Flash loans in Blockchain / Solidity

Introduction

Flash loans let you borrow money instantly without collateral, but you must pay it back in the same transaction. This helps you do quick trades or actions without needing your own funds.

You want to swap tokens quickly to take advantage of price differences.

You want to refinance a loan by paying off one and taking another instantly.

You want to perform arbitrage between different exchanges without upfront money.

You want to add or remove liquidity in a pool without holding the tokens first.

You want to execute complex multi-step operations atomically in one transaction.

Syntax

Blockchain / Solidity

contract FlashLoanReceiver is FlashLoanReceiverBase {
    function executeOperation(
        address[] calldata assets,
        uint256[] calldata amounts,
        uint256[] calldata premiums,
        address initiator,
        bytes calldata params
    ) external override returns (bool) {
        // Your logic here
        // Assume single asset at index 0
        address asset = assets[0];
        uint256 amount = amounts[0];
        uint256 premium = premiums[0];

        // Repay the loan plus fee
        uint256 totalDebt = amount + premium;
        IERC20(asset).approve(address(LENDING_POOL), totalDebt);
        return true;
    }
}

This example shows the core function you must implement to receive a flash loan.

You must repay the loan plus a small fee before the transaction ends, or the whole transaction fails.

Examples

This function is called by the lending pool after giving you the loan. You do your work and then approve repayment.

Blockchain / Solidity

function executeOperation(address[] calldata assets, uint256[] calldata amounts, uint256[] calldata premiums, address initiator, bytes calldata params) external override returns (bool) {
    // Use the borrowed amount
    // For example, arbitrage or swap tokens
    // Assume single asset
    address asset = assets[0];
    uint256 amount = amounts[0];
    uint256 premium = premiums[0];

    // Approve the Lending Pool to pull the owed amount
    uint256 totalDebt = amount + premium;
    IERC20(asset).approve(address(LENDING_POOL), totalDebt);
    return true;
}

This function requests a flash loan from the lending pool contract.

Blockchain / Solidity

function requestFlashLoan(address asset, uint256 amount) external {
    address receiverAddress = address(this);
    address[] memory assets = new address[](1);
    assets[0] = asset;
    uint256[] memory amounts = new uint256[](1);
    amounts[0] = amount;
    uint256[] memory modes = new uint256[](1);
    modes[0] = 0;
    bytes memory params = "";
    LENDING_POOL.flashLoan(receiverAddress, assets, amounts, modes, address(this), params, 0);
}

Sample Program

This contract requests a flash loan and repays it immediately without extra logic. It shows the minimal working flash loan flow.

Blockchain / Solidity

pragma solidity ^0.8.0;

import "@aave/protocol-v2/contracts/flashloan/base/FlashLoanReceiverBase.sol";
import "@aave/protocol-v2/contracts/interfaces/ILendingPool.sol";
import "@aave/protocol-v2/contracts/interfaces/ILendingPoolAddressesProvider.sol";
import "@openzeppelin/contracts/token/ERC20/IERC20.sol";

contract SimpleFlashLoan is FlashLoanReceiverBase {
    constructor(address _addressProvider) FlashLoanReceiverBase(_addressProvider) {}

    function executeOperation(
        address[] calldata assets,
        uint256[] calldata amounts,
        uint256[] calldata premiums,
        address initiator,
        bytes calldata params
    ) external override returns (bool) {
        // Example: just approve repayment for the single asset
        address asset = assets[0];
        uint256 amount = amounts[0];
        uint256 premium = premiums[0];
        uint256 totalDebt = amount + premium;
        IERC20(asset).approve(address(LENDING_POOL), totalDebt);
        return true;
    }

    function startFlashLoan(address asset, uint256 amount) external {
        address receiverAddress = address(this);
        address[] memory assets = new address[](1);
        assets[0] = asset;
        uint256[] memory amounts = new uint256[](1);
        amounts[0] = amount;
        uint256[] memory modes = new uint256[](1);
        modes[0] = 0;
        bytes memory params = "";
        LENDING_POOL.flashLoan(receiverAddress, assets, amounts, modes, address(this), params, 0);
    }
}

Important Notes

Flash loans must be repaid in the same transaction or the whole transaction fails.

They are useful for atomic operations that need temporary liquidity.

Always test flash loans on test networks before using real funds.

Summary

Flash loans let you borrow without collateral but must be repaid instantly.

They enable quick, complex operations like arbitrage or refinancing.

Implement executeOperation to use and repay the loan within one transaction.

Practice

1. What is the main feature of a flash loan in blockchain?

easy

A. You can borrow funds without collateral but must repay within the same transaction

B. You borrow funds with collateral and repay anytime

C. You borrow funds and repay after 30 days

D. You borrow funds only for staking purposes

Solution

1. Step 1: Understand flash loan basics

   Flash loans allow borrowing without collateral but require repayment in the same transaction.

2. Step 2: Compare options

   Only You can borrow funds without collateral but must repay within the same transaction correctly states no collateral and instant repayment.

