Tutorial 1: Hello World

Overview

In this tutorial, we will cover how to create a "Hello World" smart contract, deploy it, and call it.

Before starting, ensure all prerequisite tools are installed.

Create a new project

Run the following commands to create a new project:

npx @scrypt-inc/cli-btc project helloworld
cd helloworld
npm install

The resulting project will contain a sample smart contract /src/contracts/helloworld.ts, along with all the scaffolding.

For this example, let's modify it to the following code:

import {
    assert,
    ByteString,
    method,
    prop,
    sha256,
    Sha256,
    SmartContract,
} from '@scrypt-inc/scrypt-ts-btc'

export class Helloworld extends SmartContract {
    @prop()
    hash: Sha256

    constructor(hash: Sha256) {
        super(...arguments)
        this.hash = hash
    }

    @method()
    public unlock(message: ByteString) {
        assert(sha256(message) == this.hash, 'Hash does not match')
    }
}

This Helloworld contract stores the sha256 hash of a message in the contract property hash. Only a message which hashes to the value set in this.hash will unlock the contract.

Now let’s look at what is in the smart contract.

• SmartContract: all smart contracts must extend the SmartContract base class.
• @prop: the @prop decorator marks a contract property.
• @method: the @method decorator marks a contract method. A public method is an entry point to a contract.
• assert: throws an error and makes the method call fail if its first argument is false. Here it ensures the passed message hashed to the expected digest.

Compile Contract

1. Run following command to compile the Helloworld contract:

npx @scrypt-inc/cli-btc compile

This command will generate a contract artifact file at /artifacts/helloworld.json.

2. Then call the loadArtifact() function in the code:

const artifact = readArtifact(Helloworld);
Helloworld.loadArtifact(artifact)

Compile using the watch option

Monitoring for Real-time Error Detection

npx @scrypt-inc/cli-btc compile --watch

The watch option in the provided command continuously monitors errors during the sCrypt compilation process. Watch mode enables users to observe real-time updates and notifications regarding any errors specific to sCrypt, which are distinct from TypeScript errors.

Contract Deployment & Call

Before we deploy the contract, you need to generate a Bitcoin key.

npm run genprivkey

then follow the faucet instructions to fund the key.

Next, start deploying and calling the contract:

1. To deploy a smart contract, simply call its deploy method.
2. To call a smart contract, call one of its public methods.

For this example, overwrite deploy.ts in the root of the project with the following code to deploy and call the Helloworld contract:

import { Helloworld } from './src/contracts/helloworld'
import { getDefaultProvider, getDefaultSigner } from './tests/utils/txHelper';
import { readArtifact } from '@scrypt-inc/scrypt-ts-transpiler-btc';
import { call, Covenant, deploy, sha256, toByteString } from '@scrypt-inc/scrypt-ts-btc';

(async () => {

    const artifact = readArtifact(Helloworld);
    Helloworld.loadArtifact(artifact)
    const covenant = Covenant.createCovenant(new Helloworld(sha256(toByteString("hello world", true))))

    const provider = getDefaultProvider();
    const signer = getDefaultSigner();


    const deployTx = await deploy(signer, provider, covenant);

    console.log(`Helloworld contract deployed: ${deployTx.getId()}`)

    const callTx = await call(signer, provider, covenant, {
        invokeMethod: (contract: Helloworld) => {
            contract.unlock(toByteString('hello world', true));
        },
    });

    console.log('Helloworld contract `unlock` called: ', callTx.getId())
})()

Run the following command to deploy AND call our example contract.

npx ts-node deploy.ts

You will see some output like:

You can view the deployment transaction using the WhatsOnChain blockchain explorer:

You can also view the calling transaction:

Congrats! You have deployed and called your first Bitcoin smart contract.