dob
commented
7 years ago You could consider using an ipfs multiaddr as the return value for the token metadata. This would allow for a self describing IPFS hash, http url, or swarm addr in the future.
Closed dete closed 3 years ago
dob
commented
7 years ago You could consider using an ipfs multiaddr as the return value for the token metadata. This would allow for a self describing IPFS hash, http url, or swarm addr in the future.
dete
commented
7 years ago Great point, @dob. Thanks for the suggestion! I've updated the draft above.
(For reference, my original posting had suggested using the format ipfs:QmYwAP... for IPFS links.)
kylerchin
commented
7 years ago wow! awesome integration! This is better than sending the raw data over the wire. :)
Arachnid
commented
7 years ago Nice proposal! For the metadata to be useful, though, I think you need to mandate its format, rather than just recommend it.
GNSPS
commented
7 years ago A much needed EIP! 👏
ethernian
commented
7 years ago I would propose to use term "asset" instead of "nun-fungible token" to separate both terms. It is an usual mistake to take some (fungible) token as representation of some (non-fungible) physical asset. So we can make it clear from very beginning.
dete
commented
7 years ago @Arachnid: The whole metadata method is optional, but if someone is going to implement it, they should probably implement it in a way that everyone else is expecting. I anticipate using "SHOULD" language for the full specification.
@dip239: Ah naming things. Every programmer's favourite bikeshedding vortex. 😁 I bounced between three separate ideas for naming: "token" as seen above, "asset", following @dob's lead (and your suggestion!), and "NFT" which I used in the prose, but which is incredibly awkward in the API calls: nftMetadata(uint _nftId).
"Asset" does seem like a decent choice, and my first personal draft used it. I discarded it for a two key reasons:
I did spend some time trying out other alternatives ("item", "object", "thing", etc.), but none seemed right. I am happy to hear other suggestions because I am not entirely content with "token". It just seems like the best of some bad options.
silasdavis
commented
7 years ago @dete your justification for NFT sounds rational to me, but allowing myself to plunge into the vortex for a moment. Sometimes finding a word that is relatively unreserved in modern usage but means the same thing works. 'Tesserae' were ancient tokens or tiles used as theatre tickets, forms of religious authentication, etc (see: https://www2.warwick.ac.uk/fac/arts/classics/research/dept_projects/tcam/about/, https://www2.warwick.ac.uk/fac/arts/classics/research/dept_projects/tcam/blog/, https://en.wikipedia.org/wiki/Tessera_(commerce)). Seems like they did function as a kind of non-fungible potentially value-holding token, but were quite varied in their specific application, which might suit the present case. Then again maybe it is just trying a bit too hard...
Arachnid
commented
7 years ago @dete I think you SHOULD use MUST instead. Otherwise, callers have no way to know how to interpret the return value correctly.
tjayrush
commented
7 years ago Possible (but unlikely) names: ticket, badge, wafer, tile, marker
dete
commented
7 years ago @silasdavis: If this were a whole project, and not a single interface that is part of something larger, I'd jump on Tessera in a heartbeat. In that situation, a bit of an unusual name – with some history and context behind it – is pretty compelling. For something like an interface, tho, it's probably much better to stick with a term that people are already familiar with; even if it's imperfect.
@tjayrush: Thanks for the suggestions! I don't think most of them work well, but "marker" might. I'm definitely going to stew on that one a bit more. If it weren't overloaded in such common usage as a writing implement, it would probably be just about perfect.
dete
commented
7 years ago @Arachnid: Happy to follow the community lead here, but if you look at ERC-20, they use "SHOULD" language for something as foundational as emitting Transfer events. The metadata structure felt no more critical to me than Transfer...
MicahZoltu
commented
7 years ago Do not use ERC20 as an example of a good standard. A ton of people went and implemented the draft and then we had to finalize a "standard" that basically just listed what other people were doing. I don't believe anyone sees ERC20 as a good standard and everyone I have spoken to would like to see new better standards (hence why ERC223 exists).
Specifically, ERC20 used SHOULD because the standard came after a bunch of implementations and not all implementations did the same thing, so the standard couldn't say MUST without causing a bunch of ERC20-like tokens to not be ERC20.
dete
commented
7 years ago Great context, @MicahZoltu. Thank you!
