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Move Concepts

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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 assets and smart contracts. Sui's object system is implemented by adding new functionality to

Move while also imposing additional restrictions.

Move on Sui contains some important differences from

Move on other blockchains. Sui takes advantage of

Move 's security and flexibility, enhancing it with the features to vastly improve throughput, reduce delays in

finality , and make

Move programming more approachable. For full details, see the Sui Smart Contracts Platform whitepaper.

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.

Key differences ​

Key differences of

Move on Sui include:

Sui uses its own object-centric global storage

Addresses represent object IDs

Sui objects have globally unique IDs

Sui has module initializers ( init )

Sui has unique use cases for the

entry keyword

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 of a

transaction 's inputs are explicitly specified up front using these unique identifiers to allow the network 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 repurposed 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.

See the address section in The

Move Book for an overview on

addresses and refer to object . move in the Sui framework for implementation 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 UID s. Identifiers are never reused.

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. See the module initializer section in The

Move Book for more information.

Entry functions ​

The entry keyword has a specific use case in

Move on Sui. Use entry when you want some functionality to be usable by anyone on chain, but not be wrapped around other

Move logic. In other words, a function can be called in programmable transaction blocks (PTBs) but not in other Sui

packages . For example, this is utilized when using Sui's on-chain randomness standard to prevent other smart contract engineers from creating logic to essentially front-run or back-run the randomness generation. Find more information about this in the on-chain randomness page .

Make your function private (don't add the public visibility keyword) and mark it with the entry keyword, as in entry fun example_function .

In addition to this Sui-specific use case, there are other rules and restrictions for entry functions:

entry functions can only return types with the drop ability.

entry functions can only take objects as inputs if they weren't used as inputs in any non- entry functions in the same

PTB .

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Key differences

Object-centric global storage

Addresses represent object IDs

Object with key ability, globally unique IDs

Module initializers

Entry functions

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