> ## Documentation Index
> Fetch the complete documentation index at: https://docs.cow.bleu.builders/llms.txt
> Use this file to discover all available pages before exploring further.

# ERC3156Borrower

> Universal adapter for flash loan providers implementing the ERC-3156 standard

## Overview

The `ERC3156Borrower` contract enables the FlashLoanRouter to request flash loans from any lender implementing the ERC-3156 standard. It provides a universal interface for flash loan providers that follow the standard, including Maker's Flash Mint Module.

## Contract Details

* **Source**: `src/ERC3156Borrower.sol`
* **License**: GPL-3.0-or-later
* **Solidity**: ^0.8.28
* **Address**: `0x47d71b4B3336AB2729436186C216955F3C27cD04` (all networks)
* **Inherits**: `Borrower`, `IERC3156FlashBorrower`

## Compatible Lenders

The ERC3156Borrower works with any lender implementing the ERC-3156 standard, including:

* Maker's Flash Mint Module
* Yield Protocol
* Euler Finance
* Any other ERC-3156 compliant lender

## Functions

### flashLoanAndCallBack

```solidity theme={null}
function flashLoanAndCallBack(
    address lender,
    IERC20 token,
    uint256 amount,
    bytes calldata callBackData
) external onlyRouter
```

Initiates a flash loan request to an ERC-3156 compliant lender. Only callable by the registered router.

**Parameters:**

* `lender`: Address of the ERC-3156 flash loan provider
* `token`: ERC-20 token to borrow
* `amount`: Amount of tokens to borrow
* `callBackData`: Data to pass back to the router unchanged

### onFlashLoan

```solidity theme={null}
function onFlashLoan(
    address initiator,
    address token,
    uint256 amount,
    uint256 fee,
    bytes calldata data
) external returns (bytes32)
```

ERC-3156 callback invoked by the lender after transferring borrowed funds. Notifies the router and returns the required success constant.

**Parameters:**

* `initiator`: Address that initiated the flash loan
* `token`: Address of the borrowed token
* `amount`: Amount borrowed
* `fee`: Fee charged by the lender
* `data`: Encoded callback data (passed through unchanged)

**Returns:** `keccak256("ERC3156FlashBorrower.onFlashLoan")` -- the ERC-3156 success constant

### approve

```solidity theme={null}
function approve(
    IERC20 token,
    address target,
    uint256 amount
) external onlySettlementContract
```

Allows the settlement contract to authorize token spending during settlement for repayment.

### triggerFlashLoan (internal)

```solidity theme={null}
function triggerFlashLoan(
    address lender,
    IERC20 token,
    uint256 amount,
    bytes calldata callBackData
) internal override
```

Internal implementation that calls `IERC3156FlashLender.flashLoan()` and validates the result.

```solidity theme={null}
bool success = IERC3156FlashLender(lender).flashLoan(
    this,
    address(token),
    amount,
    callBackData
);
require(success, "Flash loan was unsuccessful");
```

## Execution Flow

1. Router calls `flashLoanAndCallBack()` on the ERC3156Borrower
2. ERC3156Borrower calls `IERC3156FlashLender.flashLoan()` on the lender
3. Lender transfers the requested tokens to the ERC3156Borrower
4. Lender invokes `onFlashLoan()` on the ERC3156Borrower
5. ERC3156Borrower calls `router.borrowerCallBack()` to continue execution
6. ERC3156Borrower returns the success constant to the lender
7. During settlement, the settlement contract calls `approve()` for lender repayment
8. Lender pulls the borrowed amount plus fee via `transferFrom()`

## Success Constant

The ERC-3156 standard requires the borrower to return a specific constant from `onFlashLoan()`:

```solidity theme={null}
bytes32 private constant ERC3156_ONFLASHLOAN_SUCCESS =
    keccak256("ERC3156FlashBorrower.onFlashLoan");
```

Returning any other value will cause the lender to revert the transaction.

## Security

* `flashLoanAndCallBack()`: Only callable by the registered router
* `approve()`: Only callable by the settlement contract
* `onFlashLoan()`: Called by the lender within the flash loan flow
* Validates flash loan success before proceeding
* Immutable router and settlement contract references

<Warning>
  Always verify that the lender address is a legitimate ERC-3156 compliant contract. Query the lender's `flashFee()` and `maxFlashLoan()` functions to verify fee structure and borrowing limits before use.
</Warning>

## Integration Notes

* Fees vary by lender and asset -- query `flashFee()` before constructing settlements
* Maximum loan amounts can be checked via `maxFlashLoan()`
* Some lenders may have specific token restrictions
* The settlement must include an interaction to approve the lender for repayment (principal + fee)

## Next Steps

<CardGroup cols={2}>
  <Card title="AaveBorrower" icon="coins" href="/flash-loan-router/contracts/aave-borrower">
    Aave flash loan adapter
  </Card>

  <Card title="Borrower Base" icon="code" href="/flash-loan-router/contracts/base/borrower">
    Abstract base contract
  </Card>
</CardGroup>