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Modules are the fundamental building blocks of Move code on Sui. They encapsulate types, functions, and logic into reusable components.

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Module Structure

module package_name::module_name {
    // Imports
    use sui::object::{Self, UID};
    use sui::tx_context::TxContext;

    // Friend declarations
    friend package_name::trusted_module;

    // Constants
    const ENotAuthorized: u64 = 0;

    // Structs
    public struct MyStruct has key {
        id: UID,
        value: u64,
    }

    // Functions
    public fun create(ctx: &mut TxContext): MyStruct {
        MyStruct {
            id: object::new(ctx),
            value: 0,
        }
    }

    // Init function (runs once at publish)
    fun init(ctx: &mut TxContext) {
        // Initialization logic
    }
}

​
Module Naming

Modules follow the format address::module_name: 
// Named address
module my_package::game {
    // ...
}

// Literal address
module 0x123::utils {
    // ...
}

​
Imports

​
Using Other Modules

// Import entire module
use sui::transfer;
use sui::object;

// Import specific items
use sui::object::{UID, ID};

// Import with alias
use sui::tx_context::{Self, TxContext};
use std::vector as vec;

// Import multiple items
use sui::coin::{Self, Coin, TreasuryCap};

​
Public Use (Re-exports)

/// Allows calling `.to_string()` on a vector
public use fun std::string::utf8 as vector.to_string;

​
Visibility

// Only visible within the module
fun internal_function() {
    // ...
}

struct InternalStruct {
    value: u64,
}

​
Init Function

The init function runs once when a module is published: 
public struct MY_COIN has drop {}

fun init(witness: MY_COIN, ctx: &mut TxContext) {
    let (treasury, metadata) = coin::create_currency(
        witness,
        6,
        b"MY_COIN",
        b"",
        b"",
        option::none(),
        ctx,
    );
    transfer::public_freeze_object(metadata);
    transfer::public_transfer(treasury, ctx.sender())
}
The init function receives a one-time witness (a struct with the same name as the module) that can be used for privileged operations like creating currencies.

​
Friend Modules

module my_package::bank {
    friend my_package::admin;
    
    public(package) fun privileged_operation() {
        // Only callable by friends
    }
}

module my_package::admin {
    use my_package::bank;
    
    public fun admin_action() {
        bank::privileged_operation();
    }
}

​
Package Organization

Packages contain multiple modules: 
my_package/
├── Move.toml
└── sources/
    ├── main.move
    ├── utils.move
    └── types.move

​
Move.toml

[package]
name = "MyPackage"
version = "0.0.1"
edition = "2024"

[dependencies]
Sui = { git = "https://github.com/MystenLabs/sui.git", subdir = "crates/sui-framework/packages/sui-framework", rev = "framework/mainnet" }

[addresses]
my_package = "0x0"

​
Best Practices

game_package/
├── sources/
│   ├── game.move        # Core game logic
│   ├── player.move      # Player management
│   ├── items.move       # Item system
│   └── marketplace.move # Trading

// Make functions as private as possible
fun internal_helper() { /* ... */ }  // Private
public(package) fun package_api() { /* ... */ }  // Package
public fun public_api() { /* ... */ }  // Public only when needed

/// Core game mechanics module
/// 
/// Handles player actions, game state, and rewards
module my_package::game {
    // ...
}

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