ERC-777: Token Standard
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:
-
Uses the same philosophy as Ether in that tokens are sent with
send(dest, value, data)
. -
Both contracts and regular addresses can control and reject which token they send by registering a
tokensToSend
hook. (Rejection is done byrevert
ing in the hook function.) -
Both contracts and regular addresses can control and reject which token they receive by registering a
tokensReceived
hook. (Rejection is done byrevert
ing in the hook function.) -
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. -
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.
-
Every token transaction contains
data
andoperatorData
bytes fields to be used freely to pass data from the holder and the operator, respectively. -
It is backward compatible with wallets that do not contain the
tokensReceived
hook function by deploying a proxy contract implementing thetokensReceived
hook for the wallet.
Specification
ERC777Token (Token Contract)
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 thekeccak256
hash ofERC777Token
(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
Get the name of the token, e.g., "MyToken"
.
identifier:
06fdde03
returns: Name of the token.
symbol
function
Get the symbol of the token, e.g., "MYT"
.
identifier:
95d89b41
returns: Symbol of the token.
totalSupply
function
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
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
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:
-
The granularity value MUST be set at creation time.
-
The granularity value MUST NOT be changed, ever.
-
The granularity value MUST be greater than or equal to
1
. -
All balances MUST be a multiple of the granularity.
-
Any amount of tokens (in the internal denomination) minted, sent or burned MUST be a multiple of the granularity value.
-
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 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 token contract MUST define default operators at creation time.
-
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 holder MUST be allowed to revoke a default operator (unless the holder is the default operator in question).
-
A holder MUST be allowed to re-authorize a previously revoked default operator.
-
When a default operator is explicitly authorized or revoked for a specific holder, an
AuthorizedOperator
orRevokedOperator
event (respectively) MUST be emitted.
The following rules apply to any operator:
-
An address MUST always be an operator for itself. Hence an address MUST NOT ever be revoked as its own operator.
-
If an address is an operator for a holder,
isOperatorFor
MUST returntrue
. -
If an address is not an operator for a holder,
isOperatorFor
MUST returnfalse
. -
The token contract MUST emit an
AuthorizedOperator
event with the correct values when a holder authorizes an address as its operator as defined in theAuthorizedOperator
Event. -
The token contract MUST emit a
RevokedOperator
event with the correct values when a holder revokes an address as its operator as defined in theRevokedOperator
Event.
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.
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 ofholder
.
holder
: Address of a holder which authorized theoperator
address as an operator.
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 ofholder
.
holder
: Address of a holder which revoked theoperator
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.
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
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 formsg.sender
.
revokeOperator
function
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 formsg.sender
.
Indicate whether the operator
address is an operator of the holder
address.
identifier:
d95b6371
parameters
operator
: Address which may be an operator ofholder
.
holder
: Address of a holder which may have theoperator
address as an operator.returns:
true
ifoperator
is an operator ofholder
andfalse
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:
-
Any authorized operator MAY send tokens to any recipient (except to
0x0
). -
The balance of the holder MUST be decreased by the
amount
. -
The balance of the recipient MUST be increased by the
amount
. -
The balance of the 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 theSent
Event. -
The operator MAY include information in the
operatorData
. -
The token contract MUST call the
tokensToSend
hook of the holder if the holder registers anERC777TokensSender
implementation via ERC-1820. -
The token contract MUST call the
tokensReceived
hook of the recipient if the recipient registers anERC777TokensRecipient
implementation via ERC-1820. -
The
data
andoperatorData
MUST be immutable during the entire send process—hence the samedata
andoperatorData
MUST be used to call both hooks and emit theSent
event.
The token contract MUST revert
when sending in any of the following cases:
-
The operator address is not an authorized operator for the holder.
-
The resulting holder balance or recipient balance after the send is not a multiple of the granularity defined by the token contract.
-
The recipient is a contract, and it does not implement the
ERC777TokensRecipient
interface via ERC-1820. -
The address of the holder or the recipient is
0x0
. -
Any of the resulting balances becomes negative, i.e. becomes less than zero (
0
). -
The
tokensToSend
hook of the holderrevert
s. -
The
tokensReceived
hook of the recipientrevert
s.
