Solidity Fast Track 2: Continue Learning Solidity Fast
Continuing to learn Solidity fast with the advanced basics
Previously we learned all of the basics in 20 minutes. If you are a complete beginner, start there and then come back here. Now we'll explore some more advanced concepts, but again as fast as possible.
1. Saving money with events
We all know gas prices are out of control right now, so it's more important than ever that you learn the basics of how to use events. Events are not stored in a contract like regular storage, but rather in a special logs section of a transaction. So what is this special logs section?
Under the hood of events: Logs in Ethereum
Logs are an efficient way in Ethereum to check for specific data that's not required for the contract itself, but only for logging purposes. Examples on how those logs could be used are the web3.eth.subscribe functionality or The Graph. A bloom filter is used to improve the performance for Ethereum nodes. It basically works by hashing the specific log data, then computing modulo 2048. This is done for all logs in a given transaction (there's no limit to how many logs can exist in a single transaction).
The result will be a bunch of numbers in the range of 0 to 2047. You can create a bit array (bitmap) with mostly 0's, except on those indices where you got a number for in the previous step. Given this bitmap, a node can very efficiently check for logs in a transaction:
Looking for a specific event? Hash the event, compute the hash modulo 2048 and check if there's a 1 at this index in the bitmap. Most of the times there will be a 0, so we can quickly filter out most of the non-relevant transactions. If there's indeed a 1, we have to inspect the full logs array in detail. Since a max of 2047 numbers will mean some logs give us the same number, we'll have a 0.048% chance for a false positive. That's very low, so the overhead for false positives is close to 0.
After looking through the full logs array, we either find a match to the event we we're looking for and return the data or we don't and ignore this transaction.
Events in Solidity
Now that we understand the fundamentals behind events, how do you use them in Solidity?
contract MyContract {
event MyEvent(address indexed sender, uint256 timestamp);
function sendTransactionWithEvent() external {
emit MyEvent(msg.sender, block.timestamp);
}
}You declare any event definitions at the beginning of the contract. Later you can emit events using the emit keyword and passing all parameters to it. Pretty simple and cost effective.
What's the indexed keyword?
You can add up to three parameters with the indexed keyword in one event type. Any indexed parameter will receive its own hashed place in the bitmap, meaning you'll be able to efficiently search for events with specific data in those three parameters.
2. Improving code readability with structs
A very important concept in Solidity are structs. They will allow you to greatly improve the readability of your code. Also since Solidity 0.7 the ABIEncoderV2 is the new default which fully supports structs, so you don't need to worry about passing structs to functions.
Consider our escrow example in the first fast track. It works only for one trade. We can change it to work for any number of traders by keeping the data separately using structs:
Before:
address public seller;
address public buyer;
address public arbitrator;
uint256 public purchasePrice;
uint256 public purchaseConfirmationCount;
uint256 public refundConfirmationCount;
mapping(address => bool) public hasConfirmedPurchase;
mapping(address => bool) public hasConfirmedRefund;After:
struct EscrowTrade {
address seller;
address buyer;
address arbitrator;
uint256 purchasePrice;
uint256 purchaseConfirmationCount;
uint256 refundConfirmationCount;
mapping(address => bool) hasConfirmedPurchase;
mapping(address => bool) hasConfirmedRefund;
}
mapping(uint256 => EscrowTrade) public escrowTrades;We can now create and access trades conveniently using a trade id. For example you may access the seller via escrowTrades[tradeId].seller. Alternatively you can store the whole trade struct data in its own variable first and later use the data directly:
EscrowTrade memory escrowTrade = escrowTrades[tradeId];
address seller = escrowTrade.seller;You'll see this pattern all the time in contracts, so make sure to get comfortable with it.
3. From EIPs to ERCs with the most popular example: ERC-20
One thing to understand is the difference between EIPs and ERCs. But first what are EIPs?
They stand for Ethereum Improvement Proposal and are just a repository on Github. Anyone can create such an EIP which is supposed to be for proposals regarding the core protocol specifications and client APIs. Very similar to that the ERC stands for Ethereum Request for Comments, the name coming from the RFCs internet standards. Those are proposals for smart contract standards.
As a beginner you probably don't need to care about EIPs too much. Some of them can have significant effects, such as EIP-1559, but only when it's actually implemented and used on Ethereum. By then you will likely read dozens of articles about it and what its implications are.