Arachnid
commented
7 years ago @dete I'd also argue that the format of something is more foundational than whether you emit it or not. If you don't emit a transfer event, others can't track token transfers for your token - but if you don't require the format of a field, then they can't parse it anywhere.
eordano
commented
7 years ago Hey @dete! I went ahead and coded a first draft of this. Names are slightly different. https://github.com/decentraland/land/blob/master/contracts/BasicNFT.sol
dete
commented
7 years ago So, I was talking to @flockonus (another Solidity engineer here) and I was worrying about the "lost token" problem that ERC-223 tries to solve. (Essentially, ERC-20 has no mechanism to keep users from sending their tokens to contracts that don't know how to handle them, resulting in entirely unrecoverable coins. @dexaran did some analysis estimating that something like $400k has been lost in this way!)
The solution proposed by ERC-223 is to only allow transfers to contract addresses that implement the tokenFallback method. This has the nice side effect that the receiving contract gets a chance to do some work when it receives the coins.
I talked to @flockonus because I was hesitant to add more complexity to NFTs, and he kind of floored me with a suggestion that seemed crazy at first, but is really growing on me: Just get rid of the transfer method.
So hear me out: What if the only way to transfer an NFT was for the owner to approve a recipient, and have the recipient (whether contract or non-contract) call takeOwnership (which works like transferFrom, but with msg.sender hard-coded as the _to parameter).
This solves three problems, two of which are shared with fungible tokens (and addressed by ERC-223), one that is specific to NFTs:
transfer call; the contract can't be given a token, it has to call takeOwnership.takeOwnership, the flow for sending a token to a contract would be approve(contract) followed by contract.someMethod(). someMethod that would takeOwnership, plus the kind of things you'd put into tokenFallback (if appropriate).It introduces a new problem, of course: The simplest conceivable operation (transferring ownership from user to user) now requires two transactions instead of one. But how much of a problem is this, really? As time goes on, we'll see more and more transactions mediated by exchanges, smart contracts, UIs, and automated agents.
One possible option would be to include transfer but make it optional (meaning that contracts would know not to use it), and strongly recommend that it throws if targeting a contract address. (Although that wouldn't solve the spammy NFT problem.)
I'd love to hear other folks thoughts!
Dexaran
commented
7 years ago @dete It's an interesting idea, but I think that the problems of this approach outweigh its advantages because:
Ethereum suffers bandwidth issues. I think that requiring that each token move is performed with two transactions is an irrational use of the blockchain.
A couple of transactions will require additional gas so it will be more expensive.
Approving then withdrawing is not a common pattern of sending funds in the cryptocurrency world. You can send ETH, ETC, Bitcoins, PIVX and even Doge without any confirmation from the side of the receiver. This will not be an intuitive-clear for users.
The ideology of uniformity. ETH and tokens are currencies. I think that it is better to make them behave similar.
The problems that you try to solve with this proposal are already solved with ERC223. I don't see any value in solving already-solved problems.
ryanschneider
commented
6 years ago Some low-value NFTs could end up being something akin to "spam". The easiest motivating example would be "trash" items in a video game; no game I know of allows griefers to shove items in your backpack without your consent. (This could be especially nasty in a smart-contract context where certain operations may be O(n) on the number of NFTs you hold.)
I thought about this some, and is it really that big of an issue? With the current ERC, each spam item would require its own transaction, so it would be a rather expensive attack, right?
That said, what if there were two new optional methods:
approveMultiple(address _to, uint[] _tokenIds)
transferMultipleFrom(address _from, address _to, uint[] _tokenIds)
These would allow "high volume" NFT contracts (like your MMO item example) to do "approved" bulk transfers, while the lower volume contracts would be naturally protected from spam via the requirement to send a single tokenId per transaction using the standard transfer call.
These methods would also allow one to merge/move wallets w/ 2 transactions instead of the N transactions currently required to move one item at a time.
flockonus
commented
6 years ago About the point of removing the transfer method, we've been thinking heavily about this for CryptoKitties (disclosure i'm in Dete's team), and altho we have unconditional transfer method implemented for the alpha, it's easy to see why unconditional transfer might not work in multiple cases.
To start off, we have to understand that while a similar API to ERC20 is desirable, the case it aims to solve is different. In ERC20 the abstraction is the more you have the better because the tokens have positive monetary value, and go into an indistinguishable sum.
That's not the case with non-fungible, either with a smart contract that tracks property ownership such as the one Dubai is implementing, or a game with relatively lower value assets, they might not always carry positive value or be desirable.
So even with due consideration about it being an expensive attack, when designing a mechanism we should take into account that some users will have significant more purchase power than others, and still they shouldn't be able to harm others with less.
