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

# Lens

> Read-only helper contract for simulating and querying Monolith state

# Lens Contract

`Lens` is a stateless, view-only helper that simulates interest accrual and redemption outcomes without mutating protocol state. It is useful for UI previews and off-chain integrations that need fresh values without paying gas for `accrueInterest()`.

[Contract source](https://github.com/MonolithMarket/Monolith/blob/main/src/Lens.sol)

## Functions

### previewRedeem

```solidity theme={null}
function previewRedeem(Lender _lender, address borrower, uint256 amountIn)
    external
    view
    returns (uint256 coinIn, uint256 amountOut)
```

Simulates the outcome of redeeming `amountIn` Coin against `borrower`'s collateral on `_lender`, using current state plus pending interest.

**Parameters:**

* `_lender`: Lender instance to query
* `borrower`: Borrower to redeem against (must be in redeemable mode)
* `amountIn`: Desired amount of Coin to redeem

**Returns:**

* `coinIn`: Amount of Coin that would actually be consumed (capped by the borrower's debt and by their collateral value at the current price)
* `amountOut`: Amount of collateral the redeemer would receive, in collateral token decimals

**Returns `(0, 0)` when any of the following hold:**

* `amountIn` is zero
* `borrower` is not redeemable (`isRedeemable(borrower) == false`)
* `borrower` has no collateral
* The oracle does not allow liquidations (stale/invalid price or reduce-only)
* `borrower` has no debt
* The computed `amountOut` is zero or exceeds the borrower's collateral balance

***

### getDebtOf

```solidity theme={null}
function getDebtOf(Lender _lender, address borrower) public view returns (uint256)
```

Returns `borrower`'s current debt on `_lender`, **including interest that has accrued since the last `accrueInterest()` call**. In contrast, `Lender.getDebtOf` reflects only already-accrued state.

**Parameters:**

* `_lender`: Lender instance to query
* `borrower`: Borrower address

**Returns:**

* Current debt amount (18 decimals), synchronized with `InterestModel.calculateInterest`. Zero if the borrower has no shares in the appropriate (paid or free) debt pool.

**Details:**

* For redeemable borrowers, debt is computed from `freeDebtShares` against `totalFreeDebt` (free debt does not accrue interest).
* For non-redeemable borrowers, debt is computed from `paidDebtShares` against a synced `totalPaidDebt` that includes pending interest via a `try/catch` call to `InterestModel.calculateInterest`. If the interest math would overflow, the function falls back to the unsynced value rather than reverting.