Blockchain Types

Sway is fundamentally a blockchain language, and it offers a selection of types tailored for the blockchain use case.

These are provided via the standard library (lib-std) which both add a degree of type-safety, as well as make the intention of the developer more clear.

Address Type

The Address type is a type-safe wrapper around the primitive b256 type. Unlike the EVM, an address never refers to a deployed smart contract (see the ContractId type below). An Address can be either the hash of a public key (effectively an externally owned account if you're coming from the EVM) or the hash of a predicate. Addresses own UTXOs.

An Address is implemented as follows.

pub struct Address {
    value: b256,
}

Casting between the b256 and Address types must be done explicitly:

let my_number: b256 = 0x000000000000000000000000000000000000000000000000000000000000002A;
let my_address: Address = Address::from(my_number);
let forty_two: b256 = my_address.into();

ContractId Type

The ContractId type is a type-safe wrapper around the primitive b256 type. A contract's ID is a unique, deterministic identifier analogous to a contract's address in the EVM. Contracts cannot own UTXOs but can own assets.

A ContractId is implemented as follows.

pub struct ContractId {
    value: b256,
}

Casting between the b256 and ContractId types must be done explicitly:

let my_number: b256 = 0x000000000000000000000000000000000000000000000000000000000000002A;
let my_contract_id: ContractId = ContractId::from(my_number);
let forty_two: b256 = my_contract_id.into();

Identity Type

The Identity type is an enum that allows for the handling of both Address and ContractId types. This is useful in cases where either type is accepted, e.g., receiving funds from an identified sender, but not caring if the sender is an address or a contract.

An Identity is implemented as follows.

pub enum Identity {
    Address: Address,
    ContractId: ContractId,
}

Casting to an Identity must be done explicitly:

        let raw_address: b256 = 0xddec0e7e6a9a4a4e3e57d08d080d71a299c628a46bc609aab4627695679421ca;
        let my_identity: Identity = Identity::Address(Address::from(raw_address));

A match statement can be used to return to an Address or ContractId as well as handle cases in which their execution differs.

        let my_contract_id: ContractId = match my_identity {
            Identity::ContractId(identity) => identity,
            _ => revert(0),
        };

        match my_identity {
            Identity::Address(address) => transfer_to_address(address, asset_id, amount),
            Identity::ContractId(contract_id) => force_transfer_to_contract(contract_id, asset_id, amount),
        };

A common use case for Identity is for access control. The use of Identity uniquely allows both ContractId and Address to have access control inclusively.

        let sender = msg_sender().unwrap();
        require(
            sender == storage
                .owner
                .read(),
            MyError::UnauthorizedUser(sender),
        );