midnight-compact

Midnight Compact Language (v0.22+)

Complete guide to writing privacy-preserving smart contracts with Compact.

What is Compact?

Compact is a purpose-built programming language for zero-knowledge smart contracts. Unlike adapting existing languages, Compact was designed from scratch to make privacy-preserving programming natural and secure.

Key Design Goals:

• Compile directly to efficient ZK circuits
• Type-safe cryptographic operations
• Familiar syntax for developers (TypeScript-like)
• Automatic proof generation
• Bounded for finite proving circuits

Key Features:

• Strong static typing (no bypass via unsafe casts)
• Generic type and numeric parameters
• Explicit disclosure via disclose() wrappers
• Module-based namespace management

Quick Start

Minimal Working Contract (v0.22+)

pragma language_version >= 0.20;
import CompactStandardLibrary;

// Ledger state (individual declarations)
export ledger counter: Counter;
export ledger owner: Bytes<32>;

// Witness for private data
witness local_secret_key(): Bytes<32>;

// Circuit (returns [] not Void)
export circuit increment(): [] {
  counter.increment(1);
}

Full Counter with Read/Write

pragma language_version >= 0.20;
import CompactStandardLibrary;

export ledger count: Counter;

export circuit increment(): [] {
  count.increment(1);
}

export circuit decrement(): [] {
  count.decrement(1);
}

export circuit getCount(): Uint<64> {
  return count.read();
}

export circuit setCount(newValue: Uint<64>): [] {
  count.write(newValue);
}

Core Concepts

Three-Part Contract Structure

Each Compact contract has three components:

1. Replicated component on a public ledger
2. Zero-knowledge circuit that confidentially proves correct execution
3. Local, off-chain component for arbitrary code

Program Elements

A Compact program can contain:

• module / import for namespace management
• struct / enum / type for program-defined types
• export ledger for public state storage
• witness for callback functions (private state)
• circuit for the operational core
• constructor for initialization