General Introduction to Smart Contracts

This chapter will give you a high level introduction to what smart contracts are, what they are used for, and why blockchain developers would use Cairo and Starknet. If you are already familiar with blockchain programming, feel free to skip this chapter. The last part might still be interesting though.

Smart Contracts

随着以太坊的诞生，智能合约得到了普及并变得更加广泛。智能合约本质上是部署在区块链上的程序。术语 "智能合约 "有些误导，因为它们既不 "智能 "也不是 "合约"，而只是根据特定输入执行的代码和指令。它们主要由两部分组成：存储和函数。部署后，用户可以通过启动包含执行数据的区块链交易（调用哪个函数，输入什么参数）与智能合约互动。智能合约可以修改和读取底层区块链的存储。智能合约有自己的地址，因此它是一个区块链账户，意味着它可以持有代币。

The programming language used to write smart contracts varies depending on the blockchain. For example, on Ethereum and the EVM-compatible ecosystem, the most commonly used language is Solidity, while on Starknet, it is Cairo. The way the code is compiled also differs based on the blockchain. On Ethereum, Solidity is compiled into bytecode. On Starknet, Cairo is compiled into Sierra and then into Cairo Assembly (CASM).

Smart contracts possess several unique characteristics. They are permissionless, meaning anyone can deploy a smart contract on the network (within the context of a decentralized blockchain, of course). Smart contracts are also transparent; the data stored by the smart contract is accessible to anyone. The code that composes the contract can also be transparent, enabling composability. This allows developers to write smart contracts that use other smart contracts. Smart contracts can only access and interact with data from the blockchain they are deployed on. They require third-party software (called oracles) to access external data (the price of a token for instance).

For developers to build smart contracts that can interact with each other, it is required to know what the other contracts look like. Hence, Ethereum developers started to build standards for smart contract development, the ERCxx. The two most used and famous standards are the ERC20, used to build tokens like USDC, DAI or STARK, and the ERC721, for NFTs (Non-Fungible Tokens) like CryptoPunks or Everai.

Use Cases

There are many possible use cases for smart contracts. The only limits are the technical constraints of the blockchain and the creativity of developers.

DeFi

For now, the principal use case for smart contracts is similar to that of Ethereum or Bitcoin, which is essentially handling money. In the context of the alternative payment system promised by Bitcoin, smart contracts on Ethereum enable the creation of decentralized financial applications that no longer rely on traditional financial intermediaries. This is what we call DeFi (decentralized finance). DeFi consists of various projects such as lending/borrowing applications, decentralized exchanges (DEX), on-chain derivatives, stablecoins, decentralized hedge funds, insurance, and many more.

Tokenization

智能合约可以促进现实世界资产的代币化，如房地产、艺术品或贵金属。代币化将资产划分为数字代币，可以在区块链平台上轻松交易和管理。这可以增加流动性，实现部分所有权，并简化购买和销售过程。

Voting

智能合约可用于创建安全和透明的投票系统。投票可以记录在区块链上，确保不可更改性和透明度。然后，智能合约可以自动统计票数并宣布结果，将欺诈或操纵的可能性降到最低。

Royalties

智能合约可以为艺术家、音乐家和其他内容创作者自动支付版税。当一段内容被消费或出售时，智能合约可以自动计算并将版税分配给合法的所有者，确保公平的补偿并减少对中间人的需求。

Decentralized Identities DIDs

智能合约可用于创建和管理数字身份，允许个人控制其个人信息，并与第三方安全地分享。智能合约可以验证用户身份的真实性，并根据用户的凭证自动授予或撤销对特定服务的访问。

The Rise of Starknet and Cairo

Ethereum, being the most widely used and resilient smart contract platform, became a victim of its own success. With the rapid adoption of some previously mentioned use cases, mainly DeFi, the cost of performing transactions became extremely high, rendering the network almost unusable. Engineers and researchers in the ecosystem began working on solutions to address this scalability issue.

A famous trilemma called The Blockchain Trilemma in the blockchain space states that it is hard to achieve a high level of scalability, decentralization, and security simultaneously; trade-offs must be made. Ethereum is at the intersection of decentralization and security. Eventually, it was decided that Ethereum's purpose would be to serve as a secure settlement layer, while complex computations would be offloaded to other networks built on top of Ethereum. These are called Layer 2s (L2s).

L2的两种主要类型是乐观rollup和有效性rollup。这两种方法都涉及压缩和批量处理大量交易，计算新状态，并将结果结算在以太坊（L1）上。区别在于在L1上结算结果的方式。对于乐观rollup，默认情况下新状态被认为是有效的，但节点有7天的窗口期来识别恶意交易。

与此相反，有效性rollup（如Starknet）使用加密技术来证明新状态的计算是正确的。这就是STARKs的目的，这种加密技术可以使有效性rollup的扩展能力大大超过乐观rollup。你可以从Starkware的Medium文章中了解更多关于STARKs的信息，它是一个很好的入门读物。

Starknet's architecture is thoroughly described in the Starknet Book, which is a great resource to learn more about the Starknet network.

还记得Cairo吗？事实上，它是一种专门为STARKs开发的语言，并使其具有通用性。使用Cairo，我们可以编写可证明的代码。在 Starknet 中，这可以证明从一个状态到另一个状态的计算的正确性。

大多数（也许不是全部）Starknet的竞争对手都选择使用 EVM（原版或改版）作为基础层，而Starknet则不同，它采用了自己的虚拟机。这使开发人员摆脱了 EVM 的束缚，开辟了更广阔的可能性。加上交易成本的降低，Starknet 与 Cairo 的结合为开发人员创造了一个令人兴奋的乐园。原生账户抽象使我们称之为 "智能账户" 的账户和交易流的逻辑更加复杂。新兴用例包括 透明人工智能 和机器学习应用。最后，区块链游戏 可以完全在 链上 开发。Starknet 经过专门设计，可最大限度地发挥 STARK 证明的能力，实现最佳可扩展性。

Learn more about Account Abstraction in the Starknet Book.

Cairo Programs and Starknet Contracts: What Is the Difference?

Starknet contracts are a special superset of Cairo programs, so the concepts previously learned in this book are still applicable to write Starknet contracts. As you may have already noticed, a Cairo program must always have a main function that serves as the entry point for this program:

fn main() {}

Contracts deployed on the Starknet network are essentially programs that are run by the sequencer, and as such, have access to Starknet's state. Contracts do not have a main function but one or multiple functions that can serve as entry points.

Starknet contracts are defined within modules. For a module to be handled as a contract by the compiler, it must be annotated with the #[starknet::contract] attribute.