My point is, if transfer is implemented it should be expected to throw depending on the smart contract implementation of what user decides to accept what. The business rules of accepting a token would definitely vary for each S.C. implementation.
dete
commented
6 years ago Upon further reflection and discussion with @flockonus, we are proposing to keep a straightforward (and therefore "unsafe") transfer() method. (i.e. It would work like ERC-20, and not EIP-223.)
Our reasoning is as follows:
Similarly, as @flockonus mentioned about the potential "spam" problem: This is not a problem that the community standard needs to solve. If an implementor of an ERC-721 contract feels like spam is likely to be a problem with their NFT, they are welcome to include additional functionality to make it easy for users to mark their accounts as not accepting unsolicited transfers.
ryanschneider
commented
6 years ago Is transfer() still accepting a single NFT tokenId? Any thoughts on my suggestion of defining some optional methods to support bulk transfers?
Say I decide to sell a large portion of my NFT collection to someone else, and it contains hundreds of NFTs (or more). Shouldn't there be a standard way to transfer N items atomically?
I can see the point of trying to keep the ERC simple, but am concerned that w/o a solution for bulk transfers the scope of what NFTs the ERC can be viable for is limited.
dete
commented
6 years ago Here is the new draft, which is basically the first "complete" draft. Any and all comments are welcome!
EIP: <to be assigned>
Title: Non-fungible Token Standard
Author: Dieter Shirley <dete@axiomzen.co>
Type: Standard
Category: ERC
Status: Draft
Created: 2017-09-20A standard interface for non-fungible tokens.
This standard allows for the implementation of a standard API for non-fungible tokens (henceforth referred to as "NFTs") within smart contracts. This standard provides basic functionality to track and transfer ownership of NFTs.
A standard interface allows any NFTs on Ethereum to be handled by general-purpose applications. In particular, it will allow for NFTs to be tracked in standardized wallets and traded on exchanges.
function name() constant returns (string name)OPTIONAL - It is recommend that this method is implemented for enhanced usability with wallets and exchanges, but interfaces and other contracts MUST NOT depend on the existence of this method.
Returns the name of the collection of NFTs managed by this contract. - e.g. "My Non-Fungibles".
function symbol() constant returns (string symbol)OPTIONAL - It is recommend that this method is implemented for enhanced usability with wallets and exchanges, but interfaces and other contracts MUST NOT depend on the existence of this method.
Returns a short string symbol referencing the entire collection of NFTs managed in this contract. e.g. "MNFT". This symbol SHOULD be short (3-8 characters is recommended), with no whitespace characters or new-lines and SHOULD be limited to the uppercase latin alphabet (i.e. the 26 letters used in English).
function totalSupply() constant returns (uint256 totalSupply)Returns the total number of NFTs currently tracked by this contract.
function balanceOf(address _owner) constant returns (uint256 balance)Returns the number of NFTs assigned to address _owner.
function ownerOf(uint256 _tokenId) constant returns (address owner)Returns the address currently marked as the owner of _tokenID. This method MUST throw if _tokenID does not represent an NFT currently tracked by this contract. This method MUST NOT return 0 (NFTs assigned to the zero address are considered destroyed, and queries about them should throw).
function approve(address _to, uint256 _tokenId)Grants approval for address _to to take possession of the NFT with ID _tokenId. This method MUST throw if msg.sender != ownerOf(_tokenId), or if _tokenID does not represent an NFT currently tracked by this contract, or if msg.sender == _to.
Only one address can "have approval" at any given time; calling approveTransfer with a new address revokes approval for the previous address. Calling this method with 0 as the _to argument clears approval for any address.
Successful completion of this method MUST emit an Approval event (defined below) unless the caller is attempting to clear approval when there is no pending approval. In particular, an Approval event MUST be fired if the _to address is zero and there is some outstanding approval. Additionally, an Approval event MUST be fired if _to is already the currently approved address and this call otherwise has no effect. (i.e. An approve() call that "reaffirms" an existing approval MUST fire an event.)
| Action | Prior State | _to address | New State | Event |
|---|---|---|---|---|
| Clear unset approval | Clear | 0 | Clear | None |
| Set new approval | Clear | X | Set to X | Approval(owner, X, tokenID) |
| Change approval | Set to X | Y | Set to Y | Approval(owner, Y, tokenID) |
| Reaffirm approval | Set to X | X | Set to X | Approval(owner, X, tokenID) |
| Clear approval | Set to X | 0 | Clear | Approval(owner, 0, tokenID) |
Note: ANY change of ownership of an NFT – whether directly through the transfer and transferFrom methods defined in this interface, or through any other mechanism defined in the conforming contract – MUST clear any and all approvals for the transferred NFT. The implicit clearing of approval via ownership transfer MUST also fire the event Approval(0, _tokenId) if there was an outstanding approval. (i.e. All actions that transfer ownership must emit the same Approval event, if any, as would emitted by calling approve(0, _tokenID).)