More interestingly are the ERC's which are standards that would be a good idea to follow in your contracts. Granted there are hundreds if not thousands of ERCs and not all of them are important. It can be tricky to find out which one to use.
Tthe Openzeppelin contracts are always a good reference. Let's now explore the most popular ERC of them all: ERC-20.
We already learned all the concepts in this and the previous post to build an ERC-20 contract. An ERC-20 contract is just a simple second currency (token) implementation on top of Ethereum where Ether is an existing implementation. Most popular might be the DAI token, which is a decentralized stable coin worth always about 1 USD.
At its core the the ERC-20 implementation is just this:
contract MyERC20Token {
mapping(address => uint256) public balanceOf;
function transfer(address to, uint256 amount) external returns (uint256) {
require(balanceOf[msg.sender] >= amount, "Not enough balance");
balanceOf[msg.sender] -= amount;
balanceOf[to] += amount;
return true;
}
}Pretty simple right? Now the actual standard further contains events, an approval mechanism, a totalSupply function and a few optional fields. Take a look at the standard for the details. Most importantly here the approval mechanism allows to use tokens in smart contract calls:
Imagine Alice wants to use the SuperSmartContract which requires users to pay 10 DAI. Alice can approve the DAI calling DaiToken.approve(SuperSmartContract.address, 10) and subsequently SuperSmartContract.use(). Inside the use() function, the contract can all DaiToken.transferFrom(msg.sender, address(this), 10) and since Alice set the allowance previously, this call is now allowed to transfer money on behalf of Alice to himself.
- How to implement an ERC-20 using latest frameworks: https://soliditydeveloper.com/erc-2020/
- How to securely interact with existing ERC-20 contracts: https://soliditydeveloper.com/safe-erc20/
- Other token standards: https://soliditydeveloper.com/blog?tag=ERC-20
4. Libraries
Libraries are special contracts that have no storage, but can be used only for implementation code. There's two different forms of libraries, those with public functions and those only with internal functions. They both serve very different purposes.
First let's talk about libraries with only internal functions. This is the most common form for libraries. You'll see them in packages like Openzeppelin contracts, Uniswap's own library repo or the quite popular ds-math lib.. This is the kind of library that you would use to make your code more readable and I highly recommend you do this yourself whenever it makes sense. Combine this with the using for syntax for maximum benefits.
The simplest example, which with Solidity 0.8+ new safe math feature wouldn't be necessary anymore, would be:
contract MyContract {
using MyMathLib for uint256;
function returnSub(uint256 a, uint256 b) external returns (uint256) {
return a.safeSub(b);
}
}The second form with public functions allows for reusability. To quote the Solidity documentation: 'Libraries are similar to contracts, but their purpose is that they are deployed only once at a specific address and their code is reused using the DELEGATECALL.'
So in other words, those libraries can be reused by many projects. After all you might have a lot of standard code that would just be duplicated logic in the blockchain if each contract would have its own version of it. That's at least the theory, in practice honestly the use of reusable libraries hasn't really taken off. There is at least one project aiming to provide those: https://github.com/modular-network/ethereum-libraries. Otherwise, external libraries can also help to stay below the contract size limit.
Congratulations. You've mastered level 2.
- Inheritance: Solidity supports inheritance including polymorphism. A great way to structure your contracts.
- Hacks: Always a great way to learn about smart contract security.
- Downsizing contracts to keep below the max contract size (its own EIP-170 btw)
- Avoiding stack too deep errors
- Toolings
- Study more ERC's (+ EIPs)
- Best coding practices in general! My colleague has a great blog post about naming (important) and another great resource currently 66% discounted is the book Dive Into DESIGN PATTERNS. Or really any other book you can find. Solidity might be a bit strange due to running on the blockchain, but most programming concepts still apply of course.
Solidity Developer
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How to use Polygon (Matic) in your Dapp
Deploying and onboarding users to Polygon to avoid the high gas costs
Gas costs are exploding again, ETH2.0 is still too far away and people are now looking at layer 2 solutions. Here's a good overview of existing layer 2 projects: https://github.com/Awesome-Layer-2/awesome-layer-2. Today we will take a closer look at Polygon (previously known as Matic) as a...
Migrating from Truffle to Buidler
And why you should probably keep both.
