Native Assets

The FuelVM has built-in support for working with multiple assets.

Key Differences Between EVM and FuelVM Assets

ERC-20 vs Native Asset

On the EVM, Ether is the native asset. As such, sending ETH to an address or contract is an operation built into the EVM, meaning it doesn't rely on the existence of a smart contract to update balances to track ownership as with ERC-20 tokens.

On the FuelVM, all assets are native and the process for sending any native asset is the same.

While you would still need a smart contract to handle the minting and burning of assets, the sending and receiving of these assets can be done independently of the asset contract.

Just like the EVM however, Fuel has a standard that describes a standard API for Native Assets using the Sway Language. The ERC-20 equivalent for the Sway Language is the SRC-20; Native Asset Standard.

NOTE It is important to note that Fuel does not have tokens.

ERC-721 vs Native Asset

On the EVM, an ERC-721 token or NFT is a contract that contains multiple tokens which are non-fungible with one another.

On the FuelVM, the ERC-721 equivalent is a Native Asset where each asset has a supply of one. This is defined in the SRC-20; Native Asset Standard under the Non-Fungible Asset Restrictions.

In practice, this means all NFTs are treated the same as any other Native Asset on Fuel. When writing Sway code, no additional cases for handling non-fungible and fungible assets are required.

No Token Approvals

An advantage Native Assets bring is that there is no need for token approvals; as with Ether on the EVM. With millions of dollars hacked every year due to misused token approvals, the FuelVM eliminates this attack vector.

Asset vs Coin vs Token

An "Asset" is a Native Asset on Fuel and has the associated AssetId type. Assets are distinguishable from one another. A "Coin" represents a singular unit of an Asset. Coins of the same Asset are not distinguishable from one another.

Fuel does not use tokens like other ecosystems such as Ethereum and uses Native Assets with a UTXO design instead.

The AssetId type

The AssetId type represents any Native Asset on Fuel. An AssetId is used for interacting with an asset on the network.

The AssetId of any Native Asset on Fuel is calculated by taking the SHA256 hash digest of the originating ContractId that minted the asset and a SubId i.e. sha256((contract_id, sub_id)).

Creating a New AssetId

There are 3 ways to instantiate a new AssetId:

Default

When a contract will only ever mint a single asset, it is recommended to use the DEFAULT_ASSET_ID sub id. This is referred to as the default asset of a contract.

To get the default asset from an internal contract call, call the default() function:

    let asset_id: AssetId = AssetId::default();

New

If a contract mints multiple assets or if the asset has been minted by an external contract, the new() function will be needed. The new() function takes the ContractId of the contract which minted the token as well as a SubId.

To create a new AssetId using a ContractId and SubId, call the new() function:

    let my_contract_id: ContractId = ContractId::from(0x1000000000000000000000000000000000000000000000000000000000000000);
    let my_sub_id: SubId = 0x2000000000000000000000000000000000000000000000000000000000000000;

    let asset_id: AssetId = AssetId::new(my_contract_id, my_sub_id);

From

In the case where the b256 value of an asset is already known, you may call the from() function with the b256 value.

    let asset_id: AssetId = AssetId::from(0x0000000000000000000000000000000000000000000000000000000000000000);

The SubId type

The SubId is used to differentiate between different assets that are created by the same contract. The SubId is a b256 value.

When creating a single new asset on Fuel, we recommend using the DEFAULT_SUB_ID or SubId::zero().

The Base Asset

On the Fuel Network, the base asset is Ether. This is the only asset on the Fuel Network that does not have a SubId.

The Base Asset can be returned anytime by calling the base() function of the AssetId type.

    let base_asset: AssetId = AssetId::base();

Basic Native Asset Functionality

Minting A Native Asset

To mint a new asset, the std::asset::mint() function must be called internally within a contract. A SubId and amount of coins must be provided. These newly minted coins will be owned by the contract which minted them. To mint another asset from the same contract, replace the DEFAULT_SUB_ID with your desired SubId.

        mint(DEFAULT_SUB_ID, mint_amount);

You may also mint an asset to a specific entity with the std::asset::mint_to() function. Be sure to provide a target Identity that will own the newly minted coins.

        mint_to(target_identity, DEFAULT_SUB_ID, mint_amount);

If you intend to allow external users to mint assets using your contract, the SRC-3; Mint and Burn Standard defines a standard API for minting assets. The Sway-Libs Asset Library also provides an additional library to support implementations of the SRC-3 Standard into your contract.

