MVU

MVU, short for Model-View-Update, is a software architecture for building interactive user interfaces around a unidirectional data flow. It originated with The Elm Architecture, the design pattern used by the Elm programming language, and has influenced other frontend frameworks and libraries across languages.

At its core, MVU consists of three parts: a Model that represents the application state; a View that renders the model into the user interface; and an Update function that takes a message and the current model and returns a new model. User actions or external events are represented as messages which are dispatched to the Update function. The View typically includes event handlers that generate messages when the user interacts with UI controls. The system then applies the new model and re-renders the View.

Key characteristics include a pure Update function, predictable state transitions, and a unidirectional data flow. Side effects are not performed inside Update; instead they are expressed as commands or effects that are executed by the runtime or host environment in response to updates. This separation makes reasoning, testing, and debugging easier, and supports patterns like time-travel debugging in some implementations.

MVU is frequently used in functional-programming contexts but has been ported to many languages, including Elm and F#, with libraries such as Elmish that bring MVU style to JavaScript/TypeScript. It is related to, but distinct from, MVC and MVVM, offering an alternative coordinated-state approach that emphasizes a single source of truth and explicit message-based updates.