# FlashLender

> The `FlashLender.vy` contract is an [`ERC-3156`](https://eips.ethereum.org/EIPS/eip-3156) contract that allows users to take out a flash loan for `crvUSD`. The flash loan must be repaid within the same transaction; otherwise, the transaction will revert.

# FlashLender

The `FlashLender.vy` contract is an [`ERC-3156`](https://eips.ethereum.org/EIPS/eip-3156) contract that allows users to take out a flash loan for `crvUSD`. The flash loan must be repaid within the same transaction; otherwise, the transaction will revert.

:::vyper[`FlashLender.vy`]

The source code for the `FlashLender.vy` contract can be found on [GitHub](https://github.com/curvefi/curve-stablecoin/blob/master/contracts/flashloan/FlashLender.vy). Additionally, a `DummyFlashBorrower.vy` contract showcasing a potential usage of a flash loan can also be found on [GitHub](https://github.com/curvefi/curve-stablecoin/blob/master/contracts/testing/DummyFlashBorrower.vy).

The `FlashLender.vy` is deployed on :logos-ethereum: Ethereum at [`0x26dE7861e213A5351F6ED767d00e0839930e9eE1`](https://etherscan.io/address/0x26dE7861e213A5351F6ED767d00e0839930e9eE1).

:::

The contract does not charge any fees on flash loans. The `fee` and `flashFee` methods are implemented to comply with the `ERC-3156` standard.

---

### `flashLoan`
::::description[`FlashLender.flashLoan(receiver: ERC3156FlashBorrower, token: address, amount: uint256, data: Bytes[10**5]) -> bool`]

Function to take out a flash loan of `amount` of `token` and send them to the `receiver`. The `receiver` address must be a contract that implements the `onFlashLoan(initiator: address, token: address, amount: uint256, fee: uint256, data: Bytes[10**5])` interface. A flash loan must be repaid within the same transaction; otherwise, the transaction will revert. Additionally, the method allows passing custom `data` to the `receiver` contract.

| Input      | Type                   | Description                                   |
| ---------- | ---------------------- | --------------------------------------------- |
| `receiver` | `ERC3156FlashBorrower` | Contract to receive the flash loan            |
| `token`    | `address`              | Address of the token to take the flash loan in|
| `amount`   | `uint256`              | Amount of tokens to flash loan                |
| `data`     | `Bytes[10**5]`         | Custom data to pass to the receiver contract  |

Returns: `True` (`bool`).

Emits: `FlashLoan`

<SourceCode>

The following source code includes all changes up to commit hash [`e771f43`](https://github.com/curvefi/curve-stablecoin/tree/e771f437f42fbc5ab73990866000f610bffe1df2); any changes made after this commit are not included.

```py
from vyper.interfaces import ERC20

interface Factory:
    def stablecoin() -> address: view
    def debt_ceiling_residual(_to: address) -> uint256: view

event FlashLoan:
    caller: indexed(address)
    receiver: indexed(address)
    amount: uint256

CRVUSD: immutable(address)
fee: public(constant(uint256)) = 0  # 1 == 0.01 %

@external
@nonreentrant('lock')
def flashLoan(receiver: ERC3156FlashBorrower, token: address, amount: uint256, data: Bytes[10**5]) -> bool:
    """
    @notice Loan `amount` tokens to `receiver`, and takes it back plus a `flashFee` after the callback
    @param receiver The contract receiving the tokens, needs to implement the
    `onFlashLoan(initiator: address, token: address, amount: uint256, fee: uint256, data: Bytes[10**5])` interface.
    @param token The loan currency.
    @param amount The amount of tokens lent.
    @param data A data parameter to be passed on to the `receiver` for any custom use.
    """
    assert token == CRVUSD, "FlashLender: Unsupported currency"
    ERC20(CRVUSD).transfer(receiver.address, amount)
    receiver.onFlashLoan(msg.sender, CRVUSD, amount, 0, data)
    assert ERC20(CRVUSD).balanceOf(self) >= FACTORY.debt_ceiling_residual(self), "FlashLender: Repay failed"

    log FlashLoan(msg.sender, receiver.address, amount)

    return True
```

</SourceCode>

::::

### `maxFlashLoan`
::::description[`FlashLender.maxFlashLoan(token: address) -> uint256: view`]

Getter for the maximum amount of flash-loanable tokens. This corresponds to the token balance of the contract (`token.balanceOf(FlashLender)`).