Burning a Native Asset

To burn an asset, the std::asset::burn() function must be called internally from the contract which minted them. The SubId used to mint the coins and amount must be provided. The burned coins must be owned by the contract. When an asset is burned it doesn't exist anymore.

        burn(DEFAULT_SUB_ID, burn_amount);

If you intend to allow external users to burn assets using your contract, the SRC-3; Mint and Burn Standard defines a standard API for burning assets. The Sway-Libs Asset Library also provides an additional library to support implementations of the SRC-3 Standard into your contract.

Transfer a Native Asset

To internally transfer a Native Asset, the std::asset::transfer() function must be called. A target Identity or user must be provided as well as the AssetId of the asset and an amount.

        transfer(target, asset_id, coins);

Native Asset And Transactions

Getting The Transaction Asset

To query for the Native Asset sent in a transaction, you may call the std::call_frames::msg_asset_id() function.

        let amount = msg_asset_id();

Getting The Transaction Amount

To query for the amount of coins sent in a transaction, you may call the std::context::msg_amount() function.

        let amount = msg_amount();

Native Assets and Contracts

Checking A Contract's Balance

To internally check a contract's balance, call the std::context::this_balance() function with the corresponding AssetId.

        this_balance(asset_id)

To check the balance of an external contract, call the std::context::balance_of() function with the corresponding AssetId.

        balance_of(target_contract, asset_id)

NOTE Due to the FuelVM's UTXO design, balances of Address's cannot be returned in the Sway Language. This must be done off-chain using the SDK.

Receiving Native Assets In A Contract

By default, a contract may not receive a Native Asset in a contract call. To allow transferring of assets to the contract, add the #[payable] attribute to the function.

    #[payable]
    fn deposit() {
        assert(msg_amount() > 0);
    }

Native Asset Standards

There are a number of standards developed to enable further functionality for Native Assets and help cross contract functionality. Information on standards can be found in the Sway Standards Repo.

We currently have the following standards for Native Assets:

Native Asset Libraries

Additional Libraries have been developed to allow you to quickly create an deploy dApps that follow the Sway Standards.

Single Native Asset Example

In this fully fleshed out example, we show a native asset contract which mints a single asset. This is the equivalent to the ERC-20 Standard use in Ethereum. Note there are no token approval functions.

It implements the SRC-20; Native Asset, SRC-3; Mint and Burn, and SRC-5; Ownership standards. It does not use any external libraries.

// ERC20 equivalent in Sway.
contract;

use standards::{
    src3::SRC3,
    src5::{
        SRC5, 
        State, 
        AccessError,
    },
    src20::{
        SetDecimalsEvent, 
        SetNameEvent, 
        SetSymbolEvent, 
        SRC20, 
        TotalSupplyEvent,
    },
};
use std::{
    asset::{
        burn,
        mint_to,
    },
    call_frames::msg_asset_id,
    constants::DEFAULT_SUB_ID,
    context::msg_amount,
    string::String,
};

configurable {
    DECIMALS: u8 = 9u8,
    NAME: str[7] = __to_str_array("MyAsset"),
    SYMBOL: str[5] = __to_str_array("MYTKN"),
}

storage {
    total_supply: u64 = 0,
    owner: State = State::Uninitialized,
}

// Native Asset Standard
impl SRC20 for Contract {
    #[storage(read)]
    fn total_assets() -> u64 {
        1
    }

    #[storage(read)]
    fn total_supply(asset: AssetId) -> Option<u64> {
        if asset == AssetId::default() {
            Some(storage.total_supply.read())
        } else {
            None
        }
    }

    #[storage(read)]
    fn name(asset: AssetId) -> Option<String> {
        if asset == AssetId::default() {
            Some(String::from_ascii_str(from_str_array(NAME)))
        } else {
            None
        }
    }

