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 [ERC820] 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.
2. Both contracts and regular addresses can control and reject which token they send by registering a `tokensToSend` hook. (Rejection is done by `revert`ing 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 `revert`ing in the hook function.)
4. The `tokensReceived` hook allows to send tokens to a contract and notify it in a single transaction, unlike [ERC20] which require a double call (`approve`/`transferFrom`) to achieve this.
5. The token 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 token 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.
The token contract MUST register the `ERC777Token` interface with its own address via [ERC820]. If the contract has a switch to enable or disable [ERC777] functions, every time the switch is triggered, the token MUST register or unregister the `ERC777Token` interface for its own address accordingly via [ERC820]. (Unregistering implies setting the address to `0x0`.)
The smallest unit—for all interactions with the token contract—MUST be `1`. I.e. all amounts and balances MUST be unsigned integers. The display denomination—to display any amount to the end user—MUST be 10<sup>18</sup> of the smallest.
In other words the technical denomination is similar to a wei and the display denomination is similar to an ether. It is equivalent to an [ERC20]'s `decimals` function returning `18`. E.g. if a token contract holds a balance of `500,000,000,000,000,000` (0.5×10<sup>18</sup>) for a user, the user interface SHOULD 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` (0.3×10<sup>18</sup>).
*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. (This applies as well to [tokens minted when the token contract is created][initial supply].)
- Any operation that would result in a balance that's not a multiple of the *granularity* value MUST be considered invalid, and the transaction MUST `revert`.
*NOTE*: Most of the tokens SHOULD be fully partitionable. I.e., this function SHOULD return `1` unless there is a good reason for not allowing any fraction of the token.
*NOTE*: [`defaultOperators`][defaultOperators] and [`isOperatorFor`][isOperatorFor] are also `view` functions, defined under the [operators] for consistency.
The decimals of the token MUST always be `18`. For a *pure* ERC777 token the [ERC20] `decimal` 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 [ERC20] compatible token, the `decimal` function is REQUIRED and MUST return `18`. (In [ERC20], 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 [ERC20] compatible tokens.)
When an address becomes an *operator* for a *token holder*, an `AuthorizedOperator` event MUST be emitted. The `AuthorizedOperator`'s `operator` (topic 1) and `tokenHolder` (topic 2) MUST be the addresses of the *operator* and the *token holder* respectively.
When a *token holder* revokes an *operator*, a `RevokedOperator` event MUST be emitted. The `RevokedOperator`'s `operator` (topic 1) and `tokenHolder` (topic 2) MUST be the addresses of the *operator* and the *token holder* respectively.
The token MAY define *default operators*. A *default operator* is an implicitly authorized *operator* for all *token holders*. `AuthorizedOperator` events MUST NOT be emitted when defining the *default operators*. The rules below apply to *default operators*:
- The *default operators* MUST be invariants. I.e., the token contract MUST NOT add or remove *default operators* ever.
-`AuthorizedOperator` events MUST NOT be emitted when defining *default operators*.
- A *token holder* MUST be allowed revoke a *default operator* (unless the *token holder* is the *default operator* in question).
- A *token holder* MUST be allowed to re-authorize a previously revoked *default operator*.
- When a *default operator* is explicitly authorized or revoked for a specific *token holder*, an `AuthorizedOperator` or `RevokedOperator` event (respectively) MUST be emitted.
- The token contract MUST emit an `AuthorizedOperator` event with the correct values when a *token holder* authorizes an address as its *operator* as defined in the [`AuthorizedOperator` Event][authorizedoperator].
- The token contract MUST emit a `RevokedOperator` event with the correct values when a *token holder* revokes an address as its *operator* as defined in the [`RevokedOperator` Event][revokedoperator].
*NOTE*: A *token holder* MAY authorize an already authorized *operator*. An `AuthorizedOperator` MUST be emitted each time.
*NOTE*: A *token holder* MAY revoke an already revoked *operator*. A `RevokedOperator` MUST be emitted each time.
*NOTE*: A token holder MAY have multiple *operators* at the same time.
*NOTE*: The *token 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 token holder (`msg.sender`) as an *operator* for itself (i.e. if `operator` is equal to `msg.sender`).
*NOTE*: The *token 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 token holder (`msg.sender`) as an *operator* for itself (i.e., if `operator` is equal to `msg.sender`).
*NOTE*: To know which addresses are *operators* for a given *token holder*, one MUST call `isOperatorFor` with the *token holder* for each *default operator* and parse the `AuthorizedOperator`, and `RevokedOperator` events for the *token holder* in question.
When an *operator* sends an `amount` of tokens from a *token holder* to a *recipient* with the associated `data` and `operatorData`, the token contract MUST apply the following rules:
- The balance of the *token holder* MUST be greater or equal to the `amount`—such that its resulting balance is greater or equal to zero (`0`) after the send.
- The token contract MUST emit a `Sent` event with the correct values as defined in the [`Sent` Event][sent].
- The token contract MUST call the `tokensToSend` hook of the *token holder* if the *token holder* registers an `ERC777TokensSender` implementation via [ERC820].
- The token contract MUST call the `tokensReceived` hook of the *recipient* if the *recipient* registers an `ERC777TokensRecipient` implementation via [ERC820].
- The `data` and `operatorData` MUST be immutable during the entire send process—hence the same `data` and `operatorData` MUST be used to call both hooks and emit the `Sent` event.
The token contract MUST `revert` when sending in any of the following cases:
- The *operator* address is not an authorized operator for the *token holder*.
