pTokens

Introduction

Each asset supported by the Parabox Protocol is integrated through a pToken contract, which is an EIP-20 compliant representation of balances supplied to the protocol. By minting pTokens, users (1) earn interest through the pToken's exchange rate, which increases in value relative to the underlying asset, and (2) gain the ability to use pTokens as collateral.

pTokens are the primary means of interacting with the Parabox Protocol; when a user mints, redeems, borrows, repays a borrow, liquidates a borrow, or transfers pTokens, she will do so using the pToken contract.

There are currently two types of pTokens: PErc20 and PEther. Though both types expose the EIP-20 interface, PErc20 wraps an underlying ERC-20 asset, while PEther simply wraps Ether itself. As such, the core functions which involve transferring an asset into the protocol have slightly different interfaces depending on the type, each of which is shown below.

Mint

The mint function transfers an asset into the protocol, which begins accumulating interest based on the current Supply Rate for the asset. The user receives a quantity of pTokens equal to the underlying tokens supplied, divided by the current Exchange Rate.

PErc20

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function mint(uint mintAmount) returns (uint)

• msg.sender: The account which shall supply the asset, and own the minted pTokens.

• mintAmount: The amount of the asset to be supplied, in units of the underlying asset.

• RETURN: 0 on success, otherwise an Error code Before supplying an asset, users must first approve the pToken to access their token balance.

PEther

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function mint() payable

• msg.value: The amount of ether to be supplied, in wei.

• msg.sender: The account which shall supply the ether, and own the minted pTokens.

• RETURN: No return, reverts on error.

Solidity

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Erc20 underlying = Erc20(0xToken...);     // get a handle for the underlying asset contract
PErc20 pToken = PErc20(0x3FDA...);        // get a handle for the corresponding pToken contract
underlying.approve(address(pToken), 100); // approve the transfer
assert(pToken.mint(100) == 0);            // mint the pTokens and assert there is no error

Web3 1.0

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const pToken = PEther.at(0x3FDB...);
await pToken.methods.mint().send({from: myAccount, value: 50});

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First MINIMUM_LIQUIDITY minted pTokens gets locked on address(0) to prevent totalSupply being 0, this prevents a critical hack reported by other compound forks. 
A minimal loss of 0.0001 pTokens, for example starting exchange rate of pWBTC is 1 WBTC = 50 pWBTC, loss in WBTC is 0.000002, just to give an idea its 0.2 USD if BTC is priced $100,000/BTC.

Redeem

The redeem function converts a specified quantity of pTokens into the underlying asset, and returns them to the user. The amount of underlying tokens received is equal to the quantity of pTokens redeemed, multiplied by the current Exchange Rate. The amount redeemed must be less than the user's Account Liquidity and the market's available liquidity.

PErc20 / PEther

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function redeem(uint redeemTokens) returns (uint)

• msg.sender: The account to which redeemed funds shall be transferred.

• redeemTokens: The number of pTokens to be redeemed.

• RETURN: 0 on success, otherwise an Error code

Solidity

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PEther pToken = PEther(0x3FDB...);
require(pToken.redeem(7) == 0, "something went wrong");

Web3 1.0

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const pToken = PErc20.at(0x3FDA...);
pToken.methods.redeem(1).send({from: ...});

Redeem Underlying

The redeem underlying function converts pTokens into a specified quantity of the underlying asset, and returns them to the user. The amount of pTokens redeemed is equal to the quantity of underlying tokens received, divided by the current Exchange Rate. The amount redeemed must be less than the user's Account Liquidity and the market's available liquidity.

PErc20 / PEther

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function redeemUnderlying(uint redeemAmount) returns (uint)

• msg.sender: The account to which redeemed funds shall be transferred.

• redeemAmount: The amount of underlying to be redeemed.

• RETURN: 0 on success, otherwise an Error code

Solidity

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PEther pToken = PEther(0x3FDB...);
require(pToken.redeemUnderlying(50) == 0, "something went wrong");

Web3 1.0

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const pToken = PErc20.at(0x3FDA...);
pToken.methods.redeemUnderlying(10).send({from: ...});

Borrow

The borrow function transfers an asset from the protocol to the user, and creates a borrow balance which begins accumulating interest based on the Borrow Rate for the asset. The amount borrowed must be less than the user's Account Liquidity and the market's available liquidity. To borrow Ether, the borrower must be 'payable' (solidity).

