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FA2 tokens

The FA2 standard supports several different token types, including fungible and non-fungible tokens.

Adhering to the FA2 standard allows developers to create new types of tokens while ensuring that the tokens work with existing wallets and applications. The FA2 standard leaves enough freedom for developers to define rules for transferring tokens and for how tokens behave.

Because a single FA2 contract can define multiple types of tokens, such as multiple types of fungible tokens or multiple different NFTs, each token type has an ID. If the contract has only one type of token, its ID must be 0, but if it has multiple types of tokens, the IDs can be any distinct values.

For the full details of the FA2 standard, see Tezos Improvement Proposal 12 (TZIP-12), which defines the standard.

Examples​

For examples of FA2 contracts, see Sample smart contracts.

Metadata​

Any FA2 token has some metadata that describes what the token represents. The standard provides multiple options for the structure of the metadata and it refers to other standards for how the metadata is stored. FA2 suggests that contracts store metadata according to TZIP-16. For examples of working with metadata, see the NFT-related tutorials at Create an NFT.

Operators​

Similar to allowances in FA1.2 tokens, FA2 token owners can allow other accounts to transfer tokens on their behalf. Accounts that are authorized to transfer other accounts' tokens in this way are called operators. For example, a user might want to sell a token on a marketplace, so they set the marketplace as an operator of that token type, which allows the marketplace to sell the token without getting further approval from the owner. Unlike allowances in FA1.2 tokens, operators can transfer any number of the owner's tokens of the specified type.

Minting and burning​

FA2 does not require contracts to provide entrypoints that mint (create) or burn (destroy) tokens, but it permits developers to add those entrypoints if they choose. If the contract does not have a mint entrypoint, it can create tokens in some other way or developers can initialize its storage with all of the tokens that it will ever have.

Entrypoints​

FA2 contracts must have these entrypoints:

  • transfer: Transfers tokens from a source account to one or more destination accounts. Its parameters are the address of the source account and a list of destination accounts, each with the token ID and amount to transfer.

  • balance_of: Sends information about an owner's token balance to another contract. Its parameters are a callback contract that accepts a list of token IDs and the amount that the specified account owns.

  • update_operators: Adds or removes operators for the specified token owners and token IDs. Its parameters are a list of commands to add or remove operators for token owners and IDs.

The standard defines what happens when these entrypoints are called, the format of their parameters, and error cases (see below). For information about these requirements, see TZIP-12.

FA2 contracts can add any other entrypoints in addition to the required entrypoints.

Errors​

FA2 defines a list of errors that contracts must create, such as "FA2_TOKEN_UNDEFINED" if a transaction refers to a token ID that doesn't exist. For a list of these errors, see TZIP-12.

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