30. Role-based access control

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

contract Roles {
    bytes32 public constant ADMIN = keccak256("ADMIN");
    mapping(bytes32 => mapping(address => bool)) public hasRole;
    constructor() { /* TODO */ }
    function grantRole(bytes32 role, address user) external {
        // TODO require caller hasRole(role)
    }
}