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Contract Structure

Owl Protocol contracts are designed to be re-usable as much as possible and avoid code duplication where possible to reduce attack surface. As such, all of our contract share a similar structure and also come with pre-configured utilities to enable easier integration in projects.


All contracts share the following


  • Solidity sourced stored under contracts/
  • An IMyContract.sol counterpart to the implementation, that only defines the interface. Documentation for external functions, is usually written in the IMyContract.sol and functions that implement the interface will use @inheritdoc tags to avoid duplicate documentation (except where necessary).
  • An empty constructor() {} replaced in favor of using an initialize(bytes memory data) function.
  • AccessControl for role management
  • ContractURI for contract-level-metadata
  • RouterReceiver for support
  • EIP-165 support implementing the proper interfaceId of the contract and it's parents' interfaceIds
  • EIP-1820 support, registering with the contract registry (if existing). BOTH its EIP-165 interfaceId and a custom EIP-1820 interfaceId (defined as interfaceId | 0x00000000FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF) are registered
  • OwlBase that inherits AccessControl, ContractURI, and RouterReceiver


  • Generated artifacts in src/artifacts/
  • Exported ether.js contract & factory types generated with Typechain
  • Exported web3.js contract types generated with Typechain
  • ether.js factories for regular, deterministic, and EIP-1167 proxy deployment explored under []
  • A utils file named MyContract.ts under src/utils/ defining common utilities such as initialization parameters, & exporting an EIP-165 interfaceId
  • Ethers-based deployment script under src/deploy/ that is hardhat-agnostic and only requires an ether.js instance and a signer. Removing the dependency on hardhat enables usage outside of the HRE, and therefore packaging the deployment logic as part of the npm package for usage in other environments such as in the browser or using another ether.js provider.
  • HRE-based deployment script under deploy/ designed to be used for local development with Hardhat. This usually calls the the [src/deploy] script with additional HRE-specific logic such as saving the deployment artifact.
  • Test suite under test/ for unit tests. Test suites usually use the src/deploy/ script for the relevant contract to deploy.

Creating a contract

  • We use the following subfolders to categorize contracts
  • Write the Solidity interface contract IMyContract.sol along with some documentation:
    • What data needs to be accessible by other smart contracts?
    • What instead can simply be stored as metadata in the contractURI ?
  • Write the Soldity implementation contract MyContract.sol that implements the interface:
    • What storage variables are required?
      • Can you use existing OpenZeppelin struct primitives such as Counter or EnumerableSet?
      • Write the contract's __MyContract_init_unchained that initializes ONLY the storage variables introduced by the smart contract & registers the contract with EIP-1820 registry using type(IMyContract)
    • What contracts should the contract inherit from? How should the contract be initialized?
      • Write the contract's __MyContract_init internal that intializes all parent __Parent_init_unchained functions and lastly calls __MyContract_init_unchained
      • expose __MyContract_init using initialize external initializer and proxyInitialier external onlyInitializing functions
    • What configurations can be updated in the future? The less the better while maintaing all utility requirements.
    • What permissions are required to be configured with AccessControl to allow for updates?

Make sure to have hardhat-shorthand installed globally. Compile your Solidity code incrementally as you develop with:

hh compile

Export Contract Artifacts

Contract artifacts are generated by hardhat and stored under src/artifacts/. However, hardhat generates artifacts for ALL contracts, including dependencies (eg. OpenZeppelin).

For easier usage, specific contract artifacts re-exported in src/artifacts.ts (.json) & src/artifacts-js.ts (for ESBUILD).

The duplicate artifacts files may be a bit confusing. The reason they are required is Hardhat uses ts-node to transpile the code at runtime, which expects the artifacts to be exported with .json, but when packaging as an npm library, esbuild converts to .json artifacts to .js modules. The fix to this, esbuild is patched in the resolution plugin esbuild.config.mjs which replaces the src/artifacts.ts with src/artifacts-js.ts. This enables using the artifacts both with hardhat locally (hh deploy, hh test) and distribute the generated artifacts in an npm package.

Typechain types

Typechain can be used to generate Typescript types for the smart contracts. We do NOT use the @typechain/hardhat plugin as it only supports generating types for one framework.

Generated Typechain types for ether.js and web3.js using:

npm run generate-types

ether.js factories

Once the artifacts and types have been generated & exported, you can now create the factory helpers to make it easier to deploy the contract.

The deterministicFactories.ts, proxy1167Factories.ts, and beaconProxyFactories.ts will automatically extend their types as they depend on factories.ts. No change necessary.

Contract Utilities

In additon to factories, contracts may also export a set of utilities that are specific to them for easier interaction. A common use case is exporting the EIP-165 interfaceId or exporting a key-value type definition of tuples used by contract with a parsing function to encode/decode from the tuple to key-value definition.

