : Int -> Int
absoluteValue number =
    if number < 0 then -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 
-- We can create entirely new types with the `data` keyword.
-- The following value represents a binary tree.
data 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 value left right ->
            1 + max (depth left) (depth right)


External links[edit]