function takeOwnership(uint256 _tokenId)Assigns the ownership of the NFT with ID _tokenId to msg.sender if and only if msg.sender currently has approval (via a previous call to approveTransfer). A successful transfer MUST fire the Transfer event (defined below).
This method MUST transfer ownership to msg.sender or throw, no other outcomes can be possible. Reasons for failure include (but are not limited to):
msg.sender does not have approval for _tokenId_tokenID does not represent an NFT currently tracked by this contractmsg.sender already has ownership of _tokenIdImportant: Please refer to the Note in the approveTransfer method description; a successful transfer MUST clear pending approval.
function transfer(address _to, uint256 _tokenId)Assigns the ownership of the NFT with ID _tokenId to _to if and only if msg.sender == ownerOf(_tokenId). A successful transfer MUST fire the Transfer event (defined below).
This method MUST transfer ownership to _to or throw, no other outcomes can be possible. Reasons for failure include (but are not limited to):
msg.sender is not the owner of _tokenId_tokenID does not represent an NFT currently tracked by this contract_to is 0 (Conforming contracts MAY have other methods to destroy or burn NFTs, which are conceptually "transfers to 0" and will emit Transfer events reflecting this. However, transfer(0, tokenID) MUST be treated as an error.)A conforming contract MUST allow the current owner to "transfer" a token to themselves, as a way of affirming ownership in the event stream. (i.e. it is valid for _to == ownerOf(_tokenID).) This "no-op transfer" MUST be considered a successful transfer, and therefore MUST fire a Transfer event (with the same address for _from and _to).
Important: Please refer to the Note in the approveTransfer method description; a successful transfer MUST clear pending approval. This includes no-op transfers to the current owner!
function tokenOfOwnerByIndex(address _owner, uint256 _index) constant returns (uint tokenId)OPTIONAL - It is recommend that this method is implemented for enhanced usability with wallets and exchanges, but interfaces and other contracts MUST NOT depend on the existence of this method.
Returns the nth NFT assigned to the address _owner, with n specified by the _index argument. This method MUST throw if _index >= balanceOf(_owner).
Recommended usage is as follows:
uint256 ownerBalance = nonFungibleContract.balanceOf(owner);
uint256[] memory ownerTokens = new uint256[](ownerBalance);
for (uint256 i = 0; i < ownerBalance; i++) {
ownerTokens[i] = nonFungibleContract.tokenOfOwnerByIndex(owner, i);
}Implementations MUST NOT assume that NFTs are accessed in any particular order by their callers (In particular, don't assume this method is called in a monotonically ascending loop.), and MUST ensure that calls to tokenOfOwnerByIndex are fully idempotent unless and until some non-constant function is called on this contract.
Callers of tokenOfOwnerByIndex MUST never assume that the order of NFTs is maintained outside of a single operation, or through the invocation (direct or indirect) of any non-constant contract method.
NOTE: Current limitations in Solidity mean that there is no efficient way to return a complete list of an address's NFTs with a single function call. Callers should not assume this method is implemented efficiently (from a gas standpoint) and should strenuously avoid calling this method "on-chain" (i.e. from any non-constant contract function, or from any constant contract function that is likely to be called on-chain).
function tokenMetadata(uint256 _tokenId) constant returns (string infoUrl)OPTIONAL - It is recommend that this method is implemented for enhanced usability with wallets and exchanges, but interfaces and other contracts MUST NOT depend on the existence of this method.
Returns a multiaddress string referencing an external resource bundle that contains (optionally localized) metadata about the NFT associated with _tokenId. The string MUST be an IPFS or HTTP(S) base path (without a trailing slash) to which specific subpaths are obtained through concatenation. (IPFS is the preferred format due to better scalability, persistence, and immutability.)
