midnight-compact-guide

Midnight Compact Language Reference (v0.19+)

CRITICAL: This reference is derived from actual compiling contracts in the Midnight ecosystem (MeshJS starter template). Always verify syntax against this reference before generating contracts.

Quick Start Template

Use this as a starting point - it compiles successfully:

pragma language_version >= 0.19;

import CompactStandardLibrary;

// Ledger state (individual declarations, NOT a block)
export ledger counter: Counter;
export ledger owner: Bytes<32>;

// Witness for private/off-chain data (declaration only)
witness local_secret_key(): Bytes<32>;

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

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1. Pragma (Version Declaration)

CORRECT - simple minimum version:

pragma language_version >= 0.19;

WRONG - these will cause issues:

pragma language_version >= 0.14.0;           // ❌ outdated version
pragma language_version >= 0.16 && <= 0.18;  // ❌ outdated, use >= 0.19

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2. Imports

Always import the standard library:

import CompactStandardLibrary;

For modular code:

import "path/to/module";
import { SomeType } from "other/module";

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3. Ledger Declarations

CORRECT - individual declarations with export ledger:

export ledger counter: Counter;
export ledger owner: Bytes<32>;
export ledger balances: Map<Bytes<32>, Uint<64>>;

// Private state (off-chain only)
ledger secretValue: Field;  // no export = private

WRONG - block syntax is DEPRECATED:

// ❌ This causes parse error: found "{" looking for an identifier
ledger {
  counter: Counter;
  owner: Bytes<32>;
}

Ledger Modifiers

export ledger publicData: Field;           // Public, readable by anyone
export sealed ledger immutableData: Field; // Set once in constructor, cannot change
ledger privateData: Field;                 // Private, not exported

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