- The resulting *token holder* balance or *recipient* balance after the send is not a multiple of the *granularity* defined by the token contract.
*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*: Transfer 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.
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 token 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*.
Typically, `data` is used to describe the intent behind the send. 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`.
The *operator* MUST be `msg.sender`. The value of `from` MAY be `0x0`, then the `from` (*token holder*) used for the send MUST be `msg.sender` (the `operator`).
*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).
Minting tokens is the act of producing new tokens. [ERC777] intentionally does not define specific functions to mint tokens. This intent comes from the wish not to limit the use of the [ERC777] standard as the minting process is generally specific for every token.
- The token contract MUST call the `tokensReceived` hook of the *recipient* if the *recipient* registers an `ERC777TokensRecipient` implementation via [ERC820].
- The `data` and `operatorData` MUST be immutable during the entire mint process—hence the same `data` and `operatorData` MUST be used to call the `tokensReceived` hook and emit the `Minted` event.
*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.
While a `Sent` event MUST NOT be emitted when minting, if the token contract is [ERC20] backward compatible, a `Transfer` event with the `from` parameter set to `0x0` SHOULD be emitted as defined in the [ERC20] standard.
Burning tokens is the act of destroying existing tokens. [ERC777] 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 *token holders* from burning tokens for any reason. The token contract MAY also define other functions to burn tokens.
- The token contract MUST call the `tokensToSend` hook of the *token holder* if the *token holder* registers an `ERC777TokensSender` implementation via [ERC820].
- The `operatorData` MUST be immutable during the entire burn process—hence the same `operatorData` MUST be used to call the `tokensToSend` hook and emit the `Burned` event.
While a `Sent` event MUST NOT be emitted when burning; if the token contract is [ERC20] enabled, a `Transfer` event with the `to` parameter set to `0x0` SHOULD be emitted. The [ERC20] standard does not define the concept of burning tokens, but this is a commonly accepted practice.
The *operator* MUST be `msg.sender`. The value of `from` MAY be `0x0`, then the `from` (*token holder*) used for the burn MUST be `msg.sender` (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).
The `tokensToSend` hook notifies of any decrement of balance (send and burn) for a given *token 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 [ERC820].
*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.
*NOTE*: An address can register at most one implementation at any given time for all [ERC777] 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.
This hook takes precedence over [ERC20] and MUST be called (if registered) when calling [ERC20]'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.
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 `ERC777TokensSender` interface described below via [ERC820].
- SHOULD accept if the `tokensReceived` hook is called from an ERC20 `transfer` or `transferFrom` call.
*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.
*NOTE*: Multiple *token holders* MAY use the same implementation of `ERC777TokensRecipient`.
*NOTE*: An address can register at most one implementation at any given time for all [ERC777] 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.
This hook takes precedence over [ERC20] and MUST be called (if registered) when calling [ERC20]'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.
Dapps and wallets SHOULD first estimate the gas required when sending, minting, or burning tokens—using [`eth_estimateGas`][eth_estimateGas]—to avoid running out of gas during the transaction.
The logo MAY be used, modified and adapted to promote valid [ERC777] token implementations and [ERC777] compliant technologies such as wallets and dapps.
The logo for the standard can be found in the [`/assets/eip-777/logo`][logos] 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`.
This standard solves some of the shortcomings of [ERC20] while maintaining backward compatibility with [ERC20]. It avoids the problems and vulnerabilities of [EIP223].
It goes a step further by allowing *operators* (generally contracts) which can manage the tokens in the same way that the [ERC20] with infinite `approve` was allowed. Finally, it adds hooks to provide further control to *token holders* over their tokens. Note that, the usage of [ERC820] provides backward compatibility with wallets and existing contracts without having to be redeployed thanks proxy contracts implementing the hooks.
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 [ERC20] functions `transfer`, `transferFrom`, `approve` and `allowance` alongside to make a token fully compatible with [ERC20].
Therefore a token contract MAY implement both [ERC20] and [ERC777] 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 [ERC777] and MUST be implemented: `name`, `symbol``balanceOf` and `totalSupply` (`decimal` is not part of the [ERC777] standard).
The state-modifying functions from both standards are decoupled and can operate independently from each other. Note that [ERC20] functions SHOULD be limited to only being called from old contracts.
If the token implements [ERC20], it MUST register the `ERC20Token` interface with its own address via [ERC820]. If the contract has a switch to enable or disable [ERC20] functions, every time the switch is triggered, the token MUST register or unregister its own address accordingly the `ERC20Token` interface via [ERC820]. (Unregistering implies setting the address to `0x0`.)
The difference for new contracts implementing [ERC20] is that `tokensToSend` and `tokensReceived` hooks take precedence over [ERC20]. Even with an [ERC20] `transfer` and `transferFrom` call, the token contract MUST check via [ERC820] 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 [ERC20] `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.
There is no particular action to take if `tokensToSend` is not implemented. The transfer 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 [ERC820], the token contract SHOULD emit a `Transfer` event for minting and burning and MUST emit a `Transfer` event for sending (as specified in the [ERC20] standard). During an [ERC20]'s `transfer` or `transferFrom` functions, a valid `Sent` event MUST be emitted.
Hence for any movement of tokens, two events MAY be emitted: an [ERC20] `Transfer` and an [ERC777] `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.
The GitHub repository [jacquesd/ERC777] contains the [reference implementation]. The reference implementation is also available via [npm][npm/erc777] and can be installed with `npm install erc777`.