Standard sub-paths:
name sub-path MUST contain the UTF-8 encoded name of the specific NFT (i.e. distinct from the name of the collection, as returned by the contract's name method). A name SHOULD be 50 characters or less, and unique amongst all NFTs tracked by this contract. A name MAY contain white space characters, but MUST NOT include new-line or carriage-return characters. A name MAY include a numeric component to differentiate from similar NFTs in the same contract. For example: "Happy Token #157".image sub-path exists, it MUST contain a PNG, JPEG, or SVG image with at least 300 pixels of detail in each dimension. The image aspect ratio SHOULD be between 16:9 (landscape mode) and 2:3 (portrait mode). The image SHOULD be structured with a "safe zone" such that cropping the image to a maximal, central square doesn't remove any critical information. (The easiest way to meet this requirement is simply to use a 1:1 image aspect ratio.)description sub-path exists, it MUST contain a UTF-8 encoded textual description of the asset. This description MAY contain multiple lines and SHOULD use a single new-line character to delimit explicit line-breaks, and two new-line characters to delimit paragraphs. The description MAY include CommonMark-compatible Markdown annotations for styling. The description SHOULD be 1500 characters or less.Each metadata subpath (including subpaths not defined in this standard) MUST contain a sub-path default leading to a file containing the default (i.e. unlocalized) version of the data for that metadata element. For example, an NFT with the metadata path /ipfs/QmZU8bKEG8fhcQwKoLHfjtJoKBzvUT5LFR3f8dEz86WdVe MUST contain the NFT's name as a UTF-8 encoded string available at the full path /ipfs/QmZU8bKEG8fhcQwKoLHfjtJoKBzvUT5LFR3f8dEz86WdVe/name/default. Additionally, each metadata subpath MAY have one or more localizations at a subpath of an ISO 639-1 language code (the same language codes used for HTML). For example, /ipfs/QmZU8bKEG8fhcQwKoLHfjtJoKBzvUT5LFR3f8dEz86WdVe/name/en would have the name in English, and /ipfs/QmZU8bKEG8fhcQwKoLHfjtJoKBzvUT5LFR3f8dEz86WdVe/name/fr would have the name in French (note that even localized values need to have a default entry). Consumers of NFT metadata SHOULD look for a localized value before falling back to the default value. Consumers MUST NOT assume that all metadata subpaths for a particular NFT are localized similarly. For example, it will be common for the name and image objects to not be localized even when the description is.
You can explore the metadata package referenced in this example here.
This event MUST trigger when NFT ownership is transferred via any mechanism.
Additionally, the creation of new NFTs MUST trigger a Transfer event for each newly created NFTs, with a _from address of 0 and a _to address matching the owner of the new NFT (possibly the smart contract itself). The deletion (or burn) of any NFT MUST trigger a Transfer event with a _to address of 0 and a _from address of the owner of the NFT (now former owner!).
NOTE: A Transfer event with _from == _to is valid. See the transfer() documentation for details.
event Transfer(address indexed _from, address indexed _to, uint256 _tokenId)This event MUST trigger on any successful call to approve(address _spender, uint256 _value) (unless the caller is attempting to clear approval when there is no pending approval).
See the documentation for the approve() method above for further detail.
event Approval(address indexed _owner, address indexed _approved, uint256 _tokenId)There are many proposed uses of Ethereum smart contracts that depend on tracking individual, non-fungible tokens (NFTs). Examples of existing or planned NFTs are LAND in Decentraland, the eponymous punks in CryptoPunks, and in-game items using systems like Dmarket or EnjinCoin. Future uses include tracking real-world non-fungible assets, like real-estate (as envisioned by companies like Ubitquity or Propy). It is critical in each of these cases that these items are not "lumped together" as numbers in a ledger, but instead, each token must have its ownership individually and atomically tracked. Regardless of the nature of these items, the ecosystem will be stronger if we have a standardized interface that allows for cross-functional non-fungible token management and sales platforms.
The basis of this standard is that every NFT is identified by a unique, 256-bit unsigned integer within its tracking contract. This ID number MUST NOT change for the life of the contract. The pair (contract address, asset ID) will then be a globally unique and fully-qualified identifier for a specific NFT within the Ethereum ecosystem. While some contracts may find it convenient to start with ID 0 and simply increment by one for each new NFT, callers MUST NOT assume that ID numbers have any specific pattern to them, and should treat the ID as a "black box".
This standard follows the semantics of ERC-20 as closely as possible, but can't be entirely compatible with it due to the fundamental differences between fungible and non-fungible tokens.
Example non-fungible implementations as of September 2017:
(It should be noted that "limited edition, collectable tokens" like Curio Cards and Rare Pepe are not non-fungible tokens. They're actually a collection of individual fungible tokens, each of which is tracked by its own smart contract with its own total supply (which may be 1 in extreme cases).)
