ERC-777 - Token Standard

Created 2017-11-20
Status Final
Category ERC
Type Standards Track
Authors

  • Jordi Baylina <jordi at baylina.cat>

  • Jacques Dafflon <mail at 0xjac.com>

  • Thomas Shababi <tom at truelevel.io>
Requires

Simple Summary

This EIP defines standard interfaces and behaviors for token contracts.

Abstract

This standard defines a new way to interact with a token contract while remaining backward compatible with ERC-20.

It defines advanced features to interact with tokens. Namely, operators to send tokens on behalf of another address—contract or regular account—and send/receive hooks to offer token holders more control over their tokens.

It takes advantage of ERC-1820 to find out whether and where to notify contracts and regular addresses when they receive tokens as well as to allow compatibility with already-deployed contracts.

Motivation

This standard tries to improve upon the widely used ERC-20 token standard. The main advantages of this standard are:

  1. Uses the same philosophy as Ether in that tokens are sent with send(dest, value, data).

  2. Both contracts and regular addresses can control and reject which token they send by registering a tokensToSend hook. (Rejection is done by reverting in the hook function.)

  3. Both contracts and regular addresses can control and reject which token they receive by registering a tokensReceived hook. (Rejection is done by reverting in the hook function.)

  4. The tokensReceived hook allows to send tokens to a contract and notify it in a single transaction, unlike ERC-20 which requires a double call (approve/transferFrom) to achieve this.

  5. The holder can "authorize" and "revoke" operators which can send tokens on their behalf. These operators are intended to be verified contracts such as an exchange, a cheque processor or an automatic charging system.

  6. Every token transaction contains data and operatorData bytes fields to be used freely to pass data from the holder and the operator, respectively.

  7. It is backward compatible with wallets that do not contain the tokensReceived hook function by deploying a proxy contract implementing the tokensReceived hook for the wallet.

Specification

ERC777Token (Token Contract)

interface ERC777Token {
    function name() external view returns (string memory);
    function symbol() external view returns (string memory);
    function totalSupply() external view returns (uint256);
    function balanceOf(address holder) external view returns (uint256);
    function granularity() external view returns (uint256);

    function defaultOperators() external view returns (address[] memory);
    function isOperatorFor(
        address operator,
        address holder
    ) external view returns (bool);
    function authorizeOperator(address operator) external;
    function revokeOperator(address operator) external;

    function send(address to, uint256 amount, bytes calldata data) external;
    function operatorSend(
        address from,
        address to,
        uint256 amount,
        bytes calldata data,
        bytes calldata operatorData
    ) external;

    function burn(uint256 amount, bytes calldata data) external;
    function operatorBurn(
        address from,
        uint256 amount,
        bytes calldata data,
        bytes calldata operatorData
    ) external;

    event Sent(
        address indexed operator,
        address indexed from,
        address indexed to,
        uint256 amount,
        bytes data,
        bytes operatorData
    );
    event Minted(
        address indexed operator,
        address indexed to,
        uint256 amount,
        bytes data,
        bytes operatorData
    );
    event Burned(
        address indexed operator,
        address indexed from,
        uint256 amount,
        bytes data,
        bytes operatorData
    );
    event AuthorizedOperator(
        address indexed operator,
        address indexed holder
    );
    event RevokedOperator(address indexed operator, address indexed holder);
}

The token contract MUST implement the above interface. The implementation MUST follow the specifications described below.

The token contract MUST register the ERC777Token interface with its own address via ERC-1820.

This is done by calling the setInterfaceImplementer function on the ERC-1820 registry with the token contract address as both the address and the implementer and the keccak256 hash of ERC777Token (0xac7fbab5f54a3ca8194167523c6753bfeb96a445279294b6125b68cce2177054) as the interface hash.

If the contract has a switch to enable or disable ERC-777 functions, every time the switch is triggered, the token MUST register or unregister the ERC777Token interface for its own address accordingly via ERC1820. Unregistering implies calling the setInterfaceImplementer with the token contract address as the address, the keccak256 hash of ERC777Token as the interface hash and 0x0 as the implementer. (See Set An Interface For An Address in ERC-1820 for more details.)

