A standard interface for contracts that create generic ERC-20 tokens which derive from a pool of unique ERC-721/ERC-1155 tokens.
This standard outlines a smart contract interface to wrap identifiable tokens with fungible tokens. This allows for derivative ERC-20 tokens to be minted by locking the base ERC-721 non-fungible tokens and ERC-1155 multi tokens into a pool. The derivative tokens can be burned to redeem base tokens out of the pool. These derivatives have no reference to the unique id of these base tokens, and should have a proportional rate of exchange with the base tokens. As representatives of the base tokens, these generic derivative tokens can be traded and otherwise utilized according to ERC-20, such that the unique identifier of each base token is irrelevant.
ERC-721 and ERC-1155 tokens are considered valid base, tokens because they have unique identifiers and are transferred according to similar rules. This allows for both ERC-721 NFTs and ERC-1155 Multi-Tokens to be wrapped under a single common interface.
The ERC-20 token standard is the most widespread and liquid token standard on Ethereum. ERC-721 and ERC-1155 tokens on the other hand can only be transferred by their individual ids, in whole amounts. Derivative tokens allow for exposure to the base asset while benefiting from contracts which utilize ERC-20 tokens. This allows for the base tokens to be fractionalized, traded and pooled generically on AMMs, collateralized, and be used for any other ERC-20 type contract. Several implementations of this proposal already exist without a common standard.
Given a fixed exchange rate between base and derivative tokens, the value of the derivative token is proportional to the floor price of the pooled tokens. With the derivative tokens being used in AMMs, there is opportunity for arbitrage between derived token markets and the base NFT markets. By specifying a subset of base tokens which may be pooled, the difference between the lowest and highest value token in the pool may be minimized. This allows for higher value tokens within a larger set to be poolable. Additionally, price calculations using methods such as Dutch auctions, as implemented by NFT20, allow for price discovery of subclasses of base tokens. This allows the provider of a higher value base token to receive a proportionally larger number of derivative tokens than a token worth the floor price would receive.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.
Every IWrapper compliant contract must implement the IWrapper and ERC165 interfaces :
The ERC-721/ERC-1155 tokens which are pooled are called {Base} tokens. Alternative names include:
The ERC-20 tokens which are minted/burned are called {Derivative} tokens. Alternative names include:
The function names mint and burn are borrowed from the minting and burning extensions to ERC-20. Alternative names include:
mint/redeem (NFTX)deposit/withdraw (WrappedKitties)wrap/unwrap (MoonCatsWrapped)The function names *idBurn are chosen to reduce confusion on what is being burned. That is, the {Derivative} tokens are burned in order to redeem the id(s).
The wrapper/pool itself can be called an "Index fund" according to NFTX, or a "DEX" according to NFT20. However, the {NFT20Pair} contract allows for direct NFT-NFT swaps which are out of the scope of this standard.
Minting requires the transfer of the {Base} tokens into the {Pool} in exchange for {Derivative} tokens. The {Base} tokens deposited in this way MUST NOT be transferred again except through the burning functions. This ensures the value of the {Derivative} tokens is representative of the value of the {Base} tokens.
Alternatively to transferring the {Base} tokens into the {Pool}, the tokens may be locked as collateral in exchange for {Derivative} loans, as proposed in NFTX litepaper, similarly to Maker vaults. This still follows the general minting pattern of removing transferability of the {Base} tokens in exchange for {Derivative} tokens.
Burning requires the transfer of {Base} tokens out of the {Pool} in exchange for burning {Derivative} tokens. The burn functions are distinguished by the quantity and quality of {Base} tokens redeemed.
id: burn, batchBurn.id(s): idBurn, batchIdBurn.By allowing for specific ids to be targeted, higher value {Base} tokens may be selected out of the pool. NFTX proposes an additional fee to be applied for such targeted withdrawals, to offset the desire to drain the {Pool} of {Base} tokens worth more than the floor price.
Prices should not be necessarily fixed. therefore, Mint/Burn events MUST include the ERC-20 _value minted/burned.
Existing pricing implementations are as follows (measured in base:derivative):
Due to the variety of pricing implementations, the Mint* and Burn* events MUST include the number {Derivative} tokens minted/burned.
The {Wrapper} MAY inherit from {ERC20}, in order to directly call super.mint and super.burn.
If the {Wrapper} does not inherit from {ERC20}, the {Derivative} contract MUST be limited such that the {Wrapper} has the sole power to mint, burn, and otherwise change the supply of tokens.
If not inheriting from {ERC721Receiver} and/or {ERC1155Receiver}, the pool MUST be limited such that the base tokens can only be transferred via the Wrapper's mint, burn.
There exists only one of each ERC-721 token of with a given (address, id) pair. However, ERC-1155 tokens of a given (address, id) may have quantities greater than 1. Accordingly, the meaning of "Single" and "Batch" in each standard varies. In both standards, "single" refers to a single id, and "batch" refers to multiple ids. In ERC-1155, a single id event/function may involve multiple tokens, according to the value field.
In building a common set of events and functions, we must be aware of these differences in implementation. The current implementation treats ERC-721 tokens as a special case where, in reference to the quantity of each {Base} token:
_amount, MUST be 1._amounts MUST be either an empty list or a list of 1 with the same length as _ids.This keeps a consistent enumeration of tokens along with ERC-1155. Alternative implementations include:
mintFromERC721.ERC721Wrapper, ERC1155Wrapper.The {Wrapper} MAY inherit from {ERC721} and/or {ERC1155} in order to call super.mint, directly. This is optional as minting {Base} tokens is not required in this standard. An "Initial NFT Offering" could use this to create a set of {Base} tokens within the contract, and directly distribute {Derivative} tokens.
If the {Wrapper} does not inherit from {ERC721} or {ERC1155}, it MUST include calls to {IERC721} and {IERC1155} in order to transfer {Base} tokens.
All of the underlying transfer methods are not tied to the {Wrapper}, but rather call the ERC-20/721/1155 transfer methods. Implementations of this standard MUST:
Most existing implementations inherit from ERC-20, using functions mint and burn.
Events:
Mint
Burn
ERC-3386 Reference Implementation
Wrapper contracts are RECOMMENDED to inherit from burnable ERC-20 tokens. If they are not, the supply of the {Derivative} tokens MUST be controlled by the Wrapper. Similarly, price implementations MUST ensure that the supply of {Base} tokens is reflected by the {Derivative} tokens.
With the functions idBurn, idBurns, users may target the most valuable NFT within the generic lot. If there is a significant difference between tokens values of different ids, the contract SHOULD consider creating specialized pools (NFTX) or pricing (NFT20) to account for this.
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