PErc20 / PEther

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function borrow(uint borrowAmount) returns (uint)

• msg.sender: The account to which borrowed funds shall be transferred.

• borrowAmount : The amount of the underlying asset to be borrowed.

• RETURN: 0 on success, otherwise an Error code

Solidity

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PErc20 pToken = PErc20(0x3FDA...);
require(pToken.borrow(100) == 0, "got collateral?");

Web3 1.0

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const pToken = PEther.at(0x3FDB...);
await pToken.methods.borrow(50).send({from: 0xMyAccount});

Repay Borrow

The repay function transfers an asset into the protocol, reducing the user's borrow balance.

PErc20

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function repayBorrow(uint repayAmount) returns (uint)

• msg.sender: The account which borrowed the asset, and shall repay the borrow.

• repayAmount: The amount of the underlying borrowed asset to be repaid. A value of -1 (i.e. 2^256 - 1) can be used to repay the full amount.

• RETURN: 0 on success, otherwise an Error code Before repaying an asset, users must first approve the pToken to access their token balance.

PEther

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function repayBorrow() payable

• msg.value: The amount of ether to be repaid, in wei.

• msg.sender: The account which borrowed the asset, and shall repay the borrow.

• RETURN: No return, reverts on error.

Solidity

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PEther pToken = PEther(0x3FDB...);
require(pToken.repayBorrow.value(100)() == 0, "transfer approved?");

Web3 1.0

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const pToken = PErc20.at(0x3FDA...);
pToken.methods.repayBorrow(10000).send({from: ...});

Repay Borrow Behalf

The repay function transfers an asset into the protocol, reducing the target user's borrow balance.

PErc20

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function repayBorrowBehalf(address borrower, uint repayAmount) returns (uint)

• msg.sender: The account which shall repay the borrow.

• borrower: The account which borrowed the asset to be repaid.

• repayAmount: The amount of the underlying borrowed asset to be repaid. A value of -1 (i.e. 2^256 - 1) can be used to repay the full amount.

• RETURN: 0 on success, otherwise an Error code Before repaying an asset, users must first approve the pToken to access their token balance.

PEther

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function repayBorrowBehalf(address borrower) payable

• msg.value: The amount of ether to be repaid, in wei.

• msg.sender: The account which shall repay the borrow.

• borrower: The account which borrowed the asset to be repaid.

• RETURN: No return, reverts on error.

Solidity

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PEther pToken = PEther(0x3FDB...);
require(pToken.repayBorrowBehalf.value(100)(0xBorrower) == 0, "transfer approved?");

Web3 1.0

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const pToken = PErc20.at(0x3FDA...);
await pToken.methods.repayBorrowBehalf(0xBorrower, 10000).send({from: 0xPayer});

Transfer

Transfer is an ERC-20 method that allows accounts to send tokens to other Ethereum addresses. A pToken transfer will fail if the account has entered that pToken market and the transfer would have put the account into a state of negative liquidity.

PErc20 / PEther

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function transfer(address recipient, uint256 amount) returns (bool)

• recipient: The transfer recipient address.

• amount: The amount of pTokens to transfer.

• RETURN: Returns a boolean value indicating whether or not the operation succeeded.

Solidity

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PEther pToken = PEther(0x3FDB...);
pToken.transfer(0xABCD..., 100000000000);

Web3 1.0

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const pToken = PErc20.at(0x3FDA...);
await pToken.methods.transfer(0xABCD..., 100000000000).send({from: 0xSender});

Liquidate Borrow

A user who has negative account liquidity is subject to liquidation by other users of the protocol to return his/her account liquidity back to positive (i.e. above the collateral requirement). When a liquidation occurs, a liquidator may repay some or all of an outstanding borrow on behalf of a borrower and in return receive a discounted amount of collateral held by the borrower; this discount is defined as the liquidation incentive. A liquidator may close up to a certain fixed percentage (i.e. close factor) of any individual outstanding borrow of the underwater account. Unlike in v1, liquidators must interact with each pToken contract in which they wish to repay a borrow and seize another asset as collateral. When collateral is seized, the liquidator is transferred pTokens, which they may redeem the same as if they had supplied the asset themselves. Users must approve each pToken contract before calling liquidate (i.e. on the borrowed asset which they are repaying), as they are transferring funds into the contract.

