Elm (programming language)

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. Mr. Czaplicki 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 targets HyperText Markup Language (HTML), Cascading Style Sheets (CSS), and JavaScript. The set of core tools has continued to expand, now including a read–eval–print loop (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 including community libraries.

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. 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. 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. Types are always capitalized; lowercase names are type variables. For example, a  is a list of values of unknown type. It is the type of the empty list and of the argument to, 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,  represents a (virtual) DOM tree whose event handlers all produce messages of type.

Rather than allow any value to be implicitly nullable (such as JavaScript's  or a null pointer), Elm's standard library defines 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:  which includes   and   to facilitate the use of numeric operators such as   or ,   which includes numbers, characters, strings, lists of comparable things, and tuples of comparable things to facilitate the use of comparison operators, and   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 Limits).

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. 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.

Elm has a library called elm/html that a programmer can use to write HTML and CSS within Elm. It uses a virtual DOM approach to make updates efficient.

Backend
Elm does not officially support server-side development. The core development team does not consider it as their primary goal and prefers to focus development on the enhancement of front-end development experience. Nevertheless, there are several independent projects, which attempt to explore possibilities to use Elm for the back-end. The projects are mainly stuck on Elm version 0.18.0 since newer ones do not support "native" code and some other utilized features. There are two attempts to use Elm with BEAM (Erlang virtual machine). One of the projects executes Elm directly on the environment while another one compiles it to Elixir. Also, there was an attempt to create a back-end framework for Elm powered by Node.js infrastructure. None of the projects are production-ready.

The Elm Architecture
The Elm Architecture is a software design 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
 * Update - a function that updates the model based on messages

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. In this case, all we need to store is one number, so our model can be as simple as. The  function would be defined with the   library and display the number and button. For the number to be updated, we need to be able to send a message to the  function, which is done through a custom type such as. The   value is attached to the button defined in the   function such that when the button is clicked by a user,   is passed on to the   function, which can update the model by increasing the number.

In the Elm Architecture, sending messages to   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, subscriptions to external events (window resize, system clock, JavaScript interop...) and URL changes and requests.

Limits
Elm does not support higher-kinded polymorphism, which related languages Haskell and PureScript offer, nor does Elm support the creation of type classes.

This means that, for example, Elm does not have a generic  function which works across multiple data structures such as   and. In Elm, such functions are typically invoked qualified by their module name, for example calling  and. In Haskell or PureScript, there would be only one function. This is a known feature request that is on Mr. Czaplicki's rough roadmap since at least 2015.

Another outcome is a large amount of boilerplate code in medium to large size projects as illustrated by the author of "Elm in Action" in their single page application example with almost identical fragments being repeated in update, view, subscriptions, route parsing and building functions.