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WTF Cairo: 25. Interface/ABI

We are learning Cairo, and write WTF Cairo Tutorials for Starknet newbies. The tutorials are based on Cairo 1.0.

Twitter: @0xAA_Science｜@WTFAcademy_

WTF Academy Community：Discord｜Wechat｜Website

All codes and tutorials are open-sourced on GitHub: github.com/WTFAcademy/WTF-Cairo

We are learning Cairo, and write WTF Cairo Tutorials for Starknet newbies. The tutorials are based on Cairo 1.0.

Twitter: @0xAA_Science｜@WTFAcademy_

WTF Academy Community：Discord｜Wechat｜Website

All codes and tutorials are open-sourced on GitHub: github.com/WTFAcademy/WTF-Cairo

In this chapter, we will compare the interface in Solidity and Cairo, a key element that facilitates interactions between different contracts.

Interface

In Solidity, an interface is a list of function definitions without implementation. An interface enforces a defined set of properties and functions on a contract and allows you to interact with other contracts without having their code.

Let's examine the interface of the ERC20 token standard IERC20. This interface outlines the functions that must be implemented to comply with this standard.

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

interface IERC20 {
    event Transfer(address indexed from, address indexed to, uint value);

    event Approval(address indexed owner, address indexed spender, uint value);

    function totalSupply() external view returns (uint);

    function balanceOf(address account) external view returns (uint);

    function transfer(address recipient, uint amount) external returns (bool);

    function allowance(address owner, address spender) external view returns (uint);

    function approve(address spender, uint amount) external returns (bool);

    function transferFrom( address sender, address recipient, uint amount) external returns (bool);
}

You can use the IERC20 contract to interact with the contracts following the ERC20 standard, such as USDC, without knowing its code.

Interface in Cairo

In Cairo, an interface is represented by a trait marked with the #[abi] attribute. The rules are as follows:

  1. Must explicitly declare the function's decorator.
  2. Should not have implementations.
  3. Should not declare a constructor.
  4. Should not declare state variables.
  5. Should not declare events (different from Solidity).

Let's rewrite the IERC20 contract in Cairo:

use starknet::ContractAddress;

#[abi]
trait IERC20 {
    #[view]
    fn total_supply() -> u256;

    #[view]
    fn balance_of(account: ContractAddress) -> u256;

    #[view]
    fn allowance(owner: ContractAddress, spender: ContractAddress) -> u256;

    #[external]
    fn transfer(recipient: ContractAddress, amount: u256) -> bool;

    #[external]
    fn transfer_from(sender: ContractAddress, recipient: ContractAddress, amount: u256) -> bool;

    #[external]
    fn approve(spender: ContractAddress, amount: u256) -> bool;
}

Since interfaces in Cairo cannot contain events, we don't have the Transfer and Approval events within the interface. These must be declared in the implementation contract, a topic that we'll discuss in subsequent chapters.

Summary

This chapter provided a deep-dive into the notion of interfaces in Cairo. Interfaces enforces a defined set of functions on a contract, enabling interaction with other contracts without having their source code.