Elm

Elm is a functional programming language designed for front-end web development. It compiles to JavaScript and emphasizes reliability and simplicity, aiming to prevent many classes of runtime errors through its design. Created by Evan Czaplicki in 2012, Elm has grown into a small, opinionated language with a focused ecosystem and a strong emphasis on maintainable UI code.

Elm's core is pure and stateless by default: functions have no side effects except when explicitly requested via commands or subscriptions; data is immutable. The language uses a static type system with inference, helping catch errors at compile time. It provides common data types (such as Maybe, Result, List, Dict) and allows the creation of custom types. The compiler offers helpful error messages.

The Elm Architecture, a central concept, structures applications around a Model representing state, a Msg type describing possible events, an update function to produce a new model, and a view function that renders the UI. Subscriptions connect to external events, and side effects are handled through Cmds. Elm uses a virtual DOM and a reactive update loop to efficiently refresh the user interface.

Tooling and interoperability: the compiler, elm make, builds apps into JavaScript and HTML; elm.json and the package ecosystem manage dependencies since the 0.19 release. Elm Reactor provides interactive evaluation in the browser, and ports enable controlled communication with JavaScript. The language deliberately limits certain patterns to preserve guarantees, which can constrain some kinds of integration with non-Elm code.

Impact: Elm has influenced front-end architecture, notably inspiring patterns later adopted by other ecosystems. While its ecosystem is smaller than JavaScript’s, it remains valued by developers who seek predictable code, strong type safety, and a friendly developer experience for building user interfaces.