EIP-5115: Super Composable Yield Token

Interface for wrapped yield-generating tokens.

摘要

This standard proposes an API for wrapped yield-generating tokens within smart contracts. It is an extension on the EIP-20 token that provides basic functionality for transferring, depositing, withdrawing tokens, as well as reading balances.

动机

Yield generating mechanisms are built in all shapes and sizes, necessitating a manual integration every time a protocol builds on top of another protocol’s yield generating mechanism.

EIP-4626 tackled a significant part of this fragmentation by standardizing the interfaces for vaults, a major category among various yield-generating mechanisms.

In this EIP, we’re extending the coverage to include assets beyond EIP-4626’s reach, namely:

• Yield-generating assets that have different base tokens used for minting vs accounting for the pool value.
  • This category includes AMM liquidity tokens (which are yield generating assets that yield swap fees) since the value of the pool is measured in “liquidity units” (for example, $\sqrt k$ in UniswapV2, as defined in UniswapV2 whitepaper) which can’t be deposited in (as they are not tokens).
  • This extends the flexibility in minting the yield-bearing assets. For example, there could be an ETH vault that wants to allow users to deposit cETH directly instead of ETH, for gas efficiency or UX reasons.
• Assets with reward tokens by default (e.g. COMP rewards for supplying in Compound). The reward tokens are expected to be sold to compound into the same asset.

While EIP-4626 is a well-designed and suitable standard for most vaults, there will inevitably be some yield-generating mechanisms that do not fit into their category (LP tokens for instance). A more flexible standard is required to standardize the interaction with all types of yield generating mechanisms.

Therefore, we are proposing Super Composable Yield (SCY), a flexible standard for wrapped yield generating tokens that could cover most mechanisms in DeFi. We foresee that:

• EIP-4626 will still be a popular vault standard, that most vaults should adopt.
• SCY tokens can wrap over most yield generating mechanisms in DeFi, including EIP-4626 vaults for projects built on top of interest-bearing tokens.
• Whoever needs the functionalities of SCY could integrate with the existing SCY tokens or write a new SCY (to wrap over the target interest-bearing token).

Use Cases

This EIP is designed for flexibility, aiming to accommodate as many yield generating mechanisms as possible. Particularly, this standard aims to be generalized enough that it supports the following uses cases and more:

• Money market supply positions
  • Lending DAI in Compound, getting DAI interests and COMP rewards
  • Lending ETH in BenQi, getting ETH interests and QI + AVAX rewards
  • Lending USDC in Aave, getting USDC interests and stkAAVE rewards
• AMM liquidity provision
  • Provide ETH + USDC to ETHUSDC pool in SushiSwap, getting swap fees in more ETH+USDC
  • Provide ETH + USDC to ETHUSDC pool in SushiSwap and stake it in Sushi Onsen, getting swap fees and SUSHI rewards
  • Provide USDC+DAI+USDT to 3crv pool and stake it in Convex, getting 3crv swap fees and CRV + CVX rewards
• Vault positions
  • Provide ETH into Yearn EIP-4626 vault, where the vault accrues yield from Yearn’s ETH strategy
  • Provide DAI into Harvest and staking it, getting DAI interests and FARM rewards
• Liquid staking positions
  • Holding stETH (in Lido), getting yields in more stETH
• Liquidity mining programs
  • Provide USDC in Stargate, getting STG rewards
  • Provide LOOKS in LooksRare, getting LOOKS yield and WETH rewards
• Rebasing tokens
  • Stake OHM into sOHM/gOHM, getting OHM rebase yield
  • Stake BTRFLY into xBTRFLY, getting BTRFLY rebase yield

The EIP hopes to minimize, if not possibly eliminate, the use of customized adapters in order to interact with many different forms of yield-generating token mechanisms.

规范

Generic Yield Generating Pool

We will first introduce Generic Yield Generating Pool (GYGP), a model to describe most yield generating mechanisms in DeFi. In every yield generating mechanism, there is a pool of funds, whose value is measured in assets. There are a number of users who contribute liquidity to the pool, in exchange for shares of the pool, which represents units of ownership of the pool. Over time, the value (measured in assets) of the pool grows, such that each share is worth more assets over time. The pool could earn a number of reward tokens over time, which are distributed to the users according to some logic (For example, proportionally the number of shares).

Here are the more concrete definitions of the terms:

Definitions:

• asset: Is a unit to measure the value of the pool. At time t, the pool has a total value of TotalAsset(t) assets.
• shares: Is a unit that represents ownership of the pool. At time t, there are TotalShares(t) shares in total.
• reward tokens: Over time, the pool earns $n_{rewards}$ types of reward tokens $(n_{rewards} \ge 0)$. At time t, $TotalRewards_i(t)$ is the amount of reward token **i** that has accumulated for the pool up until time t.
• exchange rate: At time t, the exchange rate ExchangeRate(t) is simply how many assets each shares is worth $ExchangeRate(t) = \frac{TotalAsset(t)}{TotalShares(t)}$
• users: At time t, each user u has $shares_u(t)$ shares in the pool, which is worth $asset_u(t) = shares_u(t) \cdot ExchangeRate(t)$ assets. Until time t, user u is entitled to receive a total of $rewards_{u_i}(t)$ reward token **i**. The sum of all users’ shares, assets and rewards should be the same as the total shares, assets and rewards of the whole pool.

State changes:

1. A user deposits assets into the pool, in exchange for new shares that will be created for the user, proportionally to the asset amount being deposited compared to the value of the pool.
2. A user withdraws assets from the pool, by burning their shares, proportionally to the asset amount being burned compared to the value of the pool
3. The pool earns some assets. The exchange rate will simply increase due to the additional assets.
4. The pool earns some reward tokens. The additional reward tokens will be distributed among the users.

Examples of GYGPs in DeFi:

Super Composable Yield Token Standard

Overview

Super Composable Yield is a token standard for all GYGPs. Each Super Composable Yield token represents shares in a GYGP and allows for interacting with the GYGP via a standard interface

All SCY tokens:

• MUST implement EIP-20 to represent shares in the underlying GYGP.
• MUST implement EIP-20’s optional metadata extensions name, symbol, and decimals, which SHOULD reflect the underlying GYGP’s accounting asset’s name, symbol, and decimals.
• MAY implement EIP-2612 to improve the UX of approving SCY tokens on various integrations.
• MAY revert on calls to transfer and transferFrom if a SCY token is to be non-transferable.
• The EIP-20 operations balanceOf, transfer, totalSupply, etc. SHOULD operate on the GYGP “shares”, which represent a claim to ownership on a fraction of the GYGP’s underlying holdings.

Definition of base tokens

Base tokens are tokens that could be deposited to mint SCY tokens (and hence, to enter the underlying GYGP), or redeemed when burning SCY tokens (and hence, exiting the underlying GYGP). Essentially, base tokens are implicitly converted into units of assets when deposited or redeemed, to conform to state change #1 and #2 of the GYGP definition above.

There could be multiple kinds of base tokens in a SCY. This allows for maximum flexibility in how to mint SCY to enter the underlying yield generating pool. For example, both BAL and WETH (and even BALWETH LP token) could be used to mint the SCY token for BALWETH Balancer pool (by providing single-sided liquidity).

As such, base tokens are not necessarily the same as the asset (which is a key difference between SCY and EIP-4626).

Interface

interface ISuperComposableYield {
    enum AssetType {
        TOKEN,
        LIQUIDITY
    }

    event Deposit(
        address indexed caller,
        address indexed receiver,
        address indexed tokenIn,
        uint256 amountDeposited,
        uint256 amountScyOut
    );

    event Redeem(
        address indexed caller,
        address indexed receiver,
        address indexed tokenOut,
        uint256 amountScyToRedeem,
        uint256 amountTokenOut
    );

    event ClaimRewards(
        address indexed caller,
        address indexed user,
        address[] rewardTokens,
        uint256[] rewardAmounts
    );

    event ExchangeRateUpdated(uint256 oldExchangeRate, uint256 newExchangeRate);

    function deposit(
        address receiver,
        address tokenIn,
        uint256 amountTokenToPull,
        uint256 minSharesOut
    ) external returns (uint256 amountSharesOut);

    function redeem(
        address receiver,
        uint256 amountSharesToPull,
        address tokenOut,
        uint256 minTokenOut
    ) external returns (uint256 amountTokenOut);

    function claimRewards(address user) external returns (uint256[] memory rewardAmounts);

    function exchangeRateCurrent() external returns (uint256);

    function exchangeRateStored() external view returns (uint256);

    function getRewardTokens() external view returns (address[] memory);

    function getBaseTokens() external view returns (address[] memory);

    function yieldToken() external view returns (address);

    function isValidBaseToken(address token) external view returns (bool);

    function assetInfo()
    external
    view
    returns (
        AssetType assetType,
        address assetAddress,
        uint8 assetDecimals
    );

    function name() external view returns (string memory);

    function symbol() external view returns (string memory);

    function decimals() external view returns (uint8);
}

Methods

function deposit(
    address receiver,
    address tokenIn,
    uint256 amountTokenToPull,
    uint256 minSharesOut
) external returns (uint256 amountSharesOut);

This method will first pull amountTokenToPull of tokenIn (a base token), and use the floating amount tokenIn in the SCY contract to deposit to mint new shares.

The ideal way to deposit is to send tokenIn in first, then call the deposit function with amountTokenToPull = 0. This pattern is similar to UniswapV2 (and UniswapV3) pools, which allow for better composability and gas efficiency by minimizing token transfers. For example, a router contract could swap from some other token to tokenIn which is sent directly to the SCY contract before deposit is called.

This function will convert the amount of tokenIn into some worth of assets and deposit this amount into the SCY contract for the recipient, who will receive amountSharesOut of SCY tokens (shares).

• MUST emit the Deposit event.
• MUST support EIP-20’s approve / transferFrom flow where tokenIn are taken from receiver directly (as msg.sender) or if the msg.sender has EIP-20 approved allowance over the base token of the receiver.
• MUST revert if $amountSharesOut \lt minSharesOut$ (due to deposit limit being reached, slippage, or the user not approving enough tokenIn **to the SCY contract, etc).

function redeem(
    address receiver,
    uint256 amountSharesToPull,
    address tokenOut,
    uint256 minTokenOut
) external returns (uint256 amountTokenOut);

This method will first pull amountSharesToPull of SCY tokens, and use the floating amount of SCY tokens in the SCY contract to redeem to tokenOut (a base token). This pattern is similar to the one in deposit

This function will redeem the exact SCY token (shares) from the SCY contract. The assets are converted into tokenOut of tokenOut.

• MUST emit the Redeem event.
• MUST support EIP-20’s approve / transferFrom flow where the shares are burned from receiver directly (as msg.sender) or if the msg.sender has EIP-20 approved allowance over the shares of the receiver.
• MUST revert if $tokenOut \lt minTokenOut$ (due to redeem limit being reached, slippage, or the user not approving enough amountSharesToPull **to the SCY contract, etc).

function claimRewards(address user) external returns (uint256[] memory rewardAmounts);

This method sends all the available claimable rewards to the user as is, with the amounts in the list being in the same order as getRewardTokens.

• MUST emit the ClaimRewards event.
• MAY return one or multiple rewards to the user.
• MAY return zero rewards to the user.

function exchangeRateCurrent() external returns (uint256);

This method updates and returns the latest exchange rate, which is the exchange rate from SCY token amount into asset amount, scaled by a fixed scaling factor of 1e18.

• MUST return $ExchangeRate(t_{now})$ such that $ExchangeRate(t_{now}) \times scyBalance / 1e18 = assetBalance$.
• MUST NOT include fees that are charged against the underlying yield token in the SCY contract.

function exchangeRateStored() external view returns (uint256);

This read-only method returns the last saved value of the exchange rate.

• MUST return the value of exchangeRateCurrent of a past timestamp where it was last updated in the contract
• MUST NOT include fees that are charged against the underlying yield token in the SCY contract.
• MUST NOT revert.

function yieldToken() external view returns (address);

This read-only method returns the underlying yield-generating token (representing a GYGP) that was wrapped into a SCY token.

• MUST return a token address that conforms to the EIP-20 interface, or zero address
• MUST NOT revert.
• MUST reflect the exact underlying yield-generating token address if the SCY token is a wrapped token.
• MAY return 0x or zero address if the SCY token is natively implemented, and not from wrapping.

function getRewardTokens() external view returns (address[] memory);

This read-only method returns the latest list of all reward tokens.

• MUST return EIP-20 token addresses.
• MUST NOT revert.
• MAY return an empty list, one, or several token addresses.
• MAY return additional reward tokens over time, depending on when underlyingYieldToken supports more or less reward tokens.

function getBaseTokens() external view returns (address[] memory);

This read-only method returns the list of all base tokens that can be used to deposit into the SCY contract.

• MUST return EIP-20 token addresses.
• MUST return at least one address.
• MUST NOT revert.

function isValidBaseToken(address token) external view returns (bool);

This read-only method checks whether a token address entered is a base token that can be used to mint SCY.

