scrypt-ts

scryptTS

scryptTS is a Typescript framework to write smart contracts on Bitcoin SV.

Installation

Use this command to install scryptTS to your project:

npm install scrypt-ts

Setup

scryptTS depends on ts-patch to provide a custom plugin support for typescript. So first we need to add scryptTS plugin and enable decorators in tsconfig.json file like:

{
  "compilerOptions": {
    ...
    "experimentalDecorators": true,
    "plugins": [
      {
        "transform": "scrypt-ts/dist/transformer",   // Required
        "transformProgram": true,                    // Required
        "outDir": "./scrypt",                        // Optional, define the auto-generated `.scrypt` files folder
        "debug": false                               // Optional, enable/disable debug log in console.
      }
    ]
  }
}

Note: Currently there is an issue with typescript version 4.9.x, so make sure to lock typescript version to 4.8.4:

That's all, you're ready to go!

Usage

Write a Contract

A contract can be written as a class that extends the SmartContract base, a simple example could be like this:

import { SmartContract, method, prop, assert } from "scrypt-ts";

class Demo extends SmartContract {
  @prop()
  x: bigint;

  constructor(x: bigint) {
    super(x);
    this.x = x;
  }

  @method
  public unlock(x: bigint) {
    assert(this.add(this.x, 1n) === x);
  }

  @method
  add(x0: bigint, x1:bigint) : bigint {
    return x0 + x1;
  }
}

Property Decorator: @prop(state=false)

Use this decorator on class properties to mark them as contract properties, which means the values would be stored on chain within tx.

This decorator can take a boolean parameter, which indicates whether it can be updated later. If it's true, the property is so called a stateful property and its value stored on chain can be updated between contract calls; otherwise, its value can not be changed since the contract deploy.

Method Decorator: @method

Use this decorator on class methods to mark them as contract methods. The logic implemented in these methods would be serialized into tx and be executed on chain.

The class methods decorated by @method have some special requirements / restrains that should be followed:

• Within these methods, only functions provided as built-ins from scrypt-ts or methods also decorated by @method can be called; Similarly, only the properties decorated by @prop can be visited.

• With public modifier, a method is marked as an entry method that could be called outside the contract class, especially during a tx building process. The main purpose of these methods is to validate / verify / check assertions for its input parameters according to its @prop decorated properties. The return value must be void.

• Without a public modifier, a method is kind of an inner function usually be called within the contract class. It can return any valid types described later.

Types

The types can be used in @prop and @method are restricted to these kinds:

• Basic types: boolean / string / bigint;

Note: the type number is not allowed in @prop because it may cause precision issues when representing a floating point number. It can only be used in a few cases specified later on.

• User types can be defined using type or interface, made of basic types. For example,

type ST = {
  a: bigint;
  b: boolean;
}

interface ST1 {
  x: ST;
  y: string;
}

• Array types must be declared using FixedArray, whose length must be known at compile time, like:

let aaa: FixedArray<bigint, 3> = [1n, 3n, 3n];

// 2d array
let abb: FixedArray<FixedArray<bigint, 2>, 3> = [[1n, 3n], [1n, 3n], [1n, 3n]];

• Other SmartContract subclasses are provided as libraries.

Statements

There are also some other restraints / rules on the statemets that could be used within the @methods besides the previously mentioned.

for statement

Because of the underlaying limitation of loop implemetion on Bitcoin script, one can only use a compile time const number as the loop iterations.

So currently if you want to build a loop inside @methods, there is only one restricted version of for statement that could be used. It's looks like:

for(let $i = 0; $i < $constNum; $i++) {
  ...
}

Note that the initial value 0 and the < operator and the post unary operator ++ are all unchangeable.

• $i can be whatever you named the induction variable;

• $constNum should be an expression of a CTC numberic value of the followings:

A number literal like:

for(let i = 0; i < 5; i++ ) ...

Or a const variable name like:

const N = 3;
for(let i = 0; i < N; i++ ) ...

Or a readonly property name like:

class X {
static readonly N = 3;
}
for(let i = 0; i < X.N; i++ ) ...

console.log statement

As described before, all Javascript/Typescript built-in functions/global variables are not allowed in @methods, with only a few exceptions.

One exceptional statement is console.log, which can be used for debugging purpose.

@method
add(x0: bigint, x1:bigint) : bigint {
  console.log(x0);
  return x0 + x1;
}

Build a Contract

Just run npx tsc, or npm run build if you have script as below declared in package.json:

{
 "scripts": {
   "build": "tsc"
 }
}

The tsc compiling process may output diagnostic informations in console about the contract class, update the source code if needed.

Test a Contract

You could write tests using tools like mocha, for example:

describe('Test SmartContract `Demo`', () => {
  before(async () => {
    await Demo.compile();
  })

  it('should pass the public method unit test successfully.', async () => {
    let demo = new Demo(1n);

    let result = demo.verify(() => demo.unlock(2n));
    expect(result.success, result.error).to.eq(true);

    expect(() => {
      demo.unlock(3n);
    }).to.throw(/Execution failed/)
  })
})

Deploy and Call a Contract

Generally speaking, if you want to deploy or call the contract to BSV network, it takes three steps:

1. Build a contract instance:

Giving proper parameters to get an up-to-date contract instance, like:

let instance = new MyContract(...args);

2. Build a tx:

Build a tx corresponding to your business logic, especially to set the tx's proper input & output script with contract instance.

Conceptually speaking, a contract instance has two kinds of relation with txs:

• the lockTo relation

A contract instance has a lockTo relation with a tx means that the instance forms the locking script in one of the tx's outputs.

From the perspective of tx, it may look like this:

tx.addOutput(new bsv.Transaction.Output({
  script: instance.lockingScript,
  ...
}))

From the perspective of instance, the binding can be declared like:

instance.lockTo = { tx, outputIndex: 0 };

• the unlockFrom relation

A contract prevInstance has a unlockFrom relation with a tx means that a call to prevInstance's public(entry) @method will form the unlocking script in one of the tx inputs.

From the perspective of tx, it may look like this:

tx.addInput(new bsv.Transaction.Input({
  script: prevInstance.getUnlockingScript( inst => inst.customEntryMethod(...args) )
  ...
}))

From the perspective of prevInstance, the binding can be declared like:

prevInstance.unlockFrom = { tx, inputIndex: 0};

3. Send the tx:

The final step is to sign and send the tx to the network. If everything is fine, the tx will be accpected by miners.

Here is a complete example code to deploy & call the Demo contract.

// compile contract to get low-level asm
await Demo.compile();

// build contract instance
const demo = new Demo(2n);
const balance = 1000;

// build contract deploy tx
const utxos = await fetchUtxos();
const unsignedDeployTx =
  new bsv.Transaction()
    .from(utxos)
    .addOutput(new bsv.Transaction.Output({
      // get the locking script for `demo` instance
      script: demo.lockingScript, 
      satoshis: balance,
    }));

// send contract deploy tx
const deployTx = await signAndSend(unsignedDeployTx);
console.log('contract deployed: ', deployTx.id)

// build contract call tx
const unsignedCallTx =
  new bsv.Transaction()
    .addInputFromPrevTx(deployTx)
    .addOutput(
      new bsv.Transaction.Output({
        script: bsv.Script.buildPublicKeyHashOut(publicKey.toAddress()),
        satoshis: balance / 2
      })
    );

// send contract call tx
const callTx = await signAndSend(unsignedCallTx);
console.log('contract called: ', callTx.id)

Documentation

The full version of scryptTS documentation is available here.