Contracts

A contract in Cadence is a collection of type definitions of interfaces, structs, resources, data (its state), and code (its functions) that lives in the contract storage area of an account in Flow.

Contracts are where all composite types like structs, resources, events, and interfaces for these types in Cadence have to be defined. Therefore, an object of one of these types cannot exist without having been defined in a deployed Cadence contract.

Contracts can be created, updated, and removed using the contracts object of authorized accounts. This functionality is covered in the next section

Contracts are types. They are similar to composite types, but are stored differently than structs or resources and cannot be used as values, copied, or moved like resources or structs.

Contracts stay in an account's contract storage area and can only be added, updated, or removed by the account owner with special commands.

Contracts are declared using the contract keyword. The keyword is followed by the name of the contract.

pub contract SomeContract {
    // ...
}

Contracts cannot be nested in each other.

pub contract Invalid {

    // Invalid: Contracts cannot be nested in any other type.
    //
    pub contract Nested {
        // ...
    }
}

One of the simplest forms of a contract would just be one with a state field, a function, and an init function that initializes the field:

pub contract HelloWorld {

    // Declare a stored state field in HelloWorld
    //
    pub let greeting: String

    // Declare a function that can be called by anyone
    // who imports the contract
    //
    pub fun hello(): String {
        return self.greeting
    }

    init() {
        self.greeting = "Hello World!"
    }
}

This contract could be deployed to an account and live permanently in the contract storage. Transactions and other contracts can interact with contracts by importing them at the beginning of a transaction or contract definition.

Anyone could call the above contract's hello function by importing the contract from the account it was deployed to and using the imported object to call the hello function.

import HelloWorld from 0x42

// Invalid: The contract does not know where hello comes from
//
log(hello())        // Error

// Valid: Using the imported contract object to call the hello
// function
//
log(HelloWorld.hello())    // prints "Hello World!"

// Valid: Using the imported contract object to read the greeting
// field.
log(HelloWorld.greeting)   // prints "Hello World!"

// Invalid: Cannot call the init function after the contract has been created.
//
HelloWorld.init()    // Error

There can be any number of contracts per account and they can include an arbitrary amount of data. This means that a contract can have any number of fields, functions, and type definitions, but they have to be in the contract and not another top-level definition.

// Invalid: Top-level declarations are restricted to only be contracts
//          or contract interfaces. Therefore, all of these would be invalid
//          if they were deployed to the account contract storage and
//          the deployment would be rejected.
//
pub resource Vault {}
pub struct Hat {}
pub fun helloWorld(): String {}
let num: Int

Another important feature of contracts is that instances of resources and events that are declared in contracts can only be created/emitted within functions or types that are declared in the same contract.

It is not possible create instances of resources and events outside the contract.

The contract below defines a resource interface Receiver and a resource Vault that implements that interface. The way this example is written, there is no way to create this resource, so it would not be usable.

// Valid
pub contract FungibleToken {

    pub resource interface Receiver {

        pub balance: Int

        pub fun deposit(from: @{Receiver}) {
            pre {
                from.balance > 0:
                    "Deposit balance needs to be positive!"
            }
            post {
                self.balance == before(self.balance) + before(from.balance):
                    "Incorrect amount removed"
            }
        }
    }

    pub resource Vault: Receiver {

        // keeps track of the total balance of the accounts tokens
        pub var balance: Int

        init(balance: Int) {
            self.balance = balance
        }

        // withdraw subtracts amount from the vaults balance and
        // returns a vault object with the subtracted balance
        pub fun withdraw(amount: Int): @Vault {
            self.balance = self.balance - amount
            return <-create Vault(balance: amount)
        }

        // deposit takes a vault object as a parameter and adds
        // its balance to the balance of the Account's vault, then
        // destroys the sent vault because its balance has been consumed
        pub fun deposit(from: @{Receiver}) {
            self.balance = self.balance + from.balance
            destroy from
        }
    }
}

If a user tried to run a transaction that created an instance of the Vault type, the type checker would not allow it because only code in the FungibleToken contract can create new Vaults.

import FungibleToken from 0x42

// Invalid: Cannot create an instance of the `Vault` type outside
// of the contract that defines `Vault`
//
let newVault <- create FungibleToken.Vault(balance: 10)

The contract would have to either define a function that creates new Vault instances or use its init function to create an instance and store it in the owner's account storage.

This brings up another key feature of contracts in Cadence. Contracts can interact with its account's storage and published objects to store resources, structs, and references. They do so by using the special self.account object that is only accessible within the contract.

Imagine that these were declared in the above FungibleToken contract.

    pub fun createVault(initialBalance: Int): @Vault {
        return <-create Vault(balance: initialBalance)
    }

    init(balance: Int) {
        let vault <- create Vault(balance: 1000)
        self.account.save(<-vault, to: /storage/initialVault)
    }

Now, any account could call the createVault function declared in the contract to create a Vault object. Or the owner could call the withdraw function on their own Vault to send new vaults to others.

import FungibleToken from 0x42

// Valid: Create an instance of the `Vault` type by calling the contract's
// `createVault` function.
//
let newVault <- create FungibleToken.createVault(initialBalance: 10)

Account access

Contracts have the implicit field let account: AuthAccount, which is the account in which the contract is deployed too. This gives the contract the ability to e.g. read and write to the account's storage.