When interacting with the token contract, all amounts and balances MUST be unsigned integers. I.e. internally, all values are stored as a denomination of 1E-18 of a token. The display denomination—to display any amount to the end user—MUST be 1018 of the internal denomination.

In other words, the internal denomination is similar to a wei and the display denomination is similar to an ether. It is equivalent to an ERC-20's decimals function returning 18. E.g. if a token contract returns a balance of 500,000,000,000,000,000 (0.5×1018) for a user, the user interface MUST show 0.5 tokens to the user. If the user wishes to send 0.3 tokens, the contract MUST be called with an amount of 300,000,000,000,000,000 (0.3×1018).

User Interfaces which are generated programmatically from the ABI of the token contract MAY use and display the internal denomination. But this MUST be made clear, for example by displaying the uint256 type.

View Functions

The view functions detailed below MUST be implemented.

name function

function name() external view returns (string memory)

Get the name of the token, e.g., "MyToken".

identifier: 06fdde03
returns: Name of the token.

symbol function

function symbol() external view returns (string memory)

Get the symbol of the token, e.g., "MYT".

identifier: 95d89b41
returns: Symbol of the token.

totalSupply function

function totalSupply() external view returns (uint256)

Get the total number of minted tokens.

NOTE: The total supply MUST be equal to the sum of the balances of all addresses—as returned by the balanceOf function.

NOTE: The total supply MUST be equal to the sum of all the minted tokens as defined in all the Minted events minus the sum of all the burned tokens as defined in all the Burned events.

identifier: 18160ddd
returns: Total supply of tokens currently in circulation.

balanceOf function

function balanceOf(address holder) external view returns (uint256)

Get the balance of the account with address holder.

The balance MUST be zero (0) or higher.

identifier: 70a08231
parameters
holder: Address for which the balance is returned.

returns: Amount of tokens held by holder in the token contract.

granularity function

function granularity() external view returns (uint256)

Get the smallest part of the token that's not divisible.

In other words, the granularity is the smallest amount of tokens (in the internal denomination) which MAY be minted, sent or burned at any time.

The following rules MUST be applied regarding the granularity:

NOTE: Most tokens SHOULD be fully partition-able. I.e., this function SHOULD return 1 unless there is a good reason for not allowing any fraction of the token.

identifier: 556f0dc7
returns: The smallest non-divisible part of the token.

NOTE: defaultOperators and isOperatorFor are also view functions, defined under the operators for consistency.

ERC-20 compatibility requirement:
The decimals of the token MUST always be 18. For a pure ERC-777 token the ERC-20 decimals function is OPTIONAL, and its existence SHALL NOT be relied upon when interacting with the token contract. (The decimal value of 18 is implied.) For an ERC-20 compatible token, the decimals function is REQUIRED and MUST return 18. (In ERC-20, the decimals function is OPTIONAL. If the function is not present, the decimals value is not clearly defined and may be assumed to be 0. Hence for compatibility reasons, decimals MUST be implemented for ERC-20 compatible tokens.)

Operators

An operator is an address which is allowed to send and burn tokens on behalf of some holder.

When an address becomes an operator for a holder, an AuthorizedOperator event MUST be emitted. The AuthorizedOperator's operator (topic 1) and holder (topic 2) MUST be the addresses of the operator and the holder respectively.

When a holder revokes an operator, a RevokedOperator event MUST be emitted. The RevokedOperator's operator (topic 1) and holder (topic 2) MUST be the addresses of the operator and the holder respectively.

NOTE: A holder MAY have multiple operators at the same time.

The token MAY define default operators. A default operator is an implicitly authorized operator for all holders. AuthorizedOperator events MUST NOT be emitted when defining the default operators. The rules below apply to default operators:

The following rules apply to any operator:

NOTE: A holder MAY authorize an already authorized operator. An AuthorizedOperator MUST be emitted each time.

NOTE: A holder MAY revoke an already revoked operator. A RevokedOperator MUST be emitted each time.

AuthorizedOperator event 

event AuthorizedOperator(address indexed operator, address indexed holder)

Indicates the authorization of operator as an operator for holder.

NOTE: This event MUST NOT be emitted outside of an operator authorization process.

parameters
operator: Address which became an operator of holder.
holder: Address of a holder which authorized the operator address as an operator.

