CW404 is a specification for fractional non-fungible tokens based on CosmWasm. The name and design is based on the ERC404 standard by the Pandora team, with some enhancements.

The specification is split into multiple sections, a contract may only implement some of this functionality, but must implement the base.

As much as possible, the ERC404 standard has been modified to be in-line with both CW20 and CW721 function signatures to optimise for usage. This is desirable as the naming on CW20 and CW721 are highly explicit as opposed to the ERC20 and ERC721 standards which are largely ambiguous. As an example:

// ERC20 transferFrom
function transferFrom(address from, address to, uint256 value) public;

// ERC721 transferFrom
function transferFrom(address from, address to, uint256 tokenId) public;

Versus

// CW20
Transfer{recipient, amount}

// CW721 transfer
TransferNft{recipient, token_id}

The net-effect of the above is that, instead of worrying about understanding the underlying transferFrom source code, users need to only use TransferNft{recipient, token_id} for transferring NFTs and Transfer{recipient, amount} for transferring CW20 tokens.

Before we begin, it is important to understand the rules of engagement of the 404 standard, so developers can plan around this to create unique mechanics:

• When you hold 1 CW404 token in your wallet, an NFT gets minted to your wallet
• Assuming you send out a fractional amount such that you now have less than 1 CW404 token in your wallet, an NFT gets burnt from your wallet
• The burn ordering follows a last-in-first-out (LIFO) ordering
• Token IDs that are burnt will not get recycled
• Assuming a max supply of 10k NFTs, it is possible to have Token IDs >= 10,000. However, because previous NFTs have been burnt, max supply is still 10k

Whitelist mechanics:

• There's a whitelist feature to allow saving of gas for core contracts/addresses
• Whitelisted contracts/addresses will not have NFTs minted into said wallet when transfer of full CW404 tokens are sent in (saves gas)
• Whitelisted contracts will mint, but will not burn NFTs from the address when CW404 tokens are sent out

Lock feature:

• There's a "lock" feature included in the contract to allow users to lock up token IDs for art that they potentially really love, and do not wish to potentially fat-finger burn them
• It will result in transaction reversions when a NFT is about to be burnt within a transaction, thus nullifying any potential burns

TransferNft{recipient, token_id} - This transfers ownership of the token to recipient account. This is designed to send to an address controlled by a private key and does not trigger any actions on the recipient if it is a contract.

Requires token_id to point to a valid token, and env.sender to be the owner of it, or have an allowance to transfer it.

SendNft{contract, token_id, msg} - This transfers ownership of the token to contract account. contract must be an address controlled by a smart contract, which implements the CW721Receiver interface. The msg will be passed to the recipient contract, along with the token_id.

Requires token_id to point to a valid token, and env.sender to be the owner of it, or have an allowance to transfer it.

Approve{spender, token_id, expires} - Grants permission to spender to transfer or send the given token. This can only be performed when env.sender is the owner of the given token_id or an operator. There can be multiple spender accounts per token, and they are cleared once the token is transferred or sent.

Revoke{spender, token_id} - This revokes a previously granted permission to transfer the given token_id. This can only be granted when env.sender is the owner of the given token_id or an operator.

ApproveAll{operator, expires} - Grant operator permission to transfer or send all tokens owned by env.sender. This approval is tied to the owner, not the tokens and applies to any future token that the owner receives as well.

RevokeAll{operator} - Revoke a previous ApproveAll permission granted to the given operator.

Transfer{recipient, amount} - Moves amount CW20 tokens from the info.sender account to the recipient account. This is designed to send to an address controlled by a private key and does not trigger any actions on the recipient if it is a contract.

Send{contract, amount, msg} - Moves amount CW20 tokens from the info.sender account to the contract account. contract must be an address of a contract that implements the Receiver interface. The msg will be passed to the recipient contract, along with the amount.

OwnerOf{token_id, include_expired} - Returns the owner of the given token, as well as anyone with approval on this particular token. If the token is unknown, returns an error. Return type is OwnerOfResponse. If include_expired is set, show expired owners in the results, otherwise, ignore them.

AllOperators{owner, include_expired, start_after, limit} - List all operators that can access all of the owner's tokens. Return type is OperatorsResponse. If include_expired is set, show expired owners in the results, otherwise, ignore them. If start_after is set, then it returns the first limit operators after the given one.

NumTokens{} - Total number of tokens issued

The counter-part to SendNft is ReceiveNft, which must be implemented by any contract that wishes to manage CW721 tokens. This is generally not implemented by any CW721 contract.

ReceiveNft{sender, token_id, msg} - This is designed to handle SendNft messages. The address of the contract is stored in env.sender so it cannot be faked. The contract should ensure the sender matches the token contract it expects to handle, and not allow arbitrary addresses.

The sender is the original account requesting to move the token and msg is a Binary data that can be decoded into a contract-specific message. This can be empty if we have only one default action, or it may be a ReceiveMsg variant to clarify the intention. For example, if I send to an exchange, I can specify the price I want to list the token for.

ContractInfo{} - This returns top-level metadata about the contract. Namely, name and symbol.

NftInfo{token_id} - This returns metadata about one particular token. The return value is based on ERC721 Metadata JSON Schema, but directly from the contract, not as a Uri. Only the image link is a Uri.

AllNftInfo{token_id} - This returns the result of both NftInfo and OwnerOf as one query as an optimization for clients, which may want both info to display one NFT.

Pagination is achieved via start_after and limit. Limit is a request set by the client, if unset, the contract will automatically set it to DefaultLimit (suggested 10). If set, it will be used up to a MaxLimit value (suggested 30). Contracts can define other DefaultLimit and MaxLimit values without violating the CW721 spec, and clients should not rely on any particular values.

If start_after is unset, the query returns the first results, ordered lexicographically by token_id. If start_after is set, then it returns the first limit tokens after the given one. This allows straightforward pagination by taking the last result returned (a token_id) and using it as the start_after value in a future query.

Tokens{owner, start_after, limit} - List all token_ids that belong to a given owner. Return type is TokensResponse{tokens: Vec<token_id>}.

AllTokens{start_after, limit} - Requires pagination. Lists all token_ids controlled by the contract.