Reference implementation forthcoming...
Copyright and related rights waived via CC0.
Arachnid
commented
6 years ago @dete You should update the initial issue with this draft, so people coming new to it don't need to scroll through all the comments.
dete
commented
6 years ago Thanks @Arachnid: I put a link in the first comment for now, I don't want to lose the older version for anyone who wants to follow the conversation.
dete
commented
6 years ago Does anyone have any thoughts on whether the _tokenID in the events should be indexed? I would think yes (it's easy to imagine that people or processes would want to watch for events relating to specific NFTs), but this comment in the Solidity docs on Events gave me pause:
NOTE: Indexed arguments will not be stored themselves. You can only search for the values, but it is impossible to retrieve the values themselves.
That seems... problematic...
Arachnid
commented
6 years ago I don't believe that's quite accurate; you should be able to retrieve the contents of an indexed event (for a fixed length type) or its hash (for string/bytes/arrays).
NoahMarconi
commented
6 years ago Any thoughts on transferring multiple tokens in a single transfer?
We ran into this needs recently and the gas costs are making many options unfeasible.
jpitts
commented
6 years ago For metadata, I would strongly recommend requiring a standard URI.
Perhaps consider a way to allow for linkage to the same metadata hosted on different storage networks (i.e. additional parameter specifying 'ipfs", 'swarm', or 'https'). This way the metadata has a better chance of persisting.
Additionally, instead of an external resource bundle, I would recommend using JSON-LD. This format allows for more complex data, has wide use and tooling (being JSON), and provides context to that data
nadavhollander
commented
6 years ago First -- greed with @jpitts on creating redundant storage of metadata on different storage networks and, more importantly IMO, being more unopinionated with respect to the data storage network. I imagine a URI format along the lines of "swarm://XYZ" / "ipfs://XYZ" with the URI prefix specifying the specific storage network queried.
Secondly, I'm working on tokenized debt issuance protocol (Dharma) and a big feature that would be extremely helpful to us is having built-in optional fungibility within each non-fungible asset. This may sound at odds with the purpose of NFTs, but I'll use Dharma as a tangible example:
Alice wants to issue a bond token using Dharma, and she wants to be able to sell individual shares in that bond to different creditors. She could ostensibly have an NFT representing the debt asset as a whole, wrap that NFT into another fungible ERC20 token contract, and sell tokens from that contract to creditors, but that would require her to incur the gas associated with deploying an entirely new token contract for a set of very generic token transfer functionality. It would be much simpler and cheaper to be able to ask the contract to mint an NFT with its own fractional supply of X tokens. In a sense, this would mean that individual debts would be non-fungible with one another, but within each debt there would be a fractional supply of tokens that are fungible with one another.
As an example, the function interface for transfer would look like this:
function transfer(address _to, uint256 _tokenId, uint246 _amount)The standard could easily be made compatible with NFTs that have no fractional supply -- their issuance would simply have a fractional supply of 1. This is an advantageous arrangement for classes of digital assets that have (1) highly generic functionality and (2) require some sort of fractional fungibility and (2) don't merit the deployment of a smart contract for each issuance event.
With all the above being said, I would definitely not say this is a must have -- this arguably extends beyond the definition of non-fungibility to a certain degree. If, however, this is a very common need among projects that have similar dynamics to the tokens they're issuing, I think this would be the appropriate standard in which to include this functionality.
dete
commented
6 years ago @NoahMarconi: I'd be curious if you could share more details about your scenario. Needless to say, specific NFTs can implement any additional functionality that meets their requirements, including bulk transfers. The question is: What is the value of putting this into shared spec? I'm currently not imagining a common scenario where someone would want to bulk transfer generic NFTs.
nadavhollander
commented
6 years ago Also -- thank you @dete for spearheading this 👍
willwarren89
commented
6 years ago Hi @dete, great job putting together this spec.
I'm in favor of preserving trasferFrom(...) and removing takeOwnership(...) for a few reasons:
trasferFrom(...) is necessary. For example, adding support for NFT tokens in 0x protocol is much easier with trasferFrom(...) present.trasferFrom(...) increases the similarity between ERC721 and ERC20 token interfaces.takeOwnership(...) is a special case of trasferFrom(...).dete
commented
6 years ago consider a way to allow for linkage to the same metadata hosted on different storage networks
Well, my motivation is to put as little metadata as possible into the smart contract. Even just storing a IPFS pointer is likely to be pretty expensive (about two words worth), and having one of these for each NFT will add up pretty darned quickly.
