Utility modules

See Utility modules for what these are for.

ISlotsModule

interface ISlotsModule is IERC165 {
    function name() external view returns (string memory);
    function version() external view returns (string memory);
 
    function onTransfer(uint256 slotId, address from, address to) external;
    function onPriceUpdate(uint256 slotId, uint256 oldPrice, uint256 newPrice) external;
    function onRelease(uint256 slotId, address from) external;
    function onSettle(
        uint256 slotId,
        address occupant,
        uint256 owed,
        uint256 paid
    ) external;
 
    function feeBps() external view returns (uint256);
    function feeRecipient() external view returns (address);
    function moduleURI() external view returns (string memory);
}

slotId is always 0 — one slot is one contract, so the calling slot is identified by msg.sender. Key your state off that.

Hooks

onSettle

The economic hook. The other three report who holds the slot; this reports that money moved.

• paid is what was actually taken. Use this for accounting.
• owed is what was due. owed - paid is non-zero exactly when the occupant has run out of deposit, which is a useful distress signal.

They diverge because a charge is capped by the remaining deposit. Reconstructing contributions from price × time computes owed, and someone can exploit that deliberately: declare a huge price with a tiny deposit, accrue enormous owed, pay almost nothing.

Unlike the other hooks, onSettle fires mid-transaction — from inside the settlement that every mutating call begins with. The slot is in its pre-operation state; during a buy, occupant() still returns the outgoing occupant. Reentry into the same slot is blocked, but treat anything you read as in flux.

Failure is silent

Module calls are gas-capped and their failures are swallowed. A module that reverts emits ModuleCallFailed and the slot proceeds.

So a module must never be the source of truth for anything financial — if a call is dropped, nothing tells it. Reduce over the TaxPaid event instead, which always fires regardless of what the module did.

Fees

function feeBps() external view returns (uint256);       // e.g. 500 = 5%
function feeRecipient() external view returns (address);

Skimmed from tax when it is collected; the remainder goes to the slot's recipient. Both are read from the module at collection time, so a module with no valid fee recipient simply takes nothing.

Shipped modules

MetadataModule

Stores a URI and structured metadata per slot, set by the occupant and cleared on release. The general-purpose module for "holding this slot means this content appears here" — ads, listings, profiles.

Takes no fee.

FeedPostModule

Grants the occupant the right to post into a feed. Takes no fee.