    #[storage(read)]
    fn symbol(asset: AssetId) -> Option<String> {
        if asset == AssetId::default() {
            Some(String::from_ascii_str(from_str_array(SYMBOL)))
        } else {
            None
        }
    }

    #[storage(read)]
    fn decimals(asset: AssetId) -> Option<u8> {
        if asset == AssetId::default() {
            Some(DECIMALS)
        } else {
            None
        }
    }
}

// Ownership Standard
impl SRC5 for Contract {
    #[storage(read)]
    fn owner() -> State {
        storage.owner.read()
    }
}

// Mint and Burn Standard
impl SRC3 for Contract {
    #[storage(read, write)]
    fn mint(recipient: Identity, sub_id: Option<SubId>, amount: u64) {
        require(sub_id.is_some() && sub_id.unwrap() == DEFAULT_SUB_ID, "incorrect-sub-id");
        require_access_owner();

        let new_supply = storage.total_supply.read() + amount;
        storage
            .total_supply
            .write(new_supply);
        mint_to(recipient, DEFAULT_SUB_ID, amount);
        
        TotalSupplyEvent::new(
            AssetId::default(), 
            new_supply, 
            msg_sender().unwrap()
        ).log();
    }

    #[storage(read, write)]
    fn burn(sub_id: SubId, amount: u64) {
        require(sub_id == DEFAULT_SUB_ID, "incorrect-sub-id");
        require(msg_amount() >= amount, "incorrect-amount-provided");
        require(
            msg_asset_id() == AssetId::default(),
            "incorrect-asset-provided",
        );
        require_access_owner();

        let new_supply = storage.total_supply.read() - amount;
        storage
            .total_supply
            .write(new_supply);
        burn(DEFAULT_SUB_ID, amount);
        
        TotalSupplyEvent::new(
            AssetId::default(), 
            new_supply, 
            msg_sender().unwrap()
        ).log();
    }
}

abi SingleAsset {
    #[storage(read, write)]
    fn constructor(owner_: Identity);
}

impl SingleAsset for Contract {
    #[storage(read, write)]
    fn constructor(owner_: Identity) {
        require(storage.owner.read() == State::Uninitialized, "owner-initialized");
        storage.owner.write(State::Initialized(owner_));
    }
}

#[storage(read)]
fn require_access_owner() {
    require(
        storage.owner.read() == State::Initialized(msg_sender().unwrap()),
        AccessError::NotOwner,
    );
}

abi EmitSRC20Events {
    fn emit_src20_events();
}

impl EmitSRC20Events for Contract {
    fn emit_src20_events() {
        // Metadata that is stored as a configurable should only be emitted once.
        let asset = AssetId::default();
        let sender = msg_sender().unwrap();
        let name = Some(String::from_ascii_str(from_str_array(NAME)));
        let symbol = Some(String::from_ascii_str(from_str_array(SYMBOL)));

        SetNameEvent::new(asset, name, sender).log();
        SetSymbolEvent::new(asset, symbol, sender).log();
        SetDecimalsEvent::new(asset, DECIMALS, sender).log();
    }
}

Multi Native Asset Example

In this fully fleshed out example, we show a native asset contract which mints multiple assets. This is the equivalent to the ERC-1155 Standard use in Ethereum. Note there are no token approval functions.

It implements the SRC-20; Native Asset, SRC-3; Mint and Burn, and SRC-5; Ownership standards. It does not use any external libraries.

// ERC1155 equivalent in Sway.
contract;

use standards::{
    src5::{
        SRC5, 
        State, 
        AccessError
    },
    src20::{
        SetDecimalsEvent, 
        SetNameEvent, 
        SetSymbolEvent, 
        SRC20, 
        TotalSupplyEvent,
    }
    src3::SRC3,
};
use std::{
    asset::{
        burn,
        mint_to,
    },
    call_frames::msg_asset_id,
    hash::{
        Hash,
    },
    context::this_balance,
    storage::storage_string::*,
    string::String
};

storage {
    total_assets: u64 = 0,
    total_supply: StorageMap<AssetId, u64> = StorageMap {},
    name: StorageMap<AssetId, StorageString> = StorageMap {},
    symbol: StorageMap<AssetId, StorageString> = StorageMap {},
    decimals: StorageMap<AssetId, u8> = StorageMap {},
    owner: State = State::Uninitialized,
}