On the other hand, I see the value. What do you think about something like this:
function tokenMetadata(uint256 _tokenId, string preferredTransport) constant returns (string infoUrl)Where preferredTransport is something like "http", "ipfs", or "swarm". The contract doesn't NEED to return a URI matching preferredTransport, but it should try if it can. (If the contract doesn't know about the preferred transport requested by the caller, it just returns whatever transport it wants.)
instead of an external resource bundle, I would recommend using JSON-LD
Hrm. I'll be honest, that seems to add a lot of complexity, both for any service emitting the metadata, as well as for consumers wanting to ingest it.
I'm trying to hold the standard to least common denominator. I can well imagine that some implementors will see good reason to use something like JSON-LD, but is that really the best choice for everyone? I tried to pick a metadata specification that would make sense for most NFT creators and would still be easy, robust, and efficient for wallet and exchange providers. Happy to hear further thoughts on this...
eordano
commented
6 years ago One thing to note: For wallets, using the balanceOf and tokenOfOwnerByIndex function might lead to race conditions where the state of owned tokens mutates while the wallet was fetching the contents. Consider adding a new function, to retrieve the full list of tokenIds, just for the purposes of queries that come from applications and not the EVM.
NoahMarconi
commented
6 years ago @dete The scenario I'm looking at is IoT related tying tokens to tangible assets. The assets are low enough value that ownership transfers are likely to happen almost exclusively in bulk. Conversely, the assets are high enough value that they warrant tracking ownership provenance.
Regardless of which spec we follow, there's still the cost to update state on the blockchain for each transfer of ownership. Given the volume of state changes required we're leaning toward side chains, perhaps, with a blockchain that natively supports NFTs.
dete
commented
6 years ago Believe it or not, I have given some thought to tokenization of NFTs! Tokenization probably doesn't matter much for our project 😼 but it's definitely something that comes to mind when tracking tangible assets.
My primary argument against including this capability in ERC-721 is simple: KISS. 😬 I feel like the goal for a standard like this is to collect the functionality shared across the overwhelming majority of potential implementers. While tokenization is hardly unthinkable, I also believe that there are many interesting NFT use-cases that don't benefit from tokenization. It's also worth pointing out that tokens issued in this way would distinctly not be ERC-20 compatible, and could never be.
What I would like to see, however, is a public TokenizeNonFungible contract, ERC-20 compatible, that can be instantiated for any specific NFT. It could take the NFT contract address and an NFT ID, and then track shares of that NFT in the typical ERC-20 fashion. I feel like this contract could be fairly simple and relatively cheap to deploy, especially if you were able to factor out some of the code into a library contract (if I dare mention that idea so soon after the most recent Parity disaster... 😳 ).
dete
commented
6 years ago @NoahMarconi Makes sense. I would encourage you to create a (probably informal) derivative standard that extends ERC-721 with bulk transfers. Remember, ERC's don't restrict you from having domain-specific functionality!
I still maintain that requiring that all ERC-721 tokens support bulk transfers isn't ideal, but I agree that if it's a likely scenario, there could be some pretty significant gas savings.
dete
commented
6 years ago BTW - In case my 👍s weren't obvious, the next draft will use transferFrom (instead of takeOwnership) and standard URIs (instead of multiaddresses).
dete
commented
6 years ago @eordano:
Consider adding a new function, to retrieve the full list of tokenIds, just for the purposes of queries that come from applications and not the EVM.
Yeah, as we've seen from implementing our own token, I think we'll need to make this change.
Storing a list of NFTs by owner address is pretty expensive (for storage) and a pain to maintain, walking through the complete list of NFTs checking ownership is also expensive (execution cost, this time) and DoS prone. I wanted to avoid returning an array because that doesn't work on-chain, but it turns out that any "normal" implementation of tokenOfOwnerByIndex can't be called efficiently on-chain anyway (and again, risks DoS).
I think it's best to replace tokenOfOwnerByIndex with getTokensOfOwner that returns a dynamic array, and simply state that this MUST never be called by a smart contract. (And even if anyone tries it, they won't get anything useful out of it...)
willwarren89
commented
6 years ago One potential issue that hasn't been discussed yet: the current ERC721 interface does not provide an easy way for other smart contracts to differentiate between an ERC20 and an ERC721. This makes it hard to write smart contracts that can handle both types of tokens.