EIP-165 interfaceId

Export the EIP-165 interfaceId of the contract. This can then be used by libraries looking to identify contracts that implement the interface.

An example from src/utils/IERC721.ts

import { utils } from 'ethers';
import { IERC721 } from '../artifacts';
import { IERC721Interface as Interface } from '../ethers/types';
import { interfaceId } from './IERC165';

export const IERC721Interface = new utils.Interface(IERC721.abi) as Interface;
export const IERC721InterfaceId = interfaceId(IERC721Interface);

Tuples as Key-Value

Export utility functions that wrap tuples as key-value objects.

The following example from src/utils/ERC721Mintable.ts shows wrapping the initializer parameters as a key-value interface ERC721MintableInitializeArgs that can then be parsed using flattenInitArgsERC721Mintable to a regular tuple used by the initialize function.

import type { ERC721Mintable } from '../ethers/types';

export interface ERC721MintableInitializeArgs {
admin: Parameters<ERC721Mintable['initialize']>[0];
contractUri: Parameters<ERC721Mintable['initialize']>[1];
gsnForwarder: Parameters<ERC721Mintable['initialize']>[2];
name: Parameters<ERC721Mintable['initialize']>[3];
symbol: Parameters<ERC721Mintable['initialize']>[4];
initBaseURI: Parameters<ERC721Mintable['initialize']>[5];
feeReceiver: Parameters<ERC721Mintable['initialize']>[6];
feeNumerator: Parameters<ERC721Mintable['initialize']>[7];

export function flattenInitArgsERC721Mintable(args: ERC721MintableInitializeArgs) {
const { admin, contractUri, gsnForwarder, name, symbol, initBaseURI, feeReceiver, feeNumerator } = args;
return [admin, contractUri, gsnForwarder, name, symbol, initBaseURI, feeReceiver, feeNumerator] as Parameters<

Deploy Script

If you updated the factories.ts file, the src/deploy/common/Implementations.ts and src/deploy/common/UpgradeableBeacon.ts deployment scripts will already be up to date to deploy an implementation smart contract and an beacon smart contract that points to it. Note that NEITHER of these contracts is usable and they should be used to then deploy EIP-1167 proxies to the implementation or beacon proxies to the beacon.

Test that your contract is deployed by trying deploying locally. The UpgradeableBeacon script depends on the Implementations script so will automatically deploy all.

hh deploy --tags UpgradeableBeacon
hardhat MyContract implementation deployed 0x0000...
hardhat MyContract beacon deployed 0x0000...

For production deployments, you will need to write your own deploy script under src/deploy/ that deploys EIP-1167 proxies to the implementation or beacon proxies to the beacon.

Mocha Test

If the regular src/deploy/common/Implementations.ts and src/deploy/common/UpgradeableBeacon.ts deployment scripts are working, you can then move on to writing Typescript unit tests for your smart contracts.

Unit tests should implement a before() clause that deploys all of the shared contracts that enable deterministic deployment, notably the EIP-2470 deployer and the [EIP1167Factory].

describe('ERC721Mintable', function () {
let signers: SignerWithAddress[];
let factories: Factories;
let deterministicFactories: InitializeFactories;
let ERC721MintableFactory: typeof deterministicFactories.ERC721Mintable;
let ERC721Mintable: ERC721Mintable;

let tokenName = 0;

let token: ERC721MintableInitializeArgs;

before(async () => {
await deployProxyNick(hre as any);
await deployProxyFactory(hre as any);

signers = await ethers.getSigners();
const signer = signers[0];
const signerAddress = signer.address;

factories = getFactories(signer);
deterministicFactories = getDeterministicInitializeFactories( factories, signerAddress);
ERC721MintableFactory = deterministicFactories.ERC721Mintable;


For each unit test, you should then deploy a EIP-1167 proxy contract with a unique salt:

beforeEach(async () => {
token = {
admin: signers[0].address,
contractUri: `token.${tokenName}.com`,
gsnForwarder: ethers.constants.AddressZero,
name: `Token ${tokenName}`,
symbol: `TK${tokenName}`,
initBaseURI: `token.${tokenName}.com/token`,
feeReceiver: signers[0].address,
feeNumerator: 0,
const initializerArgs = flattenInitArgsERC721Mintable(token);
ERC721Mintable = await ERC721MintableFactory.deploy(...initializerArgs);


Then write your individual unit tests. You should ONLY test functionality that is custom to your contracts or some parent logic that has been overriden by your contract. Duplicate tests (eg. regular EIP-721 transfer functionality) is unecessary.

it('name', async () => {
const result = await;

For more info, check out the example test for test/hardhat/assets/ERC721/ERC721Mintable.test.ts.