RevokedOperator event 

event RevokedOperator(address indexed operator, address indexed holder)

Indicates the revocation of operator as an operator for holder.

NOTE: This event MUST NOT be emitted outside of an operator revocation process.

parameters
operator: Address which was revoked as an operator of holder.
holder: Address of a holder which revoked the operator address as an operator.

The defaultOperators, authorizeOperator, revokeOperator and isOperatorFor functions described below MUST be implemented to manage operators. Token contracts MAY implement other functions to manage operators.

defaultOperators function 

function defaultOperators() external view returns (address[] memory)

Get the list of default operators as defined by the token contract.

NOTE: If the token contract does not have any default operators, this function MUST return an empty list.

identifier: 06e48538
returns: List of addresses of all the default operators.

authorizeOperator function

function authorizeOperator(address operator) external

Set a third party operator address as an operator of msg.sender to send and burn tokens on its behalf.

NOTE: The holder (msg.sender) is always an operator for itself. This right SHALL NOT be revoked. Hence this function MUST revert if it is called to authorize the holder (msg.sender) as an operator for itself (i.e. if operator is equal to msg.sender).

identifier: 959b8c3f
parameters
operator: Address to set as an operator for msg.sender.

revokeOperator function

function revokeOperator(address operator) external

Remove the right of the operator address to be an operator for msg.sender and to send and burn tokens on its behalf.

NOTE: The holder (msg.sender) is always an operator for itself. This right SHALL NOT be revoked. Hence this function MUST revert if it is called to revoke the holder (msg.sender) as an operator for itself (i.e., if operator is equal to msg.sender).

identifier: fad8b32a
parameters
operator: Address to rescind as an operator for msg.sender.

isOperatorFor function 

function isOperatorFor(
    address operator,
    address holder
) external view returns (bool)

Indicate whether the operator address is an operator of the holder address.

identifier: d95b6371
parameters
operator: Address which may be an operator of holder.
holder: Address of a holder which may have the operator address as an operator.

returns: true if operator is an operator of holder and false otherwise.

NOTE: To know which addresses are operators for a given holder, one MUST call isOperatorFor with the holder for each default operator and parse the AuthorizedOperator, and RevokedOperator events for the holder in question.

Sending Tokens

When an operator sends an amount of tokens from a holder to a recipient with the associated data and operatorData, the token contract MUST apply the following rules:

The token contract MUST revert when sending in any of the following cases:

The token contract MAY send tokens from many holders, to many recipients, or both. In this case:

NOTE: Mechanisms such as applying a fee on a send is considered as a send to multiple recipients: the intended recipient and the fee recipient.

NOTE: Movements of tokens MAY be chained. For example, if a contract upon receiving tokens sends them further to another address. In this case, the previous send rules apply to each send, in order.

NOTE: Sending an amount of zero (0) tokens is valid and MUST be treated as a regular send.

Implementation Requirement:
- The token contract MUST call the tokensToSend hook before updating the state. - The token contract MUST call the tokensReceived hook after updating the state.
I.e., tokensToSend MUST be called first, then the balances MUST be updated to reflect the send, and finally tokensReceived MUST be called afterward. Thus a balanceOf call within tokensToSend returns the balance of the address before the send and a balanceOf call within tokensReceived returns the balance of the address after the send.

NOTE: The data field contains information provided by the holder—similar to the data field in a regular ether send transaction. The tokensToSend() hook, the tokensReceived(), or both MAY use the information to decide if they wish to reject the transaction.

NOTE: The operatorData field is analogous to the data field except it SHALL be provided by the operator.

The operatorData MUST only be provided by the operator. It is intended more for logging purposes and particular cases. (Examples include payment references, cheque numbers, countersignatures and more.) In most of the cases the recipient would ignore the operatorData, or at most, it would log the operatorData.

Sent event 

event Sent(
    address indexed operator,
    address indexed from,
    address indexed to,
    uint256 amount,
    bytes data,
    bytes operatorData
)

Indicate a send of amount of tokens from the from address to the to address by the operator address.