Why Buidler? Proper debugging is a pain with Truffle. Events are way too difficult to use as logging and they don't even work for reverted transactions (when you would need them most). Buidler gives you a console.log for your contracts which is a game changer. And you'll also get stack traces...
Contract factories and clones
How to deploy contracts within contracts as easily and gas-efficient as possible
The factory design pattern is a pretty common pattern used in programming. The idea is simple, instead of creating objects directly, you have an object (the factory) that creates objects for you. In the case of Solidity, an object is a smart contract and so a factory will deploy new contracts for...
How to use IPFS in your Dapp?
Using the interplanetary file system in your frontend and contracts
You may have heard about IPFS before, the Interplanetary File System. The concept has existed for quite some time now, but with IPFS you'll get a more reliable data storage, thanks to their internal use of blockchain technology. Filecoin is a new system that is incentivizing storage for IPFS...
Downsizing contracts to fight the contract size limit
What can you do to prevent your contracts from getting too large?
Why is there a limit? On November 22, 2016 the Spurious Dragon hard-fork introduced EIP-170 which added a smart contract size limit of 24.576 kb. For you as a Solidity developer this means when you add more and more functionality to your contract, at some point you will reach the limit and when...
Using EXTCODEHASH to secure your systems
How to safely integrate anyone's smart contract
What is the EXTCODEHASH? The EVM opcode EXTCODEHASH was added on February 28, 2019 via EIP-1052. Not only does it help to reduce external function calls for compiled Solidity contracts, it also adds additional functionality. It gives you the hash of the code from an address. Since only contract...
Using the new Uniswap v2 in your contracts
What's new in Uniswap v2 and how to integrate Uniswap v2
Note : For Uniswap 3 check out the tutorial here. What is UniSwap? If you're not familiar with Uniswap yet, it's a fully decentralized protocol for automated liquidity provision on Ethereum. An easier-to-understand description would be that it's a decentralized exchange (DEX) relying on external...
Solidity and Truffle Continuous Integration Setup
How to setup Travis or Circle CI for Truffle testing along with useful plugins.
Continuous integration (CI) with Truffle is great for developing once you have a basic set of tests implemented. It allows you to run very long tests, ensure all tests pass before merging a pull request and to keep track of various statistics using additional tools. We will use the Truffle...
Upcoming Devcon 2021 and other events
The Ethereum Foundation just announced the next Devcon in 2021 in Colombia
Biggest virtual hackathon almost finished First of all, the current HackMoney event has come to an end and it has been a massive success. One can only imagine what kind of cool projects people have built in a 30 days hackathon. All final projects can be seen at:...
The Year of the 20: Creating an ERC20 in 2020
How to use the latest and best tools to create an ERC-20 token contract
You know what an ERC-20 is, you probably have created your own versions of it several times (if not, have a look at: ERC-20). But how would you start in 2020 using the latest tools? Let's create a new ERC-2020 token contract with some basic functionality which focuses on simplicity and latest...
How to get a Solidity developer job?
There are many ways to get a Solidity job and it might be easier than you think!
You have mastered the basics of Solidity, created your first few useful projects and now want to get your hands on some real-world projects. Getting a Solidity developer job might be easier than you think. There are generally plenty of options to choose from and often times not a lot of...
Design Pattern Solidity: Mock contracts for testing
Why you should make fun of your contracts
Mock objects are a common design pattern in object-oriented programming. Coming from the old French word 'mocquer' with the meaning of 'making fun of', it evolved to 'imitating something real' which is actually what we are doing in programming. Please only make fun of your smart contracts if you...
Kickstart your Dapp frontend development with create-eth-app
An overview on how to use the app and its features
Last time we looked at the big picture of Solidity and already mentioned the create-eth-app. Now you will find out how to use it, what features are integrated and additional ideas on how to expand on it. Started by Paul Razvan Berg, the founder of sablier, this app will kickstart your frontend...
The big picture of Solidity and Blockchain development in 2020
Overview of the most important technologies, services and tools that you need to know
Now, I do not know about you, but I remember when I first started with Solidity development being very confused by all the tools and services and how they work in connection with one another. If you are like me, this overview will help you understand the big picture of Solidity development. As I...
Design Pattern Solidity: Free up unused storage
Why you should clean up after yourself
You may or may not be used to a garbage collectors in your previous programming language. There is no such thing in Solidity and even if there was a similar concept, you would still be better off managing state data yourself. Only you as a programmer can know exactly which data will not be used...
