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:

1. Pragma - Language version (pragma language_version 0.18;)
2. Ledger - On-chain state declarations
3. 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