NOTE: This event MUST NOT be emitted outside of a send or an ERC-20 transfer process.

parameters
operator: Address which triggered the send.
from: Holder whose tokens were sent.
to: Recipient of the tokens.
amount: Number of tokens sent.
data: Information provided by the holder.
operatorData: Information provided by the operator.

The send and operatorSend functions described below MUST be implemented to send tokens. Token contracts MAY implement other functions to send tokens.

send function

function send(address to, uint256 amount, bytes calldata data) external

Send the amount of tokens from the address msg.sender to the address to.

The operator and the holder MUST both be the msg.sender.

identifier: 9bd9bbc6
parameters
to: Recipient of the tokens.
amount: Number of tokens to send.
data: Information provided by the holder.

operatorSend function

function operatorSend(
    address from,
    address to,
    uint256 amount,
    bytes calldata data,
    bytes calldata operatorData
) external

Send the amount of tokens on behalf of the address from to the address to.

Reminder: If the operator address is not an authorized operator of the from address, then the send process MUST revert.

NOTE: from and msg.sender MAY be the same address. I.e., an address MAY call operatorSend for itself. This call MUST be equivalent to send with the addition that the operator MAY specify an explicit value for operatorData (which cannot be done with the send function).

identifier: 62ad1b83
parameters
from: Holder whose tokens are being sent.
to: Recipient of the tokens.
amount: Number of tokens to send.
data: Information provided by the holder.
operatorData: Information provided by the operator.

Minting Tokens

Minting tokens is the act of producing new tokens. ERC-777 intentionally does not define specific functions to mint tokens. This intent comes from the wish not to limit the use of the ERC-777 standard as the minting process is generally specific for every token.

Nonetheless, the rules below MUST be respected when minting for a recipient:

The token contract MUST revert when minting in any of the following cases:

NOTE: The initial token supply at the creation of the token contract MUST be considered as minting for the amount of the initial supply to the address(es) receiving the initial supply. This means one or more Minted events must be emitted and the tokensReceived hook of the recipient(s) MUST be called.

ERC-20 compatibility requirement:
While a Sent event MUST NOT be emitted when minting, if the token contract is ERC-20 backward compatible, a Transfer event with the from parameter set to 0x0 SHOULD be emitted as defined in the ERC-20 standard.

The token contract MAY mint tokens for multiple recipients at once. In this case:

NOTE: Minting an amount of zero (0) tokens is valid and MUST be treated as a regular mint.

NOTE: While during a send or a burn, the data is provided by the holder, it is inapplicable for a mint. In this case the data MAY be provided by the token contract or the operator, for example to ensure a successful minting to a holder expecting specific data.

NOTE: The operatorData field contains information provided by the operator—similar to the data field in a regular ether send transaction. The tokensReceived() hooks MAY use the information to decide if it wish to reject the transaction.

Minted event 

event Minted(
    address indexed operator,
    address indexed to,
    uint256 amount,
    bytes data,
    bytes operatorData
)

Indicate the minting of amount of tokens to the to address by the operator address.

NOTE: This event MUST NOT be emitted outside of a mint process.

parameters
operator: Address which triggered the mint.
to: Recipient of the tokens.
amount: Number of tokens minted.
data: Information provided for the recipient.
operatorData: Information provided by the operator.

Burning Tokens

Burning tokens is the act of destroying existing tokens. ERC-777 explicitly defines two functions to burn tokens (burn and operatorBurn). These functions facilitate the integration of the burning process in wallets and dapps. However, the token contract MAY prevent some or all holders from burning tokens for any reason. The token contract MAY also define other functions to burn tokens.

The rules below MUST be respected when burning the tokens of a holder:

The token contract MUST revert when burning in any of the following cases:

ERC-20 compatibility requirement:
While a Sent event MUST NOT be emitted when burning; if the token contract is ERC-20 enabled, a Transfer event with the to parameter set to 0x0 SHOULD be emitted. The ERC-20 standard does not define the concept of burning tokens, but this is a commonly accepted practice.

The token contract MAY burn tokens for multiple holders at once. In this case:

NOTE: Burning an amount of zero (0) tokens is valid and MUST be treated as a regular burn.

