Appendix D - Common Types & Traits and the Cairo Prelude
Prelude
The Cairo prelude is a collection of commonly used modules, functions, data types, and traits that are automatically brought into scope of every module in a Cairo crate without needing explicit import statements. Cairo's prelude provides the basic building blocks developers need to start Cairo programs and writing smart contracts.
The core library prelude is defined in the lib.cairo file of the corelib crate and contains Cairo's primitive data types, traits, operators, and utility functions. This includes:
- Data types: integers, bools, arrays, dicts, etc.
- Traits: behaviors for arithmetic, comparison, and serialization operations
- Operators: arithmetic, logical, bitwise
- Utility functions - helpers for arrays, maps, boxing, etc.
The core library prelude delivers the fundamental programming
constructs and operations needed for basic Cairo programs, without requiring the
explicit import of elements. Since the core library prelude is automatically
imported, its contents are available for use in any Cairo crate without explicit
imports. This prevents repetition and provides a better devX. This is what
allows you to use ArrayTrait::append() or the Default trait without bringing
them explicitly into scope.
You can choose which prelude to use. For example, adding edition = "2023_11" in the Scarb.toml configuration file will load the prelude from November 2023. Note that when you create a new project using scarb new command, the Scarb.toml file will automatically include edition = "2023_11".
The compiler currently exposes 2 different versions of the prelude:
- A general version, with a lot of traits that are made available, corresponding to
edition = "2023_01". - A restricted version, including the most essential traits needed for general cairo programming, corresponding to
edition = 2023_11.
List of Common Types and Traits
The following section provides a brief overview of commonly used types and traits when developing Cairo programs. Most of these are included in the prelude and are therefore not required to be imported explicitly - but not all of them.
| Import | Path | Usage |
|---|---|---|
OptionTrait | core::option::OptionTrait | OptionTrait<T> defines a set of methods required to manipulate optional values. |
ResultTrait | core::result::ResultTrait | ResultTrait<T, E> defines a set of methods required to manipulate Result enum. |
ContractAddress | starknet::ContractAddress | ContractAddress is a type to represent the smart contract address. |
ContractAddressZeroable | starknet::contract_address::ContractAddressZeroable | ContractAddressZeroable is the implementation of the trait Zeroable for the ContractAddress type. It is required to check whether a value of t:ContractAddress is zero or not. |
contract_address_const | starknet::contract_address_const | The contract_address_const function allows instantiating constant contract address values. |
Into | traits::Into | Into<T> is a trait used for conversion between types. If there is an implementation of Into<T,S> for the types T and S, you can convert T into S. |
TryInto | traits::TryInto | TryInto<T> is a trait used for conversion between types. If there is an implementation of TryInto<T,S> for the types T and S, you can try convert T into S if it is possible. |
get_caller_address | starknet::get_caller_address | get_caller_address() is a function that returns the address of the caller of the contract. It can be used to identify the caller of a contract function. |
get_contract_address | starknet::info::get_contract_address | get_contract_address() is a function that returns the address of the current contract. It can be used to obtain the address of the contract being executed. |
This is not an exhaustive list, but it covers some of the commonly used types and traits in contract development. For more details, refer to the official documentation and explore the available libraries and frameworks.