It would be ideal if the ERC721 interface included a flag isERC721, but it would require some funky calling behavior to check for this flag on an ERC20 token without an exception being thrown. The following code would need to be used:
if(tokenAddress.call(bytes4(sha3("isERC721()"))) { ... };
If tokenAddress is an ERC721 token that includes the flag and sets it to true, the above should return true. If tokenAddress is an ERC20 token that does not contain this flag in its interface, the above should return false rather than throw an exception.
dete
commented
6 years ago If anyone is interested in seeing the source code for our implementation of ERC-721, please check here: https://github.com/axiomzen/cryptokitties-bounty/blob/master/contracts/KittyOwnership.sol
willwarren89
commented
6 years ago Regarding the addition of a flag isERC721 or implementsERC721: @phABC has pointed out that a more general solution to this issue has already been proposed: Pseudo-Introspection, or standard interface detection. I'm in favor of adopting this more general solution.
abandeali1
commented
6 years ago I think it's worth considering adding a data parameter to all of the transfer functions, similar to ERC #223 (or at least overload the current transfer functions). Since we're creating a new standard from scratch, no reason to hold off on features.
I also want to stress the importance of having some sort of ERC721 identifier in the contract. NFTs act in a very similar way to ERC20, but they should be handled completely differently by other contracts. I can think of at least a few examples of dangerous behavior resulting from contracts assuming that NFTs act the same way as an ERC20/223 token. It might actually be desirable for the standard interface to not resemble ERC20 (although forcing every standard to have a different interface probably isn't sustainable).
The nice thing about adding a simple flag is that it is completely backwards compatible (ERC20 will throw when trying to access the nonexistent flag). Something like EIP #165 would result in false negatives for existing tokens, since they don't already include support for pseudo introspection.
jbaylina
commented
6 years ago What about just use a standard token (ERC20, ERC223, or whatever) with a total supply of only 1 wei ?
If you want, then you just create a "Factory contract" to create assets that are mainly ProxyContracts with a simple delegate call to a library contract that just acts as template.
dete
commented
6 years ago Thanks for thinking about creative solutions for this, @jbaylina!
We just went live with our implementation of ERC-721, and our users have created more than 2k NFTs in one day. I feel like issuing a new contract for each of these items would have been incredibly expensive.
But even if I'm overestimating the cost of contract creation, the functionality of our NFTs would have been much more expensive if every interaction between two NFTs ("breeding" in our case) would have required multiple cross-contract calls.
Finally, the complexity of building common wallet and exchange software seems like it would be much trickier, also. Not only would these things need some mechanism for enumerating all contracts that are "supposed" to be part of a particular NFT collection, it would need a mechanism for rejecting contracts that were not issued properly. Solvable, certainly, but also complex and potentially expensive.
eordano
commented
6 years ago In the spirit of standardization, I've turned Decentraland's Land contract compatible with these methods. Some notes:
tokenOfOwnerByIndex is efficient. Creating tokens takes ~120,000 gas.getAllTokens(address owner) public constant returns (uint[]) function for querying all of a users' tokens.tokenMetadata(uint tokenId) function doesn't take the preferredMethod parameter, in the spirit of KISS.Hope this is helpful, looking forward to hearing your feedback @dete!
eordano
commented
6 years ago Also, I think we should be wary of race conditions on calling approve, and block reorgs.
Sources: https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729 https://github.com/Giveth/minime/blob/master/contracts/MiniMeToken.sol#L253
PhABC
commented
6 years ago @dete Do you mind updating the OP with the newest draft (published 18 days ago)? Would be easier for people to understand that that's the current specification.
superphly
commented
6 years ago Not all NFTs are positive. What if, and stay with me here, an NFT had a negative effect. With tokens, we treat them as net positives. The logic of "who would be mad if someone deposited money into your bank account" rings true. But what if that NFT was something like a debt or in the case of a game a "negative spell". Another example might be "I don't want ownership of that piece of property, I can't afford the tax bill" or "I have a moral objection to owning a gun".
Sorry, this is a concern about not being able to refuse a NFT. Maybe make this a feature of the contract? Some can be claimable.
This proposal has been accepted and merged as a draft standard, please see the officially tracked version for the current draft.
Please see PR #841 for the discussions leading up to this draft, and use this thread (#721) for further discussion. (Or, if you have a concrete proposal, consider opening a new PR with your proposed changes.)
Original Draft (Sep 20, 2017)
## Preamble EIP:Second Draft (Nov 9, 2017)
## Preamble EIP: