Glossary

Application backend

A Fabulous integration that turns widgets into controls from a UI framework. The maintained backends are Fabulous.MauiControls for .NET MAUI and Fabulous.Avalonia for Avalonia.

Attribute

The compiled representation of a widget value, child widget, widget collection, environment value, or event handler. During reconciliation, attributes describe the changes to apply to an existing native view.

Command (`Cmd<'msg>`)

A list of effects that may dispatch messages. Commands represent work requested by init or update, such as asynchronous operations, without performing that work inside the pure state transition.

Command message (`CmdMsg`)

An application-defined discriminated union describing requested effects as data. A separate function maps each command message to a Cmd<'msg>. This pattern keeps init and update straightforward to unit test.

Component

A stateful, composable Fabulous UI unit. Components can own local state, bind environment values, receive external events, and produce widgets.

Dispatch

A function of type 'msg -> unit that sends a message into the program's processing loop.

Effect

A function that receives dispatch and may perform work or dispatch messages. Cmd<'msg> is a list of effects.

Environment

Values propagated through a component/widget tree without being passed explicitly through every intermediate function.

Marker type

A compile-time interface used by WidgetBuilder to constrain which constructors, children, and modifiers are valid for a widget. Marker types do not represent runtime controls.

Model

The immutable F# value representing application or component state at a point in time.

Modifier

An extension method that returns a new WidgetBuilder with an additional or replaced attribute. Modifiers configure properties, events, layout metadata, references, and other widget behavior.

Model-View-Update (MVU)

The architecture used by Fabulous programs:

The model stores state.
The view describes UI for that state.
Messages describe events.
The update function produces the next model and optional commands.

Program

The functions and configuration required to run an MVU loop: initialization, update, subscriptions, logging, exception handling, and, for a rendered program, the view function.

Reconciliation

The process of comparing the previous and next widget trees and applying only the required changes to native views.

Runner

The runtime host that initializes a program, executes commands/subscriptions, processes messages, reconciles views, and handles errors.

Subscription (`Sub<'msg>`)

A long-lived external event source associated with a stable identifier. The runner starts and disposes subscriptions as the model changes.

View function

A function that converts the current model into a widget description. A view function describes desired UI; it should not mutate native controls directly.

View node (`IViewNode` / `ViewNode`)

The runtime object connecting a compiled widget to its native view. It stores tree context, handlers, component state, and child-node relationships used during reconciliation.

View reference (`ViewRef<'T>`)

An escape hatch that provides access to the native control created for a widget. Prefer declarative widgets and modifiers; use a view reference when an imperative platform API has no declarative equivalent.

An immutable, compiled description of a native view, including its registered widget key and attributes. Widgets are compared during reconciliation.

Widget builder (`WidgetBuilder<'msg, 'marker>`)

The typed F# value returned by Fabulous constructors and modifiers. Compiling a widget builder produces a widget for reconciliation.