Compact Smart Contracts

Compact (being renamed to Minokawa) is Midnight's domain-specific language for privacy-preserving smart contracts. Contracts compile to ZK-SNARKs, enabling selective disclosure of data.

Note: As of compiler v0.26.0, the language is being renamed from "Compact" to "Minokawa" under the Linux Foundation Decentralized Trust. The toolchain commands still use compact .

Quick Start

pragma language_version 0.18;

export ledger message: Opaque<"string">;

export circuit setMessage(input: Opaque<"string">): [] { message = disclose(input); // Makes private input public }

Contract Structure

Every Compact contract has three parts:

• Pragma - Language version (pragma language_version 0.18; )

• Ledger - On-chain state declarations

• Circuits - ZK-proven functions

Core Concepts

Privacy Model

Level Syntax Visibility

Private const x = input;

Only prover

Disclosed disclose(value)

Allowed to become public

Proven disclose(a >= b)

Public boolean only

Witness witness f(...): T;

Private, DApp-provided

Notes:

• Circuit arguments and witness returns are treated as potentially private (“witness data”).

• disclose(...) is a compiler acknowledgement: it does not itself publish anything, it just permits an expression to flow into public outputs (ledger writes / exported circuit returns / cross-contract comms).

• Only Opaque<"string"> and Opaque<"Uint8Array"> are currently supported.

Ledger Types

ledger counter: Counter; // Auto-incrementing ledger balances: Map<Bytes<32>, Uint<64>>; // Key-value ledger members: Set<Field>; // Membership tracking ledger tree: MerkleTree<20, Field>; // Cryptographic proofs

Reference Files

Topic Resource

Type System references/types.md - Full type reference

Standard Library references/stdlib.md - Hashing, coins, EC ops

VS Code extension references/vscode-extension.md - Editor setup and tasks

Ledger Patterns references/ledger-patterns.md - State management

Advanced Patterns references/advanced-patterns.md - Access control, state machines

Detailed API Patterns references/detailed-api-patterns.md - API, code

Templates

Template Description

assets/basic-contract.compact Simple ledger + circuit

assets/token-contract.compact Token with transfers

assets/private-voting.compact Anonymous voting

assets/commitment-reveal.compact Commit-reveal pattern

Compilation

Compile contract

compact compile contracts/my-contract.compact contracts/managed/my-contract

Output structure

contracts/managed/my-contract/ ├── contract/ # JSON artifacts ├── keys/ # ZK proving/verifying keys └── zkir/ # ZK Intermediate Representation

Common Errors

Error Cause Fix

Type mismatch

Wrong bit width Use correct Uint<N> size

Cannot assign private to public

Missing disclose Add disclose() wrapper

Undefined symbol

Import missing Check pragma and imports

Best Practices

• ✅ Start with pragma language_version 0.18;

• ✅ Use witness for private inputs that need proofs

• ✅ Choose smallest Uint<N> that fits your data

• ✅ Use persistentHash for on-chain data, transientHash for temp

• ❌ Don't expose secrets via disclose() unnecessarily

• ❌ Avoid large state (increases gas costs)

Resources

• Writing a contract

• Language reference

• Explicit disclosure

• Compact standard library

• Ledger ADTs

• Opaque data types

• Examples Repository