A fast-paced racing simulator where you tune a kart, race for the top spot, and climb an on-chain leaderboard.
Under the hood
The game. BlockKart is a single-player score-attack racer: you submit a car build plus item loadout, and a deterministic race runs against three AI bots. The client renders the race in Three.js (native WebGL canvas) with a CRT-style post-processing pass.
Chains. BlockKart pairs Arbitrum with Midnight. Game inputs go to an Arbitrum PaimaL2Contract (deployed with fee = 0); the deterministic race simulation runs in the EffectStream node; and Midnight holds a tiny anchor contract. Arbitrum is the chosen EVM today, but the EVM layer is standard PaimaL2Contract - point it at any EVM chain and the rest is unchanged.
Running on EffectStream. The EffectStream state machine ingests the EVM PaimaGameInteraction events: a play input records the race entry and schedules an executeRace ~10 blocks later, which replays the seeded simulation, records the result, and awards achievements. The batcher has both an EVM adapter and a Midnight adapter; the Midnight side writes the anchor.
Leaderboard & achievements. Results land in Postgres and are ranked by wins on the /metrics/leaderboard endpoint. BlockKart defines win, loss, and per-surface (dirt/ice/asphalt) achievements, shown in the Honors panel.
The Compact contract
</>View source on GitHub→Source: packages/shared/contracts/midnight-contracts/contract-midnight-data/src/midnight-data.compact
Here's the teaching moment: the Midnight contract is intentionally tiny. All of the racing logic lives on Arbitrum and in the EffectStream node; Midnight is just an immutable anchor. Not every Compact contract should model the whole game.
The "anchor only" pattern
The entire contract is 11 lines: a single ledger payload: Opaque<"string"> plus a storeValue circuit that discloses and writes it. No enums, no maps, no witnesses, no commit-reveal. The EffectStream node calls storeValue to notarize game-event data on Midnight. Reach for this pattern when you need tamper-evident anchoring but not zero-knowledge - the cheaper the on-chain piece, the cheaper every update.
CompactFull contract (11 lines)
pragma language_version >= 0.18.0;
import CompactStandardLibrary;
constructor() {
}
export ledger payload: Opaque<"string">;
export circuit storeValue(_payload: Opaque<"string">): [] {
payload = disclose(_payload);
}