A contract that rewards its first caller after the Merge.

MergeReward.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.13;

import {SafeTransferLib} from "solmate/utils/SafeTransferLib.sol";

/// @title Merge Reward
/// @author Miguel Piedrafita
/// @notice A contract that rewards its first caller after the Merge
contract MergeReward {
    /// @notice Thrown when attempting to claim the reward while in POW
    error StillPow();

    /// @notice Thrown when the reward has already been claimed
    error AlreadyClaimed();

    /// @notice Whether the reward has been claimed
    bool public hasClaimed;

    /// @notice Claim a reward, when in POS
    function claim() public {
        if (hasClaimed) revert AlreadyClaimed();
        if (!haveWeMergedYet()) revert StillPow();

        hasClaimed = true;

        SafeTransferLib.safeTransferETH(msg.sender, address(this).balance);
    }

    /// @notice Determine whether we're running in Proof of Work or Proof of Stake
    /// @dev Post-merge, the DIFFICULTY opcode gets renamed to PREVRANDAO, and stores the prevRandao field from the beacon chain state.
    function haveWeMergedYet() public view returns (bool) {
        return block.difficulty > 2**64;
    }
}