Write Smart Contracts with Sui Move

Welcome to the Sui tutorial for building smart contracts with Sui Move. This tutorial provides a brief explanation of the Sui Move language, and includes concrete examples to demonstrate how you can use Move in Sui.

About Sui Move

Sui Move is an open source language for writing safe smart contracts. It is Sui's dialect of Move, which was originally developed at Facebook to power the Diem blockchain.

Sui Move can define, create, and manage programmable Sui objects representing user-level assets. Sui's object system is implemented by adding new functionality while also imposing additional restrictions to the original Move. This is one of the major differences in the Sui Move dialect that makes certain parts of the original Move documentation not applicable to smart contract development in Sui. Consequently, it's best to follow this tutorial and the relevant Move documentation links within.

The documentation for the original Move language is available at the Move GitHub repository which includes a tutorial and a book describing language features in detail. These are invaluable resources to deepen your understanding of the Move language but not strict prerequisites to following this tutorial.

Before looking at examples of Sui Move, let's talk briefly about code organization.

Sui Move code organization

Sui Move shares the same code organization concepts as Move. The main unit of Move code organization (and distribution) is a package. A package consists of a set of modules defined in separate files with the .move extension. These files include Move functions and type definitions. A package must include the Move.toml manifest file describing package configuration, such as package name, metadata and dependencies. See Move.toml for more information about package manifest files in Sui Move. Packages also include an auto-generated Move.lock file. The Move.lock file is similar in format to the package manifest, but is not meant for users to edit directly. See Move.lock for more information about the lock file in Sui Move.

The minimal package source directory structure looks as follows and contains the manifest file, the lock file, and the sources subdirectory where one or more module files are located:

my_move_package
├── Move.lock
├── Move.toml
├── sources
    ├── my_module.move

See Package Layout and Manifest Syntax for more information on package layout.

It's now time to look at some Sui Move code. You can either keep reading for an introductory description of the main Sui Move language constructs or you can jump straight into the code by writing a simple Sui Move package. Also you can check out additional code examples in Sui by Example.

First look at Move source code

The Sui platform includes the Sui Framework, which includes the core on-chain libraries that Sui Move developers need to bootstrap Sui operations. For example, Sui supports user-defined coin types, which are custom assets. Sui Framework contains the Coin module supporting creation and management of custom coins. The Coin module is located in the coin.move file. As you might expect, the manifest file describing how to build the package containing the Coin module is located in the corresponding Move.toml file.

Let's see how module definition appears in the Coin module file:

module sui::coin {
...
}

Don't worry about the rest of the module contents for now; you can read more about modules in the Move book later.

Important: In Sui Move, package names are always in PascalCase, while the address alias is lowercase, for example sui = 0x2 and std = 0x1. So: Sui = name of the imported package (Sui = sui framework), sui = address alias of 0x2, sui::sui = module sui under the address 0x2, and sui::sui::SUI = type in the module above.

When you define a module, specify the module name (coin) preceded by the name of the package where this module resides (sui). The combination of the package name and the module name uniquely identifies a module in Sui Move source code. The package name is globally unique, but different packages can contain modules with the same name. While module names are not unique, when they combine with their unique package name they result in a unique combination.

For example, if you have a published package "P", you cannot publish an entirely different package also named "P". At the same time you can have module "P1::M1", "P2::M1", and "P1::M2" but not another, say, "P1::M1" in the system at the same time.

While you can't name different packages the same, you can upgrade a package on chain with updated code using the same package name.

In addition to having a presence at the source code level, as discussed in Sui Move code organization, a package in Sui is also a Sui object and must have a unique numeric ID in addition to a unique name, which is assigned in the manifest file:

[addresses]
sui = "0x2"

Sui Move structs

The Coin module defines the Coin struct type that you can use to represent different types of user-defined coins as Sui objects:

struct Coin<phantom T> has key, store {
    id: UID,
    value: u64
}

Sui Move's struct type is similar to struct types defined in other programming languages, such as C or C++, and contains a name and a set of typed fields. In particular, struct fields can be of a primitive type, such as an integer type, or of a struct type.

You can read more about Move primitive types and structs in the Move book.

For a Sui Move struct type to define a Sui object type, such as Coin, its first field must be id: UID. UID is a struct type defined in the object module. The Move struct type must also have the key ability, which allows Sui's global storage to persist the object. Abilities of a Move struct are listed after the has keyword in the struct definition, and their existence (or lack thereof) helps the compiler enforce various properties on a definition or on instances of a given struct.

You can read more about struct abilities in the Move book.

The reason that the Coin struct can represent different types of coin is that the struct definition is parameterized with a type parameter. When you create an instance of the Coin struct, you can pass it an arbitrary concrete Move type (e.g. another struct type) to distinguish different types of coins from one another.

Learn about Move type parameters known as generics and the optional phantom keyword at your leisure.

In particular, one type of custom coin already defined in Sui is Coin<SUI>, which represents a token used to pay for Sui computations (more generally known as gas) - in this case, the concrete type used to parameterize the Coin struct is the SUI struct in the SUI module:

struct SUI has drop {}

The Write a Sui Move Package topic shows how to define and instantiate custom structs.

Move functions

Similar to other popular programming languages, the main unit of computation in Move is a function. Let us look at one of the simplest functions defined in the Coin module, that is the value function.

public fun value<T>(self: &Coin<T>): u64 {
    self.value
}

Functions in other modules can call this public function to return the unsigned integer value currently stored in a given instance of the Coin struct. The Move compiler allows direct access to fields of a struct only within the module defining a given struct, as described in Privileged Struct Operations. The body of the function simply retrieves the value field from the Coin struct instance parameter and returns it. The coin parameter is a read-only reference to the Coin struct instance, indicated by the & preceding the parameter type. Move's type system enforces an invariant that struct instance arguments passed by read-only references (as opposed to mutable references) cannot be modified in the body of a function.

You can read more about Move references in the Move book.

The Write a Sui Move Package topic shows how to call Move functions from other functions and how to define the new ones.

The Sui dialect of the Move language also defines entry functions. These must satisfy a certain set of properties and you can call them directly from Sui (e.g., from a Sui application written in a different language).

Entry functions

One of the basic operations in Sui is a gas object transfer between addresses representing individual users. The gas object transfer implementation in the SUI module is also an example of the use of an entry function:

public entry fun transfer(c: coin::Coin<SUI>, recipient: address, _ctx: &mut TxContext) {
    ...
}

Don't worry about the function body for now - because the function is part of Sui framework, you can trust that it will do what it is intended to do.

In general, an entry function must satisfy the following properties:

  • Has the entry modifier. The visibility does not matter. The function can be public, public(friend), or internal.
  • Has no return value
  • (Optional) Has a mutable reference to an instance of the TxContext struct defined in the TxContext module as the last parameter.

More concretely, the transfer function is public, has no return value, and has three parameters:

  • c - Represents a gas object whose ownership is to be transferred.
  • recipient - The address of the intended recipient
  • _ctx - A mutable reference to an instance of the TxContext struct (in this particular case, this parameter is not actually used in the function's body as indicated by its name starting with _). Because it is unused, the parameter could be removed. The mutable reference to the TxContext is optional for entry functions.
Calling Move code describes how to call the transfer function from the Sui CLI client.