Skip to main content
Version: v1

Create your first Midnight contract

In this tutorial, you'll write your first Compact smart contract, deploy it to a local test network and write a Hello World message to the blockchain using privacy-preserving logic.

By the end of this tutorial, you'll:

  • Create a Compact smart contract with state storage
  • Compile the contract into zero-knowledge circuits
  • Deploy your contract to a local test network
  • Write a Hello World message to the blockchain

Prerequisites​

Before you begin, ensure you have:

1

Set up project​

Clone the starter repo:

git clone https://github.com/midnightntwrk/example-hello-world.git

Install dependencies:

yarn install
2

Create the contract file​

Create a new file named hello-world.compact in the contracts directory:

touch contracts/hello-world.compact

Open this file in VS Code:

code .
3

Create the Compact Smart Contract​

pragma language_version >= 0.22;

export ledger message: Opaque<"string">;

export circuit storeMessage(newMessage: Opaque<"string">): [] {
  message = disclose(newMessage);
}
  • pragma language_version specifies which version of Compact your contract uses.
  • ledger message creates a state variable named message that stores a string value in the on-chain state. On-chain state is public and persistent on the blockchain.
  • circuit storeMessage is a Compact circuit (function) that defines the logic to modify on-chain state.
  • newMessage: Opaque<"string"> is the input parameter. Circuit parameters are always private by default. The disclose() function marks the private value as safe to store publicly. Without it, trying to assign newMessage directly to the ledger returns a compiler error.
4

Compile the contract​

Compiling transforms your Compact code into zero-knowledge circuits, generates cryptographic keys, and creates TypeScript APIs and a JavaScript implementation for the contract to be used by DApps.

Run the compiler from the contracts folder:

compact compile hello-world.compact managed/hello-world.compact

You should see the following output:

Compiling 1 circuits:
  circuit "storeMessage" (k=6, rows=26)

The compilation process will:

  1. Parse and validate your Compact code.
  2. Generate zero-knowledge circuits from your logic.
  3. Create proving and verifying keys for the circuits.
  4. Generate the TypeScript API and JavaScript implementation for the contract.

When compilation completes, you'll see a new directory structure:

contracts/
├── managed/
|   └── hello-world/
|        ├── compiler/
|        ├── contract/
|        ├── keys/
|        └── zkir/
└── hello-world.compact
└── index.ts

Here's what each directory contains:

  • contract/: The compiled contract artifacts, which includes the JavaScript implementation and type definitions.
  • keys/: Cryptographic proving and verifying keys that enable zero-knowledge proofs.
  • zkir/: Zero-Knowledge Intermediate Representation—the bridge between Compact and the ZK backend.
  • compiler/: Compiler-generated JSON output that other tools can use to understand the contract structure.
5

Deploy Contract to Local Devnet​

Now that your contract is compiled, it needs to be deployed to the blockchain so that you can interact with it.

Be sure the Docker engine is running and in a separate terminal start the proof server from the project root:

yarn env:up

Leave the proof server running for the following steps.

To deploy the contract, you'll need a wallet. The local devnet package comes with 3 pre-funded wallets.

Run the Hello World test script:

yarn test:local

The test script will begin to show output from your local devnet and will progress the contract deployment and interaction programatically:

[12:46:12.694] INFO (22064): Wallet sync complete after 23 emissions
[12:46:12.703] INFO (22064): Providers initialized. Ready to test
[12:46:12.707] INFO (22064): Creating private state...
[12:46:32.347] INFO (22064): Setting the contract address...
[12:46:32.347] INFO (22064): Contract deployed at: bba6579743ae23b44301d4a9f8df30dbd5244d63a59d8fbc2c9fc7ea521a04f8
 ✓ src/test/hw.test.ts (2 tests) 39112ms
   ✓ Hello World Contract > Deploys the contract  19649ms
   ✓ Hello World Contract > Stores a message  18184ms

Next steps​

Explore the Tutorials for detailed explanations on building Midnight DApps. For less detailed walkthroughs of DApps that provide additional content see Examples.