3. Final Answer:

   You can borrow funds without collateral but must repay within the same transaction -> Option A

4. Quick Check:

   Flash loan = no collateral + instant repayment [OK]

Hint: Flash loans = borrow now, repay instantly [OK]

Common Mistakes:

• Thinking collateral is required
• Assuming repayment can be delayed
• Confusing flash loans with regular loans

2. Which of the following is the correct Solidity function signature to implement a flash loan callback?

easy

A. function repayLoan(uint256 amount) external

B. function flashLoan(address borrower, uint256 amount) public

C. function startLoan(address asset, uint256 amount) external returns (bool)

D. function executeOperation(address[] calldata assets, uint256[] calldata amounts, uint256[] calldata premiums, address initiator, bytes calldata params) external returns (bool)

Solution

1. Step 1: Identify the standard flash loan callback

   The Aave protocol requires implementing executeOperation with specific parameters.

2. Step 2: Match function signature

   function executeOperation(address[] calldata assets, uint256[] calldata amounts, uint256[] calldata premiums, address initiator, bytes calldata params) external returns (bool) matches the exact signature needed for flash loan execution and repayment.

3. Final Answer:

   function executeOperation(address[] calldata assets, uint256[] calldata amounts, uint256[] calldata premiums, address initiator, bytes calldata params) external returns (bool) -> Option D

4. Quick Check:

   executeOperation signature = function executeOperation(address[] calldata assets, uint256[] calldata amounts, uint256[] calldata premiums, address initiator, bytes calldata params) external returns (bool) [OK]

Hint: Flash loan callback is always executeOperation with specific params [OK]

Common Mistakes:

• Using incorrect function names
• Missing required parameters
• Wrong return type

3. Given this simplified Solidity snippet inside executeOperation:

uint256 amountOwing = amounts[0] + premiums[0];
IERC20(assets[0]).approve(address(LENDING_POOL), amountOwing);
return true;

What does this code do?

medium

A. Transfers the loan amount to the borrower

B. Approves the lending pool to withdraw the loan plus fee for repayment

C. Withdraws the loan amount from the lending pool

D. Rejects the flash loan request

Solution

1. Step 1: Understand the approval call

   The code approves the lending pool contract to spend the loan amount plus premium from this contract.

2. Step 2: Interpret the purpose

   This approval is necessary so the lending pool can pull repayment automatically after the operation.

3. Final Answer:

   Approves the lending pool to withdraw the loan plus fee for repayment -> Option B

4. Quick Check:

   approve() = allow repayment withdrawal [OK]

Hint: approve() lets lending pool pull repayment [OK]

Common Mistakes:

• Confusing approve with transfer
• Thinking it sends funds to borrower
• Missing the premium fee in amount

4. Identify the error in this simplified flash loan executeOperation snippet:

function executeOperation(address[] calldata assets, uint256[] calldata amounts, uint256[] calldata premiums, address initiator, bytes calldata params) external returns (bool) {
    uint256 amountOwing = amounts[0] + premiums[0];
    IERC20(assets[0]).transferFrom(msg.sender, address(this), amountOwing);
    return true;
}

medium

A. Incorrect function parameters

B. Missing return statement

C. Using transferFrom instead of approve for repayment

D. Not calling the lending pool to borrow funds

Solution

1. Step 1: Analyze repayment method

   The code tries to pull repayment using transferFrom from msg.sender, which is incorrect.

2. Step 2: Correct repayment approach

   Flash loans require approving the lending pool to pull funds, not transferring from msg.sender.

3. Final Answer:

   Using transferFrom instead of approve for repayment -> Option C

4. Quick Check:

   Repayment needs approve(), not transferFrom() [OK]

Hint: Repay by approve(), not transferFrom() [OK]

Common Mistakes:

• Confusing transferFrom with approve
• Forgetting to approve lending pool
• Misunderstanding msg.sender role

5. You want to use a flash loan to perform arbitrage between two decentralized exchanges (DEXs). Which sequence correctly describes the steps inside executeOperation to profit and repay the loan?

hard

A. Borrow funds -> Buy low on DEX1 -> Sell high on DEX2 -> Approve repayment -> Return true

B. Borrow funds -> Approve repayment -> Buy low on DEX1 -> Sell high on DEX2 -> Return true

C. Approve repayment -> Borrow funds -> Buy low on DEX1 -> Sell high on DEX2 -> Return true

D. Buy low on DEX1 -> Borrow funds -> Sell high on DEX2 -> Approve repayment -> Return true

Solution

1. Step 1: Understand flash loan flow

   You first borrow funds, then use them to buy low on one DEX and sell high on another to gain profit.

2. Step 2: Approve repayment and finish

   After trading, approve the lending pool to pull the loan plus fee, then return true to complete.

3. Final Answer:

   Borrow funds -> Buy low on DEX1 -> Sell high on DEX2 -> Approve repayment -> Return true -> Option A

4. Quick Check:

   Arbitrage flow = borrow -> trade -> approve -> return [OK]

Hint: Trade first, then approve repayment [OK]

Common Mistakes:

• Approving repayment before trading
• Trying to trade before borrowing
• Not approving repayment at all