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", made possible by the Elm compiler's static type checking.
History
Elm was initially designed by Evan Czaplicki as his thesis in 2012. The first release of Elm came with many examples and an online editor that made it easy to try out in a web browser. Evan joined Prezi in 2013 to work on Elm, and in 2016 moved to NoRedInk as an Open Source Engineer, also starting the Elm Software Foundation.
The initial implementation of the Elm compiler targeted HTML, CSS, and JavaScript. The set of core tools has continued to expand, now including a REPL, package manager, time-travelling debugger, and installers for macOS and Windows. Elm also has an ecosystem of community created libraries and Ellie, an advanced online editor that allows saved work and inclusion of community libraries.
Features
Elm has a small set of language constructs, including traditional if-expressions, let-expressions for local state, and case-expressions for pattern matching. As a functional language, it supports anonymous functions, functions as arguments, and functions can return functions, the latter often by partial application of curried functions. Functions are called by value. Its semantics include immutable values, 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
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 arrays, sets, and dictionaries in the standard library.
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 round : Float -> Int. Custom types allow the programmer to create custom types to represent data in a way that matches the problem domain.
Types can refer to other types, for example a List Int. Types are always capitalized; lowercase names are type variables. For example, a List a is a list of values of unknown type. It is the type of the empty list and of the argument to List.length, 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, Html Msg represents a (virtual) DOM tree whose event handlers all produce messages of type Msg.
Rather than allow any value to be implicitly nullable (such as JavaScript's undefined or a null pointer), Elm's standard library defines a Maybe a 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 classes: number which includes Int and Float to facilitate the use of numeric operators such as (+) or (*), comparable which includes numbers, characters, strings, lists of comparable things, and tuples of comparable things to facilitate the use of comparison operators, and appendable which includes strings and lists to facilitate concatenation with (++). Elm does not provide a mechanism to include custom types into these type classes or create new type classes (see Limitations section).
Module system
Elm has a 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 Bitwise.and. Third party libraries (or packages) consist of one or more modules, and are available from the Elm Public Library. All libraries are versioned according to 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
Elm uses an abstraction called ports to communicate with JavaScript. It allows values to flow in and out of Elm programs, making it possible to communicate between Elm and JavaScript.