// Native Asset Standard
impl SRC20 for Contract {
    #[storage(read)]
    fn total_assets() -> u64 {
        storage.total_assets.read()
    }

    #[storage(read)]
    fn total_supply(asset: AssetId) -> Option<u64> {
        storage.total_supply.get(asset).try_read()
    }

    #[storage(read)]
    fn name(asset: AssetId) -> Option<String> {
        storage.name.get(asset).read_slice()
    }
    
    #[storage(read)]
    fn symbol(asset: AssetId) -> Option<String> {
        storage.symbol.get(asset).read_slice()
    }

    #[storage(read)]
    fn decimals(asset: AssetId) -> Option<u8> {
        storage.decimals.get(asset).try_read()
    }
}

// Mint and Burn Standard
impl SRC3 for Contract {
    #[storage(read, write)]
    fn mint(recipient: Identity, sub_id: Option<SubId>, amount: u64) {
        require(sub_id.is_some(), "Error: SubId is None");
        require_access_owner();

        let asset_id = AssetId::new(ContractId::this(), sub_id.unwrap());
        let supply = storage.total_supply.get(asset_id).try_read();
        if supply.is_none() {
            storage.total_assets.write(storage.total_assets.try_read().unwrap_or(0) + 1);
        }
        let new_supply = supply.unwrap_or(0) + amount;
        storage.total_supply.insert(asset_id, new_supply);
        mint_to(recipient, sub_id, amount);
        
        TotalSupplyEvent::new(
            asset_id, 
            new_supply, 
            msg_sender().unwrap()
        ).log();
    }
    
    #[storage(read, write)]
    fn burn(sub_id: SubId, amount: u64) {
        require_access_owner();
        let asset_id = AssetId::new(ContractId::this(), sub_id);
        require(this_balance(asset_id) >= amount, "not-enough-coins");
        
        let supply = storage.total_supply.get(asset_id).try_read();
        let new_supply = supply.unwrap_or(0) - amount;
        storage.total_supply.insert(asset_id, new_supply);
        burn(sub_id, amount);

        TotalSupplyEvent::new(
            asset_id, 
            new_supply, 
            msg_sender().unwrap()
        ).log();
    }
}

abi MultiAsset {
    #[storage(read, write)]
    fn constructor(owner_: Identity);
    
    #[storage(read, write)]
    fn set_name(asset: AssetId, name: Option<String>);

    #[storage(read, write)]
    fn set_symbol(asset: AssetId, symbol: Option<String>);

    #[storage(read, write)]
    fn set_decimals(asset: AssetId, decimals: u8);
}

impl MultiAsset for Contract {
    #[storage(read, write)]
    fn constructor(owner_: Identity) {
        require(storage.owner.read() == State::Uninitialized, "owner-initialized");
        storage.owner.write(State::Initialized(owner_));
    }
    
    #[storage(read, write)]
    fn set_name(asset: AssetId, name: Option<String>) {
        require_access_owner();
        storage.name.insert(asset, StorageString {});
        storage.name.get(asset).write_slice(name);

        SetNameEvent::new(asset, name, msg_sender().unwrap()).log();
    }

    #[storage(read, write)]
    fn set_symbol(asset: AssetId, symbol: Option<String>) {
        require_access_owner();
        storage.symbol.insert(asset, StorageString {});
        storage.symbol.get(asset).write_slice(symbol);

        SetSymbolEvent::new(asset, symbol, msg_sender().unwrap()).log();
    }

    #[storage(read, write)]
    fn set_decimals(asset: AssetId, decimals: u8) {
        require_access_owner();
        storage.decimals.insert(asset, decimals);

        SetDecimalsEvent::new(asset, decimals, msg_sender().unwrap()).log();
    }
}

#[storage(read)]
fn require_access_owner() {
    require(
        storage.owner.read() == State::Initialized(msg_sender().unwrap()),
        AccessError::NotOwner,
    );
}