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Add a "fees" section to the ERC4626 guide (#4054)

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Co-authored-by: Francisco Giordano <fg@frang.io>
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Hadrien Croubois
2023-02-24 15:49:10 +01:00
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docs/modules/ROOT/pages/erc4626.adoc
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@ -191,3 +191,25 @@ stem:[\delta = 3], stem:[a_0 = 100], stem:[a_1 = 10^5]
image::erc4626-attack-6.png[Inflation attack without offset=6]
stem:[\delta = 6], stem:[a_0 = 1], stem:[a_1 = 10^5]
[[fees]]
== Custom behavior: Adding fees to the vault
In an ERC4626 vaults, fees can be captured during the deposit/mint and/or during the withdraw/redeem steps. In both cases it is essential to remain compliant with the ERC4626 requirements with regard to the preview functions.
For example, if calling `deposit(100, receiver)`, the caller should deposit exactly 100 underlying tokens, including fees, and the receiver should receive a number of shares that matches the value returned by `previewDeposit(100)`. Similarly, `previewMint` should account for the fees that the user will have to pay on top of share's cost.
As for the `Deposit` event, while this is less clear in the EIP spec itself, there seems to be consensus that it should include the number of assets paid for by the user, including the fees.
On the other hand, when withdrawing assets, the number given by the user should correspond to what he receives. Any fees should be added to the quote (in shares) performed by `previewWithdraw`.
The `Withdraw` event should include the number of shares the user burns (including fees) and the number of assets the user actually receives (after fees are deducted).
The consequence of this design is that both the `Deposit` and `Withdraw` events will describe two exchange rates. The spread between the "Buy-in" and the "Exit" prices correspond to the fees taken by the vault.
The following example describes how fees proportional to the deposited/withdrawn amount can be implemented:
```solidity
include::api:example$ERC4626Fees.sol[]
```