Returns: maximum flash-loanable amount (`uint256`).

| Input   | Type      | Description                                                  |
| ------- | --------- | ------------------------------------------------------------ |
| `token` | `address` | Token address to check the maximum flash-loanable amount for |

<SourceCode>

The following source code includes all changes up to commit hash [`e771f43`](https://github.com/curvefi/curve-stablecoin/tree/e771f437f42fbc5ab73990866000f610bffe1df2); any changes made after this commit are not included.

```py
CRVUSD: immutable(address)
fee: public(constant(uint256)) = 0  # 1 == 0.01 %

@external
@view
def maxFlashLoan(token: address) -> uint256:
    """
    @notice The amount of currency available to be lent.
    @param token The loan currency.
    @return The amount of `token` that can be borrowed.
    """
    if token == CRVUSD:
        return ERC20(CRVUSD).balanceOf(self)
    else:
        return 0
```

</SourceCode>

<Example>

Calling the function with the `crvUSD` address as input will return the flash-loanable amount. Calling the function with any other token than `crvUSD` will return `0`. 

```shell
>>> FlashLender.maxFlashLoan('0xf939E0A03FB07F59A73314E73794Be0E57ac1b4E')
1000000000000000000000000
```

</Example>

::::

### `fee`
::::description[`FlashLender.fee() -> uint256: view`]

Getter for the fee charged on the flash loan. This variable is a constant set to `0` and cannot be changed. The fees for the contract will always remain at `0`.

Returns: fee (`uint256`).

<SourceCode>

The following source code includes all changes up to commit hash [`e771f43`](https://github.com/curvefi/curve-stablecoin/tree/e771f437f42fbc5ab73990866000f610bffe1df2); any changes made after this commit are not included.

```py
fee: public(constant(uint256)) = 0  # 1 == 0.01 %
```

</SourceCode>

<Example>

```shell
>>> FlashLender.fee()
0
```

</Example>

::::

### `flashFee`
::::description[`FlashLender.flashFee(token: address, amount: uint256) -> uint256: view`]

Getter for the flash fee when taking out a flash loan of `amount` of `token`. This method will always return `0` and will always revert if `token != CRVUSD`.

Returns: total fee charged on the flashloan (`uint256`).

| Input    | Type      | Description                               |
| -------- | --------- | ----------------------------------------- |
| `token`  | `address` | Address of the token for the flash loan   |
| `amount` | `uint256` | Amount of tokens to be flash loaned       |

<SourceCode>

The following source code includes all changes up to commit hash [`e771f43`](https://github.com/curvefi/curve-stablecoin/tree/e771f437f42fbc5ab73990866000f610bffe1df2); any changes made after this commit are not included.

```py
CRVUSD: immutable(address)

@external
@view
def flashFee(token: address, amount: uint256) -> uint256:
    """
    @notice The fee to be charged for a given loan.
    @param token The loan currency.
    @param amount The amount of tokens lent.
    @return The amount of `token` to be charged for the loan, on top of the returned principal.
    """
    assert token == CRVUSD, "FlashLender: Unsupported currency"
    return 0
```

</SourceCode>

<Example>

This method will always return `0` for any amount of flash loaned `crvUSD` because the `fee` is `0`. If the function is called with any token other than `crvUSD`, the function will revert.

```shell
>>> FlashLender.flashFee('0xf939E0A03FB07F59A73314E73794Be0E57ac1b4E', 100000 * 10**18)
0

>>> FlashLender.flashFee('0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48', 100000 * 10**18)
reverts: "FlashLender: Unsupported currency"
```

</Example>

::::

### `supportedTokens`
::::description[`FlashLender.supportedTokens(token: address) -> bool: view`]

Getter for the supported token by the `FlashLender`. The only supported token is `crvUSD`. Due to the immutability of the contract, no further supported tokens can be added.

Returns: `True` or `False` (`bool`).

| Input   | Type      | Description                               |
| ------- | --------- | ----------------------------------------- |
| `token` | `address` | Token address to check support status for |

<SourceCode>

The following source code includes all changes up to commit hash [`e771f43`](https://github.com/curvefi/curve-stablecoin/tree/e771f437f42fbc5ab73990866000f610bffe1df2); any changes made after this commit are not included.

```py
CRVUSD: immutable(address)

@external
@view
def supportedTokens(token: address) -> bool:
    return token == CRVUSD
```

</SourceCode>

<Example>

This method returns a `bool` based on whether the token is supported or not.

```shell
>>> FlashLender.supportedTokens('0xf939E0A03FB07F59A73314E73794Be0E57ac1b4E')   # crvusd
'true'

>>> FlashLender.supportedTokens('0xf939E0A03FB07F59A73314E73794Be0E57ac1b4E')   # usdc
'false'
```

</Example>

::::

### `version`
::::description[`FlashLender.version() -> String[8]: view`]

Getter for the version of the contract.

Returns: contract version (`String[8]`).

<SourceCode>

The following source code includes all changes up to commit hash [`e771f43`](https://github.com/curvefi/curve-stablecoin/tree/e771f437f42fbc5ab73990866000f610bffe1df2); any changes made after this commit are not included.

```py
version: public(constant(String[8])) = "1.0.0"  # Initial
```

</SourceCode>

<Example>

```shell
>>> FlashLender.version()
"1.0.0"
```

</Example>

::::