Knopper

A functional-reactive terminal UI framework with a machine-centered programming model — designed multi-user from the ground up.

Knopper is a Rust framework for building terminal user interfaces as machines: self-contained units that project a reactive Scene from local Model state and replicated Shared state. A terminal-native scene algebra composes layout; a single rendering pipeline lowers scenes to backend commands with structural diffing. The Notcurses backend (behind a feature flag) targets high-performance terminal rendering.

Knopper is the UI substrate for Industrial Algebra's collaborative tools — Wallace, Dominic, and Tsume compose and render multi-participant sessions through it.

Why Knopper?

• Machine-first, not widget-first. You implement one trait — Machine — with four associated types (Context, Msg, Model, Shared) and four functions (init, update, project, cursor_position). There is no widget tree to manage; the scene is a pure projection of state.
• The Model/Shared split is the collaboration seam. Model is participant-local (focus, scroll, input buffers — never synced). Shared is host-pushed projection state (rosters, transcripts, task boards). Multi-user support isn't bolted on; it's the type system.
• Principled focus. Focus order derives from the scene, traverses with scope policies (Wrap / Trap / Local / Passthrough), and modals trap focus declaratively — no imperative key routing.
• Embeddable by contract. Knopper owns no event loop and no process. A host drives Runtime turn-by-turn: dispatch keys, send messages, set_shared projections, commit diffs. The embedding contract is validated against the code (see Embedding Contract).

A taste

#![allow(unused)]
fn main() {
use knopper::{Effect, PureMachine, Runtime, Scene};

let machine = PureMachine::new(
    |_ctx: &()| 0_i32,                                    // init
    |model: &mut i32, msg: bool, _ctx: &()| {             // update
        if msg { *model += 1; }
        Effect::None
    },
    |model: &i32, _shared: &(), _ctx: &()| {              // project
        Scene::text(1_u64, format!("count: {model}"))
            .on_activate(true)
    },
);

let mut runtime = Runtime::new(machine, (), ());
// Host drives turn-by-turn: dispatch events, read diffs, commit to a backend.
}

Status

0.1.0 is the first experimental release: coherent, tested (130 tests), and clippy-clean — but unstable. Expect refinement in 0.2.0, especially around the collaboration seam and demo organization. See the roadmap.