NOTE: The data field contains information provided by the holder—similar to the data field in a regular ether send transaction. The tokensToSend() hook, the tokensReceived(), or both MAY use the information to decide if they wish to reject the transaction.

NOTE: The operatorData field is analogous to the data field except it SHALL be provided by the operator.

Burned event 

event Burned(
    address indexed operator,
    address indexed from,
    uint256 amount,
    bytes data,
    bytes operatorData
);

Indicate the burning of amount of tokens from the from address by the operator address.

NOTE: This event MUST NOT be emitted outside of a burn process.

parameters
operator: Address which triggered the burn.
from: Holder whose tokens were burned.
amount: Number of tokens burned.
data: Information provided by the holder.
operatorData: Information provided by the operator.

The burn and operatorBurn functions described below MUST be implemented to burn tokens. Token contracts MAY implement other functions to burn tokens.

burn function

function burn(uint256 amount, bytes calldata data) external

Burn the amount of tokens from the address msg.sender.

The operator and the holder MUST both be the msg.sender.

identifier: fe9d9303
parameters
amount: Number of tokens to burn.
data: Information provided by the holder.

operatorBurn function

function operatorBurn(
    address from,
    uint256 amount,
    bytes calldata data,
    bytes calldata operatorData
) external

Burn the amount of tokens on behalf of the address from.

Reminder: If the operator address is not an authorized operator of the from address, then the burn process MUST revert.

identifier: fc673c4f
parameters
from: Holder whose tokens will be burned.
amount: Number of tokens to burn.
data: Information provided by the holder.
operatorData: Information provided by the operator.

NOTE: The operator MAY pass any information via operatorData. The operatorData MUST only be provided by the operator.

NOTE: from and msg.sender MAY be the same address. I.e., an address MAY call operatorBurn for itself. This call MUST be equivalent to burn with the addition that the operator MAY specify an explicit value for operatorData (which cannot be done with the burn function).

ERC777TokensSender And The tokensToSend Hook

The tokensToSend hook notifies of any request to decrement the balance (send and burn) for a given holder. Any address (regular or contract) wishing to be notified of token debits from their address MAY register the address of a contract implementing the ERC777TokensSender interface described below via ERC-1820.

This is done by calling the setInterfaceImplementer function on the ERC-1820 registry with the holder address as the address, the keccak256 hash of ERC777TokensSender (0x29ddb589b1fb5fc7cf394961c1adf5f8c6454761adf795e67fe149f658abe895) as the interface hash, and the address of the contract implementing the ERC777TokensSender as the implementer.

interface ERC777TokensSender {
    function tokensToSend(
        address operator,
        address from,
        address to,
        uint256 amount,
        bytes calldata userData,
        bytes calldata operatorData
    ) external;
}

NOTE: A regular address MAY register a different address—the address of a contract—implementing the interface on its behalf. A contract MAY register either its address or the address of another contract but said address MUST implement the interface on its behalf.

tokensToSend

function tokensToSend(
    address operator,
    address from,
    address to,
    uint256 amount,
    bytes calldata userData,
    bytes calldata operatorData
) external

Notify a request to send or burn (if to is 0x0) an amount tokens from the from address to the to address by the operator address.

NOTE: This function MUST NOT be called outside of a burn, send or ERC-20 transfer process.

identifier: 75ab9782
parameters
operator: Address which triggered the balance decrease (through sending or burning).
from: Holder whose tokens were sent.
to: Recipient of the tokens for a send (or 0x0 for a burn).
amount: Number of tokens the holder balance is decreased by.
data: Information provided by the holder.
operatorData: Information provided by the operator.

The following rules apply when calling the tokensToSend hook:

NOTE: Multiple holders MAY use the same implementation of ERC777TokensSender.

NOTE: An address can register at most one implementation at any given time for all ERC-777 tokens. Hence the ERC777TokensSender MUST expect to be called by different token contracts. The msg.sender of the tokensToSend call is expected to be the address of the token contract.

ERC-20 compatibility requirement:
This hook takes precedence over ERC-20 and MUST be called (if registered) when calling ERC-20's transfer and transferFrom event. When called from a transfer, operator MUST be the same value as the from. When called from a transferFrom, operator MUST be the address which issued the transferFrom call.