Deploying, Updating, and Removing Contracts

In order for a contract to be used in Cadence, it needs to be deployed to an account. The deployed contracts of an account can be accessed through the contracts object.

Deployed Contracts

Accounts store "deployed contracts", that is, the code of the contract:

struct DeployedContract {
    let name: String
    let code: [UInt8]
}

Note that this is not the contract instance that can be acquired by importing it.

Deploying a New Contract

A new contract can be deployed to an account using the add function:

fun add(
    name: String,
    code: [UInt8],
    ... contractInitializerArguments
): DeployedContract

Adds the given contract to the account.

The code parameter is the UTF-8 encoded representation of the source code. The code must contain exactly one contract or contract interface, which must have the same name as the name parameter.

All additional arguments that are given are passed further to the initializer of the contract that is being deployed.

Fails if a contract/contract interface with the given name already exists in the account, if the given code does not declare exactly one contract or contract interface, or if the given name does not match the name of the contract/contract interface declaration in the code.

Returns the deployed contract.

For example, assuming the following contract code should be deployed:

pub contract Test {
    pub let message: String

    init(message: String) {
        self.message = message
    }
}

The contract can be deployed as follows:

// Decode the hex-encoded source code into a byte array
// using the built-in function `decodeHex`.
//
// (The ellipsis ... indicates the remainder of the string)
//
let code = "70756220636f6e...".decodeHex()

// `code` has type `[UInt8]`

let signer: AuthAccount = ...
signer.contracts.add(
    name: "Test",
    code: code,
    message: "I'm a new contract in an existing account"
)

Updating a Deployed Contract

🚧 Status: Updating contracts is experimental.

Updating contracts is currently limited to maintain data consistency. Read more details on valid changes and restrictions imposed on updating contracts in the contract updatability section.

A deployed contract can be updated using the update__experimental function:

fun update__experimental(name: String, code: [UInt8]): DeployedContract

Updates the code for the contract/contract interface in the account.

The code parameter is the UTF-8 encoded representation of the source code. The code must contain exactly one contract or contract interface, which must have the same name as the name parameter.

Does not run the initializer of the contract/contract interface again. The contract instance in the world state stays as is.

Fails if no contract/contract interface with the given name exists in the account, if the given code does not declare exactly one contract or contract interface, or if the given name does not match the name of the contract/contract interface declaration in the code.

Returns the deployed contract for the updated contract.

For example, assuming that a contract named Test is already deployed to the account and it should be updated with the following contract code:

pub contract Test {
    pub let message: String

    init(message: String) {
        self.message = message
    }
}

The contract can be updated as follows:

// Decode the hex-encoded source code into a byte array
// using the built-in function `decodeHex`.
//
// (The ellipsis ... indicates the remainder of the string)
//
let code = "70756220636f6e...".decodeHex()

// `code` has type `[UInt8]`

let signer: AuthAccount = ...
signer.contracts.update__experimental(name: "Test", code: code)

Getting a Deployed Contract

A deployed contract can be get from an account using the get function:

fun get(name: String): DeployedContract?

Returns the deployed contract for the contract/contract interface with the given name in the account, if any.

Returns nil if no contract/contract interface with the given name exists in the account.

For example, assuming that a contract named Test is deployed to an account, the contract can be retrieved as follows:

let signer: AuthAccount = ...
let contract = signer.contracts.get(name: "Test")

Removing a Deployed Contract

A deployed contract can be removed from an account using the remove function:

fun remove(name: String): DeployedContract?

Removes the contract/contract interface from the account which has the given name, if any.

Returns the removed deployed contract, if any.

Returns nil if no contract/contract interface with the given name exist in the account.

For example, assuming that a contract named Test is deployed to an account, the contract can be removed as follows:

let signer: AuthAccount = ...
let contract = signer.contracts.remove(name: "Test")

Contract Interfaces

Like composite types, contracts can have interfaces that specify rules about their behavior, their types, and the behavior of their types.

Contract interfaces have to be declared globally. Declarations cannot be nested in other types.

If a contract interface declares a concrete type, implementations of it must also declare the same concrete type conforming to the type requirement.

If a contract interface declares an interface type, the implementing contract does not have to also define that interface. They can refer to that nested interface by saying {ContractInterfaceName}.{NestedInterfaceName}

// Declare a contract interface that declares an interface and a resource
// that needs to implement that interface in the contract implementation.
//
pub contract interface InterfaceExample {

    // Implementations do not need to declare this
    // They refer to it as InterfaceExample.NestedInterface
    //
    pub resource interface NestedInterface {}

    // Implementations must declare this type
    //
    pub resource Composite: NestedInterface {}
}

pub contract ExampleContract: InterfaceExample {

    // The contract doesn't need to redeclare the `NestedInterface` interface
    // because it is already declared in the contract interface

    // The resource has to refer to the resource interface using the name
    // of the contract interface to access it
    //
    pub resource Composite: InterfaceExample.NestedInterface {
    }
}