How to setup Solidity Developer Environment on Windows
What you need to know about developing on Windows
Using Windows for development, especially for Solidity development, can be a pain sometimes, but it does not have to be. Once you have configured your environment properly, it can actually be extremely efficient and Windows is a very, very stable OS, so your overall experience can be amazing. The...
Avoiding out of gas for Truffle tests
How you do not have to worry about gas in tests anymore
You have probably seen this error message a lot of times: Error: VM Exception while processing transaction: out of gas Disclaimer : Unfortunately, this does not always actually mean what it is saying when using Truffle , especially for older versions. It can occur for various reasons and might be...
Design Pattern Solidity: Stages
How you can design stages in your contract
Closely related to the concept of finite-state machines, this pattern will help you restrict functions in your contract. You will find a lot of situations where it might be useful. Any time a contract should allow function calls only in certain stages. Let's look at an example: contract Pool {...
Web3 1.2.5: Revert reason strings
How to use the new feature
A new Web3 version was just released and it comes with a new feature that should make your life easier. With the latest version 1.2.5, you can now see the the revert reason if you use the new handleRevert option. You can activate it easily by using web3.eth.handleRevert = true . Now when you use...
Gaining back control of the internet
How Ocelot is decentralizing cloud computing
I recently came across an ambitious company that will completely redefine the way we are using the internet. Or rather, the way we are using its underlying infrastructure which ultimately is the internet. While looking at their offering, I also learned how to get anonymous cloud machines, you...
Devcon 5 - Review
Impressions from the conference
I had a lot to catch up on after Devcon. Also things didn't go quite as planned, so please excuse my delayed review! This year's Devcon was certainly stormy with a big typhoon warning already on day 1. Luckily (for us, not the people in Tokyo), it went right past Osaka. Nevertheless, a lot of...
Devcon 5 - Information, Events, Links, Telegram
What you need to know
Devcon 5 is coming up soon and there are already lots of events available, information about Osaka and more. Here is a short overview: Events Events Calendar Events Google Docs Events Kickback Most events are in all three, but if you really want to see all, you will have to look at all three...
Design Pattern Solidity: Off-chain beats on-chain
Why you should do as much as possible off-chain
As you might have realized, Ethereum transactions are anything but cheap. In particular, if you are computing complex things or storing a lot of data. That means sometimes we cannot put all logic inside Solidity. Instead, we can utilize off-chain computations to help us. A very simple example...
Design Pattern Solidity: Initialize Contract after Deployment
How to use the Initializable pattern
There are a few reasons why you might want to initialize a contract after deployment and not directly by passing constructor arguments. But first let's look at an example: contract MyCrowdsale { uint256 rate; function initialize(uint256 _rate) public { rate = _rate; } } What's the advantage over...
Consensys Blockchain Jobs Report
What the current blockchain job market looks like
Consensys published their blockchain jobs report which you can checkout in their Blockchain Developer Job Kit. The most interesting aspects are Blockchain developer jobs have been growing at a rate of 33x of the previous year according to LinkedIns jobs report Typical salary is about...
Provable — Randomness Oracle
How the Oraclize random number generator works
One particularly interesting approach by Provable is the usage of a hardware security device, namely the Ledger Nano S. It uses a trusted execution environment to generate random numbers and provides a Provable Connector Contract as interface. How to use the Provable Randomness Oracle? Use the...
Solidity Design Patterns: Multiply before Dividing
Why the correct order matters!
There has been a lot of progress since the beginning of Ethereum about best practices in Solidity. Unfortunately, I have the feeling that most of the knowledge is within the circle of experienced people and there aren’t that many online resources about it. That is why I would like to start this...
Devcon 5 Applications closing in one week
Devcon 5 Applications closing
Watch out for the Devcon 5 applications. You only have one week left to apply either as Buidler Student Scholarship Press Devcon is by far the biggest and most impressive Ethereum conference in the world. And it's full of developers! I am especially excited about the cool location this year in...
Randomness and the Blockchain
How to achieve secure randomness for Solidity smart contracts?
Update 2023 : Ethereum transitioned to Proof of Stake! If you are interested in the randomness there, you can now use the updated info over at https://soliditydeveloper.com/prevrandao. When we talk about randomness and blockchain, these are really two problems: How to generate randomness in smart...