ERC777TokensRecipient And The tokensReceived Hook

The tokensReceived hook notifies of any increment of the balance (send and mint) for a given recipient. Any address (regular or contract) wishing to be notified of token credits to their address MAY register the address of a contract implementing the ERC777TokensRecipient interface described below via ERC-1820.

This is done by calling the setInterfaceImplementer function on the ERC-1820 registry with the recipient address as the address, the keccak256 hash of ERC777TokensRecipient (0xb281fc8c12954d22544db45de3159a39272895b169a852b314f9cc762e44c53b) as the interface hash, and the address of the contract implementing the ERC777TokensRecipient as the implementer.

interface ERC777TokensRecipient {
    function tokensReceived(
        address operator,
        address from,
        address to,
        uint256 amount,
        bytes calldata data,
        bytes calldata operatorData
    ) external;
}

If the recipient is a contract, which has not registered an ERC777TokensRecipient implementation; then the token contract:

NOTE: A regular address MAY register a different address—the address of a contract—implementing the interface on its behalf. A contract MUST register either its address or the address of another contract but said address MUST implement the interface on its behalf.

tokensReceived

function tokensReceived(
    address operator,
    address from,
    address to,
    uint256 amount,
    bytes calldata data,
    bytes calldata operatorData
) external

Notify a send or mint (if from is 0x0) of amount tokens from the from address to the to address by the operator address.

NOTE: This function MUST NOT be called outside of a mint, send or ERC-20 transfer process.

identifier: 0023de29
parameters
operator: Address which triggered the balance increase (through sending or minting).
from: Holder whose tokens were sent (or 0x0 for a mint).
to: Recipient of the tokens.
amount: Number of tokens the recipient balance is increased by.
data: Information provided by the holder.
operatorData: Information provided by the operator.

The following rules apply when calling the tokensReceived hook:

NOTE: Multiple holders MAY use the same implementation of ERC777TokensRecipient.

NOTE: An address can register at most one implementation at any given time for all ERC-777 tokens. Hence the ERC777TokensRecipient MUST expect to be called by different token contracts. The msg.sender of the tokensReceived call is expected to be the address of the token contract.

ERC-20 compatibility requirement:
This hook takes precedence over ERC-20 and MUST be called (if registered) when calling ERC-20's transfer and transferFrom event. When called from a transfer, operator MUST be the same value as the from. When called from a transferFrom, operator MUST be the address which issued the transferFrom call.

Note On Gas Consumption

Dapps and wallets SHOULD first estimate the gas required when sending, minting, or burning tokens—using eth_estimateGas—to avoid running out of gas during the transaction.

Logo

Image beige logo white logo light grey logo dark grey logo black logo
Color beige white light grey dark grey black
Hex #C99D66 #FFFFFF #EBEFF0 #3C3C3D #000000

The logo MAY be used, modified and adapted to promote valid ERC-777 token implementations and ERC-777 compliant technologies such as wallets and dapps.

ERC-777 token contract authors MAY create a specific logo for their token based on this logo.

The logo MUST NOT be used to advertise, promote or associate in any way technology—such as tokens—which is not ERC-777 compliant.

The logo for the standard can be found in the /static/assets/eip-777/logo folder in SVG and PNG formats. The PNG version of the logo offers a few sizes in pixels. If needed, other sizes MAY be created by converting from SVG into PNG.

Rationale

The principal intent for this standard is to solve some of the shortcomings of ERC-20 while maintaining backward compatibility with ERC-20, and avoiding the problems and vulnerabilities of EIP-223.

Below are the rationales for the decisions regarding the main aspects of the standards.

NOTE: Jacques Dafflon (0xjac), one of the authors of the standard, conjointly wrote his master thesis on the standard, which goes in more details than could reasonably fit directly within the standard, and can provide further clarifications regarding certain aspects or decisions.

Lifecycle

More than just sending tokens, ERC-777 defines the entire lifecycle of a token, starting with the minting process, followed by the sending process and terminating with the burn process.

Having a lifecycle clearly defined is important for consistency and accuracy, especially when value is derived from scarcity. In contrast when looking at some ERC-20 tokens, a discrepancy can be observed between the value returned by the totalSupply and the actual circulating supply, as the standard does not clearly define a process to create and destroy tokens.

