FUNCTOR

Game development from an alternate timeline.

A free, open-source game engine.

Put the fun in functional programming.

Features

Editing a value in the live editor grows the orbs on screen instantly — no reload

Instant Feedback

Edit your game while it runs. The runtime swaps your code under the live model — no rebuild, no restart, no losing your place.

Rewinding a physics scene and extrapolating its future along the timeline

Travel Through Time

Rewind while editing, make changes, or extrapolate forward to see how your edits ripple through gameplay.

A rigged character streamed from a CDN, animating with an idle/walk/run blend

Batteries Included

Asset management, networking, physics, and spatial audio — in the box, ready to use.

Live values from the running game appearing inline in the code as time scrubs

Introspective

Analyze the live values coursing through your game at any moment — nothing is hidden behind opaque binary state.

An MCP client launching a game, pausing it, injecting a keypress, and stepping a jump four frames at a time while reading the model back as JSON

AI Native

Register functor mcp and any coding agent can build, run, inspect, and rewind your game — real Functor code, deterministic, on the runtime that ships to native and web.

How does it work?

Functor is a functional language built for game development, inspired by Elm and F#. Pure functions, interpreted, no compilation. Functor lives in a Rust runtime that does the heavy lifting — graphics and physics.

Build and run anywhere

Native

Windows · macOS

Browser

WebAssembly · no install

VR

Meta Quest

Build a game as a pure function.

Nothing to install — try in the browser now.

▶ Open the sandbox