Hooks

The protocol uses two kinds of hook: transfer-restriction hooks, consulted on token transfers and register changes, and a Uniswap v4 hook for LiquiLeX pools.

Transfer-restriction hooks

ITransferRestrictionHook

interface ITransferRestrictionHook {
    // Delivery: may the token move between these addresses?
    function checkTransferRestriction(
        address from,
        address to,
        uint256 tokenId,   // token id for LETs; amount for scrip
        bytes memory data
    ) external view returns (bool allowed, string memory reason);

    // Registration: may the holder of record change from `from` to `to`?
    function checkLegalTransferRestriction(
        address from,      // current holder of record (never address(0))
        address to,        // incoming holder of record
        uint256 tokenId,
        bytes memory data
    ) external view returns (bool allowed, string memory reason);
}

The two checks answer different questions. checkTransferRestriction governs possession; checkLegalTransferRestriction governs the register and is never called at original issuance. BaseTransferHook answers the registration check with the delivery answer unless a hook overrides it, so a registration is at least as restricted as a delivery. A hook used by a v5 LET contract must implement both functions: a hook that implements only checkTransferRestriction reverts when the LET contract asks it about a registration.

LedgerEntryToken, CyberScrip and CyberShares consult these hooks:

• CyberScrip holds an array of hooks (setRestrictionHook replaces the whole list). Every hook must allow the transfer, or it reverts RestrictedTransfer(reason). The hooks run on transfers and mints (any move to a nonzero address), so scripifying to a recipient needs that recipient to pass them. Burns skip them.
• LedgerEntryToken holds per-LET hooks (setRestrictionHook(id, hook)) and a globalRestrictionHook (setGlobalRestrictionHook). It calls checkTransferRestriction when a token moves between two nonzero addresses and checkLegalTransferRestriction when the holder of record changes after issuance, alongside its own stop-transfer flags (set with setGlobalLegalTransferable / setTokenLegalTransferable). A failing hook reverts TransferRestricted(reason).
• CyberShares checks its global hook on transfer and transferFrom.

Implementations

In src/hooks/transfer/:

Hook	Purpose
BaseTransferHook	Abstract base: an admin setEnabled switch (a disabled hook allows everything) and the default registration check.
WhitelistTransferHook	Allows a transfer when both sender and recipient are whitelisted, and a mint when the recipient is whitelisted. Admin functions setWhitelisted / batchSetWhitelisted.

For per-LET switches, use the LET contract's own setTokenTransferable (delivery) and setTokenLegalTransferable (registration); see LedgerEntryToken.

Uniswap v4 hook

MetalexIssuerFeeHook

In src/hooks/uniswap/. A Uniswap v4 hook for LiquiLeX AMM pools that splits swap fees between MetaLeX and the issuer, so a scrip/stablecoin pool pays the issuer onchain on every trade.

• Per-pool configuration through setPoolConfig (onlyAdmin): a PoolFeeConfig with the MetaLeX and issuer recipients, metalexFeeBps / issuerFeeBps (their sum capped at 10 000 bps), and an enabled flag.
• Registers beforeSwap / afterSwap permissions with return deltas and handles all four swap flows: exact-input swaps are charged in beforeSwap and exact-output swaps in afterSwap, in both directions (zeroForOne and oneForZero).

Uniswap v4 also places requirements on the hook's address; the source documents them.