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SIP009: the NFT standard

(The full SIP can be found here: https://github.com/stacksgov/sips/blob/main/sips/sip-009/sip-009-nft-standard.md.)

NFTs—short for _non-fungible tokens_—are digital assets with unique identifiers. Non-fungible means that the tokens are not equivalent to one another. Think of a stamp or trading card collection. Even though they are all stamps, one particular stamp can be rarer and thus more valuable compared to a more common one. A concert ticket is another example. They all give you access to the venue but some are for seats in the front whilst others put you in the back. Trading your front-row ticket for one in the back is not an equal exchange. NFTs are the crypto equivalent, they are digital collectibles.

The concept of NFTs as we know them today were first pioneered on the Ethereum blockchain. You have probably already come across the acronym ERC-721, which is the technical designation for an EIP721-compliant NFT on Ethereum¹. Reading up on its history is a fun exercise in itself.

NFTs are first-class members in Clarity. Built-in functions exist to define, mint, transfer, and burn NFTs. However, these built-ins do not define the public interface of NFT contracts. That is where SIP009 comes in. The standard enables interoperability so that different contracts and apps can interact with NFTs contracts seamlessly. Trustless marketplaces are made possible thanks to token standards. (In fact, we will build one in a later chapter.)

The SIP009 NFT trait

Creating a SIP009-compliant NFT comes down implementing a trait. We start by taking a look at the trait and then dissecting it piece by piece.

(define-trait sip009-nft-trait
    (
        ;; Last token ID, limited to uint range
        (get-last-token-id () (response uint uint))

        ;; URI for metadata associated with the token 
        (get-token-uri (uint) (response (optional (string-ascii 256)) uint))

        ;; Owner of a given token identifier
        (get-owner (uint) (response (optional principal) uint))

        ;; Transfer from the sender to a new principal
        (transfer (uint principal principal) (response bool uint))
    )
)

get-last-token-id

A read-only function that returns the token ID of the last NFT that was created by the contract. The ID can be used to iterate through the list and get an idea of how many NFTs are in existence—as long as the contract does not burn tokens. It should never return an err response.

get-token-uri

A read-only function that takes a token ID and returns a valid URI (a link) which resolves to a metadata file for that particular NFT. The idea is that you might want to provide some information about an NFT that you cannot or do not want to store on-chain. For example, the URI for an NFT that represents a piece of digital artwork could contain a link to the image file, the author, and so on. The SIP009 standard does not provide a specification for what should exist at the URI returned by get-token-uri. It is supposed to be covered by a future SIP.

If the supplied token ID does not exist, it returns none.

get-owner

A read-only function that takes a token ID and returns the principal that owns the specified NFT. If the supplied token ID does not exist, it returns none.

transfer

A public function that transfers ownership from one principal to another. If the token ID does not exist, then it must return an err response.