Data

The mint, send and burn processes can all make use of a data and operatorData fields which are passed to any movement (mint, send or burn). Those fields may be empty for simple use cases, or they may contain valuable information related to the movement of tokens, similar to information attached to a bank transfer by the sender or the bank itself.

The use of a data field is equally present in other standard proposals such as EIP-223, and was requested by multiple members of the community who reviewed this standard.

Hooks

In most cases, ERC-20 requires two calls to safely transfer tokens to a contract without locking them. A call from the sender, using the approve function and a call from the recipient using transferFrom. Furthermore, this requires extra communication between the parties which is not clearly defined. Finally, holders can get confused between transfer and approve/transferFrom. Using the former to transfer tokens to a contract will most likely result in locked tokens.

Hooks allow streamlining of the sending process and offer a single way to send tokens to any recipient. Thanks to the tokensReceived hook, contracts are able to react and prevent locking tokens upon reception.

Greater Control For Holders

The tokensReceived hook also allows holders to reject the reception of some tokens. This gives greater control to holders who can accept or reject incoming tokens based on some parameters, for example located in the data or operatorData fields.

Following the same intentions and based on suggestions from the community, the tokensToSend hook was added to give control over and prevent the movement of outgoing tokens.

ERC-1820 Registry

The ERC-1820 Registry allows holders to register their hooks. Other alternatives were examined beforehand to link hooks and holders.

The first was for hooks to be defined at the sender's or recipient's address. This approach is similar to EIP-223 which proposes a tokenFallback function on recipient contracts to be called when receiving tokens, but improves on it by relying on ERC-165 for interface detection. While straightforward to implement, this approach imposes several limitations. In particular, the sender and recipient must be contracts in order to provide their implementation of the hooks. Preventing externally owned addresses to benefit from hooks. Existing contracts have a strong probability not to be compatible, as they undoubtedly were unaware and do not define the new hooks. Consequently existing smart contract infrastructure such as multisig wallets which potentially hold large amounts of ether and tokens would need to be migrated to new updated contracts.

The second approach considered was to use ERC-672 which offered pseudo-introspection for addresses using reverse-ENS. However, this approach relied heavily on ENS, on top of which reverse lookup would need to be implemented. Analysis of this approach promptly revealed a certain degree of complexity and security concerns which would transcend the benefits of approach.

The third solution—used in this standard—is to rely on a unique registry where any address can register the addresses of contracts implementing the hooks on its behalf. This approach has the advantage that externally owned accounts and contracts can benefit from hooks, including existing contracts which can rely on hooks deployed on proxy contracts.

The decision was made to keep this registry in a separate EIP, as to not over complicate this standard. More importantly, the registry is designed in a flexible fashion, such that other EIPs and smart contract infrastructures can benefit from it for their own use cases, outside the realm of ERC-777 and tokens. The first proposal for this registry was ERC-820. Unfortunately, issues emanating from upgrades in the Solidity language to versions 0.5 and above resulted in a bug in a separated part of the registry, which required changes. This was discovered right after the last call period. Attempts made to avoid creating a separate EIP, such as ERC-820a, were rejected. Hence the standard for the registry used for ERC-777 became ERC-1820. ERC-1820 and ERC-820 are functionally equivalent. ERC-1820 simply contains the fix for newer versions of Solidity.

Operators

The standard defines the concept of operators as any address which moves tokens. While intuitively every address moves its own tokens, separating the concepts of holder and operator allows for greater flexibility. Primarily, this originates from the fact that the standard defines a mechanism for holders to let other addresses become their operators. Moreover, unlike the approve calls in ERC-20 where the role of an approved address is not clearly defined, ERC-777 details the intent of and interactions with operators, including an obligation for operators to be approved, and an irrevocable right for any holder to revoke operators.

Default Operators

Default operators were added based on community demand for pre-approved operators. That is operators which are approved for all holders by default. For obvious security reasons, the list of default operators is defined at the token contract creation time, and cannot be changed. Any holder still has the right to revoke default operators. One of the obvious advantages of default operators is to allow ether-less movements of tokens. Default operators offer other usability advantages, such as allowing token providers to offer functionality in a modular way, and to reduce the complexity for holders to use features provided through operators.