• MUST NOT revert.

function assetInfo()
    external
    view
    returns (
        AssetType assetType,
        address assetAddress,
        uint8 assetDecimals
    );

This read-only function returns useful information about the asset, intended for front-ends or off-chain systems to display balances and information about the asset.

decimals is the decimals to format asset balances.

Convention for assetType and format of the info field: 1) If asset is an EIP-20 token, assetType = 0, assetAddress is the address of the token; 2) If asset is liquidity of an AMM (like $\sqrt{k}$ in UniswapV2 forks), assetType = 1, assetAddress is the address of the LP token.

• MUST reflect the underlying asset’s decimals if at all possible in order to eliminate any possible source of confusion or be deemed malicious.
• MUST conform to the conventions for assetType and info.
• MUST NOT revert.

Events

event Deposit(
    address indexed caller,
    address indexed receiver,
    address indexed tokenIn,
    uint256 amountDeposited,
    uint256 amountScyOut
);

caller has converted exact base tokens into SCY (shares) and transferred those SCY to receiver.

• MUST be emitted when base tokens are deposited into the SCY contract via deposit method.

event Redeem(
    address indexed caller,
    address indexed receiver,
    address indexed tokenOut,
    uint256 amountScyToRedeem,
    uint256 amountTokenOut
);

caller has converted exact SCY (shares) into base tokens and transferred those base tokens to receiver.

• MUST be emitted when base tokens are redeemed from the SCY contract via redeem method.

event ClaimRewards(
    address indexed caller,
    address indexed user,
    address[] rewardTokens,
    uint256[] rewardAmounts
);

caller has claimed user rewards and transferred them to the user.

• MUST be emitted when rewards are claimed from the SCY contract via claimRewards method.

event ExchangeRateUpdated(uint256 oldExchangeRate, uint256 newExchangeRate);

The exchangeRateCurrent is updated to the latest exchange rate.

• MUST be emitted when the exchange rate is updated in the SCY contract via exchangeRateCurrent method.

“SCY” Word Choice:

“SCY” (pronunciation: /sʌɪ/), an abbreviation of Super Composable Yield, was found to be appropriate to describe a broad universe of composable yield-bearing digital assets.

基本原理

EIP-20 is enforced because implementation details such as transfer, token approvals, and balance calculation directly carry over to the SCY tokens. This standardization makes the SCY tokens immediately compatible with all EIP-20 use cases.

EIP-165 is not explicitly mentioned to be supported as there are no optional methods in this standard. It is expected for all methods defined in the interface to be implemented.

EIP-2612 can optionally be implemented in order to improve the UX of approving SCY tokens on various integrations.

The exchangeRateStored read-only method serves as a rough estimate of the prevalent exchange rate since the last update. It is included for frontends, wallets, and applications that need an estimate on the exchange rate of SCY tokens into assets, not an exact value possibly including slippage or other fees as this would require them doing a state update and spending gas. For applications that need an exact exchange rate, the exchangeRateCurrent mutable function can be used.

向后兼容性

This EIP is fully backwards compatible as its implementation extends the functionality of EIP-20, however the optional metadata extensions, namely name, decimals, and symbol semantics MUST be implemented for all SCY token implementations.

Security Considerations

Malicious implementations which conform to the interface can put users at risk. It is recommended that all integrators (such as wallets, aggregators, or other smart contract protocols) review the implementation to avoid possible exploits and users losing funds.

The method exchangeRateStored returns an outdated estimated value and does not confer the exact current exchange rate of asset per share. Should accuracy be needed, exchangeRateCurrent should be used instead (which additionally updates exchangeRateStored.)

decimals in assetInfo must strongly reflect the underlying asset’s decimals if at all possible in order to eliminate any possible source of confusion or be deemed malicious.

yieldToken must strongly reflect the address of the underlying wrapped yield-generating token. For a native implementation wherein the SCY token does not wrap a yield-generating token, but natively represents a GYGP share, then the address returned MAY be a zero address. Otherwise, for wrapped tokens, you may introduce confusion on what the SCY token represents, or may be deemed malicious.

版权声明

Copyright and related rights waived via CC0.

参考文献

Please cite this document as:

Vu Nguyen, Long Vuong, Anton Buenavista, "EIP-5115: Super Composable Yield Token [DRAFT]," Ethereum Improvement Proposals, no. 5115, May 2022. [Online serial]. Available: https://eips.ethereum.org/EIPS/eip-5115.