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Move Concepts

Move is an open source language for writing safe packages to manipulate on-chain objects (sometimes referred to as "smart contracts"). Move is a platform-agnostic language to enable common libraries, tooling, and developer communities across blockchains with vastly different data and execution models. Move is adaptable to meet the needs of the blockchain the code operates on, see Move on Sui to review enhancements made to Move for optimization on the Sui blockchain.

You can use Move to define, create, and manage programmable Sui objects representing user-level assets. Sui's object system is implemented by adding new functionality to Move while also imposing additional restrictions. See Object Model for more details.

Move on Sui​

Move on Sui contains some important differences from Move on other blockchains. Sui takes advantage of Move's security and flexibility and enhances it with the features described in the following sections to vastly improve throughput, reduce delays in finality, and make Move programming more approachable. For full details, see the Sui Smart Contracts Platform white paper.

tip

Where the Sui documentation refers to the Move language, the content is documenting the specific Move implementation on the Sui blockchain. If relevant, the documentation expressly refers to the original use case for the Move language as Move on Diem.

In general, Move on Diem code written for other systems works in Sui with these exceptions:

Key differences​

Key differences with Move on Sui include:

Object-centric global storage​

In Move on Diem, global storage is part of the programming model. Resources and modules are held in global storage, owned by an account which has an address. Transactions are free to access resources from any account in global storage when they run, using special operations such as move_to and move_from.

This approach introduces a scaling issue, as it is not possible to statically determine which transactions are contending over the same resource and which are not. This is similar to the scaling issues faced by other blockchains where smart contracts typically store account information in large, internal mappings, which limit throughput.

Move on Sui addresses the scaling issue by not having global storage, or its related operations. When objects (in contrast to resources) and packages (sets of modules) are stored on Sui, they are each given unique identifiers. All a transaction's inputs are explicitly specified up-front using these unique identifiers, to allow the chain to schedule transactions with non-overlapping inputs in parallel.

Addresses represent Object IDs​

In Move on Diem, there is a 16-byte address type used to represent account addresses in global storage. A 16 byte address is sufficient for the Move on Diem security model.

Sui doesn't have global storage, so address is re-purposed as a 32-byte identifier used for both objects and accounts. Each transaction is signed by an account (the "sender") that is accessible from the transaction context, and each object stores its address wrapped in its id: UID field. (Refer to object.move in the Sui framework for details).

Object with key ability, globally unique IDs​

In Move on Diem, the key ability indicates that the type is a resource, meaning it (along with an account address) can be used as a key in global storage.

On Sui, the key ability indicates that a struct is an object type and comes with an additional requirement that the first field of the struct has signature id: UID, to contain the object's unique address on-chain. Sui's bytecode verifier ensures that new objects are always assigned fresh UIDs (identifiers are never re-used).

Module initializers​

As described in Object-centric global storage, you publish Move modules into Sui storage. The Sui runtime executes a special initializer function you optionally define in a module only once at the time of module publication to pre-initialize module-specific data (for example, creating singleton objects).

Entry points take object references as input​

You can call public functions from Sui transactions (called programmable transaction blocks). These functions can take objects by value, by immutable reference, or by mutable reference. If taken by value, you can destroy the object, wrap it (in another object), or transfer it (to a Sui ID specified by an address). If taken by mutable reference, the modified version of the object saves to storage without any change in ownership. In any case, the Sui network authenticates the object and declares it's usage as a part of the transaction.

In addition to calling public functions, you can call a function that is marked entry even if it is private as long as other non-entry functions have not used its inputs.

Explore concepts​

Some of the features of Move are defined in this section using commented code examples.

Init​

The init function is a special function that executes only once - when you publish the associated module. See Init for details.

Entry functions​

The entry modifier for functions enables safe and direct invocation of module functions, much like scripts. See Entry for details.

Strings​

Move does not have a native type for strings, but it has a useful wrapper. See Strings for examples.

One-time witness​

A one-time witness (OTW) is a special instance of a type created in the module initializer and guaranteed to be unique with only one instance. See One-Time Witness for an example.