PErc20

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function liquidateBorrow(address borrower, uint amount, address collateral) returns (uint)

• msg.sender: The account which shall liquidate the borrower by repaying their debt and seizing their collateral.

• borrower: The account with negative account liquidity that shall be liquidated.

• repayAmount: The amount of the borrowed asset to be repaid and converted into collateral, specified in units of the underlying borrowed asset.

• pTokenCollateral: The address of the pToken currently held as collateral by a borrower, that the liquidator shall seize.

• RETURN: 0 on success, otherwise an Error code Before supplying an asset, users must first approve the pToken to access their token balance.

PEther

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function liquidateBorrow(address borrower, address pTokenCollateral) payable

• msg.value: The amount of ether to be repaid and converted into collateral, in wei.

• msg.sender: The account which shall liquidate the borrower by repaying their debt and seizing their collateral.

• borrower: The account with negative account liquidity that shall be liquidated.

• pTokenCollateral: The address of the pToken currently held as collateral by a borrower, that the liquidator shall seize.

• RETURN: No return, reverts on error.

Solidity

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PEther pToken = PEther(0x3FDB...);
PErc20 pTokenCollateral = PErc20(0x3FDA...);
require(pToken.liquidateBorrow.value(100)(0xBorrower, pTokenCollateral) == 0, "borrower underwater??");

Web3 1.0

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const pToken = PErc20.at(0x3FDA...);
const pTokenCollateral = PEther.at(0x3FDB...);
await pToken.methods.liquidateBorrow(0xBorrower, 33, pTokenCollateral).send({from: 0xLiquidator});

Key Events

EventDescription

Mint(address minter, uint mintAmount, uint mintTokens)

Emitted upon a successful Mint.

Redeem(address redeemer, uint redeemAmount, uint redeemTokens)

Emitted upon a successful Redeem.

Borrow(address borrower, uint borrowAmount, uint accountBorrows, uint totalBorrows)

Emitted upon a successful Borrow.

RepayBorrow(address payer, address borrower, uint repayAmount, uint accountBorrows, uint totalBorrows)

Emitted upon a successful Repay Borrow.

LiquidateBorrow(address liquidator, address borrower, uint repayAmount, address pTokenCollateral, uint seizeTokens)

Emitted upon a successful Liquidate Borrow.

Exchange Rate

Each pToken is convertible into an ever increasing quantity of the underlying asset, as interest accrues in the market. The exchange rate between a pToken and the underlying asset is equal to:

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exchangeRate = (getCash() + totalBorrows() - totalReserves()) / totalSupply()

PErc20 / PEther

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function exchangeRateCurrent() returns (uint)

• RETURN: The current exchange rate as an unsigned integer, scaled by 1 * 10^(18 - 8 + Underlying Token Decimals).

Solidity

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PErc20 pToken = pToken(0x3FDA...);
uint exchangeRateMantissa = pToken.exchangeRateCurrent();

Web3 1.0

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const pToken = PEther.at(0x3FDB...);
const exchangeRate = (await pToken.methods.exchangeRateCurrent().call()) / 1e18;

Tip: note the use of call vs. send to invoke the function from off-chain without incurring gas costs.

Get Cash

Cash is the amount of underlying balance owned by this pToken contract. One may query the total amount of cash currently available to this market.

PErc20 / PEther

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function getCash() returns (uint)

• RETURN: The quantity of underlying asset owned by the contract.

Solidity

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PErc20 pToken = pToken(0x3FDA...);
uint cash = pToken.getCash();

Web3 1.0

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const pToken = PEther.at(0x3FDB...);
const cash = (await pToken.methods.getCash().call());

Total Borrows

Total Borrows is the amount of underlying currently loaned out by the market, and the amount upon which interest is accumulated to suppliers of the market.

PErc20 / PEther

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function totalBorrowsCurrent() returns (uint)

• RETURN: The total amount of borrowed underlying, with interest.

