Writing your first contract on Optimism

This tutorial assumes that you know how to program, but that you've never dealt with smart contracts, much less written one. If you are already familiar with Solidity go here for a tutorial that teaches you only how to use OP Mainnet and OP Goerli with the tools you already know.

Setup

Wallet

Before you can use Ethereum you need wallet software. In this tutorial we use MetaMask, a browser extension, as our wallet. You can also use MetaMask on a mobile device (iOS or Android), but for software development it is best to use a notebook or desktop computer.

1. Make sure you have either Chrome, Firefox, Brave, or Edge.

2. Browse to MetaMask, scroll down and choose to install MetaMask for your browser.

3. Click Get Started.

4. Click Create a wallet and then I agree.

5. Type a password, accept the terms, and click Create.

6. Click Next, reveal the secret words, and write them down somewhere safe. We plan to only use this wallet for testing, so the key phrase should not matter that much, but it's best to develop good habits from the beginning.

7. Let MetaMask help you confirm you have the correct written key phrase.

8. Copy your address, you'll need it soon.

Blockchain

In this tutorial we'll use OP Mainnet, an Ethereum Layer 2 blockchain, to run our contracts.

Deploying contracts and interacting with them costs gas, which has to be purchased with Ether (also known as ETH). On the production network that Ether costs money, it is part of the security mechanism of Ethereum. To avoid spending real money, we'll use OP Goerli, a test network built on top of Goerli. The test network also requires ETH, but it is test ETH you can get for free.

1. Use this faucet to obtain OP Goerli test ETH. Follow five github users or organizations to get past the anti-Sybil defense.

2. Go here to add OP Goerli to your wallet. Click Connect on the web page and then Approve in MetaMask.

3. Permit the network switch.

4. See if you have anything on OP Goerli.

Interacting with a Solidity contract

The most common smart contract development language is called Solidity. Here we will use using an integrated development environment (IDE) called Remix. It has the significant advantage of requiring minimal set up because it runs in your browser.

1. Go to Remix.

2. Click the Files icon () and open contracts > 1_Storage.sol.

3. Click the Compiler icon ()

4. Click Compile 1_Storage.sol.

5. Click the Run icon ().

6. Select the Environment Injected Provider.

7. In MetaMask, click Next and then Connect to accept the connection in the wallet.

8. To add the smart contract into the blockchain, click Deploy (in Remix) and Confirm the transaction in MetaMask.

9. Expand the contract STORAGE AT ... and scroll down to see what you can do.

10. To retrieve the current stored number, click retrieve. Note that as this is a read-only operation, it does not cost anything and does not require MetaMask confirmation.

11. See the retrieved number right below the retrieve button.

    

12. To change the number, type a value next to store and then click it. Because this is a transaction that affects the blockchain state it costs ETH and requires you to Confirm on MetaMask.

    

13. In the Remix console (bottom right quadrant of the window), expand the bottom entry to see the transaction information.

    

14. To verify that the transaction is now part of the permanent blockchain record:

    1. Copy the transaction hash from the remix console.

    2. Open OP Goerli on Etherscan.

    3. Search for the transaction hash. Note that it may take a few minutes for Etherscan to get updated.

15. Back in Remix, click retrieve again to see that the value has changed.

    

How does it work?

To understand the Storage contract, let's go over it line by line

// SPDX-License-Identifier: GPL-3.0

The Solidity compiler expects every source file to have a license comment. See the list of supported licenses.

pragma solidity >=0.7.0 <0.9.0;

This line specifies the acceptable versions of the Solidity programming language. In this case we only allow 0.7.x and 0.8.x versions. This is important because Solidity is new and still developing rapidly, and therefore breaking changes are necessary from time to time.

/**
 * @title Storage
 * @dev Store & retrieve value in a variable
 */
contract Storage {

Define a contract called Storage. A contract functions in many ways like a class in Object Oriented Programming. It contains variables and methods.

    uint256 number;

This is a field variable. The type, uint, is a 256 bit unsigned integer. This is the standard type for evm variables.

Note that all the fields in a contract are effectively public. Ethereum code is executed by multiple computers in multiple locations, and can be verified by anybody. This would be impossible if some part of the contract state had been unreadable. A field variable being private only means it cannot be ready by onchain code.

    /**
     * @dev Store value in variable
     * @param num value to store
     */
    function store(uint256 num) public {

This is a Solidity function definition (for a function without a return value). It includes:

• Function name (store)
• Parameters (in this case there is only one, num)
• A visibility modifier (in this case public, which makes the function callable from anywhere).

        number = num;
    }

The rest of the function is standard for languages derived from C.

    /**
     * @dev Return value 
     * @return value of 'number'
     */
    function retrieve() public view returns (uint256){

This is another function definition, with a few differences:

• The function is defined as a view, which means it is a read-only function that doesn't change the state. As you have seen, calling this kind of function is free.

        return number;
    }
}

To learn more Solidity we have a list of resources. If you learn best by reading source code, try here.