Backward Compatibility

This EIP does not introduce backward incompatibilities and is backward compatible with the older ERC-20 token standard.

This EIP does not use transfer and transferFrom and uses send and operatorSend to avoid confusion and mistakes when deciphering which token standard is being used.

This standard allows the implementation of ERC-20 functions transfer, transferFrom, approve and allowance alongside to make a token fully compatible with ERC-20.

The token MAY implement decimals() for backward compatibility with ERC-20. If implemented, it MUST always return 18.

Therefore a token contract MAY implement both ERC-20 and ERC-777 in parallel. The specification of the view functions (such as name, symbol, balanceOf, totalSupply) and internal data (such as the mapping of balances) overlap without problems. Note however that the following functions are mandatory in ERC-777 and MUST be implemented: name, symbol balanceOf and totalSupply (decimals is not part of the ERC-777 standard).

The state-modifying functions from both standards are decoupled and can operate independently from each other. Note that ERC-20 functions SHOULD be limited to only being called from old contracts.

If the token implements ERC-20, it MUST register the ERC20Token interface with its own address via ERC-1820. This is done by calling the setInterfaceImplementer function on the ERC-1820 registry with the token contract address as both the address and the implementer and the keccak256 hash of ERC20Token (0xaea199e31a596269b42cdafd93407f14436db6e4cad65417994c2eb37381e05a) as the interface hash.

If the contract has a switch to enable or disable ERC-20 functions, every time the switch is triggered, the token MUST register or unregister the ERC20Token interface for its own address accordingly via ERC1820. Unregistering implies calling the setInterfaceImplementer with the token contract address as the address, the keccak256 hash of ERC20Token as the interface hash and 0x0 as the implementer. (See Set An Interface For An Address in ERC-1820 for more details.)

The difference for new contracts implementing ERC-20 is that tokensToSend and tokensReceived hooks take precedence over ERC-20. Even with an ERC-20 transfer and transferFrom call, the token contract MUST check via ERC-1820 if the from and the to address implement tokensToSend and tokensReceived hook respectively. If any hook is implemented, it MUST be called. Note that when calling ERC-20 transfer on a contract, if the contract does not implement tokensReceived, the transfer call SHOULD still be accepted even if this means the tokens will probably be locked.

The table below summarizes the different actions the token contract MUST take when sending, minting and transferring token via ERC-777 and ERC-20:

ERC1820 to address ERC777 Sending And Minting ERC20 transfer/transferFrom
ERC777TokensRecipient
registered 		regular address MUST call tokensReceived
contract
ERC777TokensRecipient
not registered 		regular address continue
contract MUST revert SHOULD continue1

1. The transaction SHOULD continue for clarity as ERC20 is not aware of hooks.
However, this can result in accidentally locked tokens. If avoiding accidentally locked tokens is paramount, the transaction MAY revert.

There is no particular action to take if tokensToSend is not implemented. The movement MUST proceed and only be canceled if another condition is not respected such as lack of funds or a revert in tokensReceived (if present).

During a send, mint and burn, the respective Sent, Minted and Burned events MUST be emitted. Furthermore, if the token contract declares that it implements ERC20Token via ERC-1820, the token contract SHOULD emit a Transfer event for minting and burning and MUST emit a Transfer event for sending (as specified in the ERC-20 standard). During an ERC-20's transfer or transferFrom functions, a valid Sent event MUST be emitted.

Hence for any movement of tokens, two events MAY be emitted: an ERC-20 Transfer and an ERC-777 Sent, Minted or Burned (depending on the type of movement). Third-party developers MUST be careful not to consider both events as separate movements. As a general rule, if an application considers the token as an ERC20 token, then only the Transfer event MUST be taken into account. If the application considers the token as an ERC777 token, then only the Sent, Minted and Burned events MUST be considered.

Test Cases

The repository with the reference implementation contains all the tests.

Implementation

The GitHub repository 0xjac/ERC777 contains the reference implementation. The reference implementation is also available via npm and can be installed with npm install erc777.

Copyright

Copyright and related rights waived via CC0.