Solidity

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PErc20 pToken = pToken(0x3FDA...);
uint borrows = pToken.totalBorrowsCurrent();

Web3 1.0

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const pToken = PEther.at(0x3FDB...);
const borrows = (await pToken.methods.totalBorrowsCurrent().call());

Borrow Balance

A user who borrows assets from the protocol is subject to accumulated interest based on the current borrow rate. Interest is accumulated every block and integrations may use this function to obtain the current value of a user's borrow balance with interest.

PErc20 / PEther

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function borrowBalanceCurrent(address account) returns (uint)

• account: The account which borrowed the assets.

• RETURN: The user's current borrow balance (with interest) in units of the underlying asset.

Solidity

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PErc20 pToken = pToken(0x3FDA...);
uint borrows = pToken.borrowBalanceCurrent(msg.caller);

Web3 1.0

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const pToken = PEther.at(0x3FDB...);
const borrows = await pToken.methods.borrowBalanceCurrent(account).call();

Borrow Rate

At any point in time one may query the contract to get the current borrow rate per block.

PErc20 / PEther

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function borrowRatePerBlock() returns (uint)

• RETURN: The current borrow rate as an unsigned integer, scaled by 1e18.

Solidity

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PErc20 pToken = pToken(0x3FDA...);
uint borrowRateMantissa = pToken.borrowRatePerBlock();

Web3 1.0

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const pToken = PEther.at(0x3FDB...);
const borrowRate = (await pToken.methods.borrowRatePerBlock().call()) / 1e18;

Total Supply

Total Supply is the number of tokens currently in circulation in this pToken market. It is part of the EIP-20 interface of the pToken contract.

PErc20 / PEther

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function totalSupply() returns (uint)

• RETURN: The total number of tokens in circulation for the market.

Solidity

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PErc20 pToken = pToken(0x3FDA...);
uint tokens = pToken.totalSupply();

Web3 1.0

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const pToken = PEther.at(0x3FDB...);
const tokens = (await pToken.methods.totalSupply().call());

Underlying Balance

The user's underlying balance, representing their assets in the protocol, is equal to the user's pToken balance multiplied by the Exchange Rate.

PErc20 / PEther

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function balanceOfUnderlying(address account) returns (uint)

• account: The account to get the underlying balance of.

• RETURN: The amount of underlying currently owned by the account.

Solidity

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PErc20 pToken = pToken(0x3FDA...);
uint tokens = pToken.balanceOfUnderlying(msg.caller);

Web3 1.0

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const pToken = PEther.at(0x3FDB...);
const tokens = await pToken.methods.balanceOfUnderlying(account).call();

Supply Rate

At any point in time one may query the contract to get the current supply rate per block. The supply rate is derived from the borrow rate, reserve factor and the amount of total borrows.

PErc20 / PEther

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function supplyRatePerBlock() returns (uint)

• RETURN: The current supply rate as an unsigned integer, scaled by 1e18.

Solidity

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PErc20 pToken = pToken(0x3FDA...);
uint supplyRateMantissa = pToken.supplyRatePerBlock();

Web3 1.0

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const pToken = PEther.at(0x3FDB...);
const supplyRate = (await pToken.methods.supplyRatePerBlock().call()) / 1e18;

Total Reserves

Reserves are an accounting entry in each pToken contract that represents a portion of historical interest set aside as cash which can be withdrawn or transferred through the protocol's governance. A small portion of borrower interest accrues into the protocol, determined by the reserve factor.

PErc20 / PEther

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function totalReserves() returns (uint)

• RETURN: The total amount of reserves held in the market.

Solidity

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PErc20 pToken = pToken(0x3FDA...);
uint reserves = pToken.totalReserves();

Web3 1.0

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const pToken = PEther.at(0x3FDB...);
const reserves = (await pToken.methods.totalReserves().call());

Reserve Factor

The reserve factor defines the portion of borrower interest that is converted into reserves.

PErc20 / PEther

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function reserveFactorMantissa() returns (uint)

• RETURN: The current reserve factor as an unsigned integer, scaled by 1e18.

Solidity

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PErc20 pToken = pToken(0x3FDA...);
uint reserveFactorMantissa = pToken.reserveFactorMantissa();

Web3 1.0

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const pToken = PEther.at(0x3FDB...);
const reserveFactor = (await pToken.methods.reserveFactorMantissa().call()) / 1e18;