The token contract MAY send tokens from many holders, to many recipients, or both. In this case:
- The previous send rules MUST apply to all the holders and all the recipients.
- The sum of all the balances incremented MUST be equal to the total sent
amount
. - The sum of all the balances decremented MUST be equal to the total sent
amount
. - A
Sent
event MUST be emitted for every holder and recipient pair with the corresponding amount for each pair. - The sum of all the amounts from the
Sent
event MUST be equal to the total sentamount
.
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 finallytokensReceived
MUST be called afterward. Thus abalanceOf
call withintokensToSend
returns the balance of the address before the send and abalanceOf
call withintokensReceived
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
.
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
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
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:
-
Tokens MAY be minted for any recipient address (except
0x0
). -
The total supply MUST be increased by the amount of tokens minted.
-
The balance of
0x0
MUST NOT be decreased. -
The balance of the recipient MUST be increased by the amount of tokens minted.
-
The token contract MUST emit a
Minted
event with the correct values as defined in theMinted
Event. -
The token contract MUST call the
tokensReceived
hook of the recipient if the recipient registers anERC777TokensRecipient
implementation via ERC-1820. -
The
data
andoperatorData
MUST be immutable during the entire mint process—hence the samedata
andoperatorData
MUST be used to call thetokensReceived
hook and emit theMinted
event.
The token contract MUST revert
when minting in any of the following cases:
- The resulting recipient balance after the mint is not a multiple of the granularity defined by the token contract.
- The recipient is a contract, and it does not implement the
ERC777TokensRecipient
interface via ERC-1820. - The address of the recipient is
0x0
. - The
tokensReceived
hook of the recipientrevert
s.
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:
- The previous mint rules MUST apply to all the recipients.
- The sum of all the balances incremented MUST be equal to the total minted amount.
- A
Minted
event MUST be emitted for every recipient with the corresponding amount for each recipient. - The sum of all the amounts from the
Minted
event MUST be equal to the total mintedamount
.
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.
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:
-
Tokens MAY be burned from any holder address (except
0x0
). -
The total supply MUST be decreased by the amount of tokens burned.
-
The balance of
0x0
MUST NOT be increased. -
The balance of the holder MUST be decreased by amount of tokens burned.
-
The token contract MUST emit a
Burned
event with the correct values as defined in theBurned
Event. -
The token contract MUST call the
tokensToSend
hook of the holder if the holder registers anERC777TokensSender
implementation via ERC-1820. -
The
operatorData
MUST be immutable during the entire burn process—hence the sameoperatorData
MUST be used to call thetokensToSend
hook and emit theBurned
event.
The token contract MUST revert
when burning in any of the following cases:
-
The operator address is not an authorized operator for the holder.
-
The resulting holder balance after the burn is not a multiple of the granularity defined by the token contract.
-
The balance of holder is inferior to the amount of tokens to burn (i.e., resulting in a negative balance for the holder).
-
The address of the holder is
0x0
. -
The
tokensToSend
hook of the holderrevert
s.
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:
- The previous burn rules MUST apply to each holders.
- The sum of all the balances decremented MUST be equal to the total burned amount.
- A
Burned
event MUST be emitted for every holder with the corresponding amount for each holder. - The sum of all the amounts from the
Burned
event MUST be equal to the total burnedamount
.
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.
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
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
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, thekeccak256
hash ofERC777TokensSender
(0x29ddb589b1fb5fc7cf394961c1adf5f8c6454761adf795e67fe149f658abe895
) as the interface hash, and the address of the contract implementing theERC777TokensSender
as the implementer.
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
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 (or0x0
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:
-
The
tokensToSend
hook MUST be called for every send and burn processes. -
The
tokensToSend
hook MUST be called before the state is updated—i.e. before the balance is decremented. -
operator
MUST be the address which triggered the send or burn process. -
from
MUST be the address of the holder whose tokens are sent or burned. -
to
MUST be the address of the recipient which receives the tokens for a send. -
to
MUST be0x0
for a burn. -
amount
MUST be the number of tokens the holder sent or burned. -
data
MUST contain the extra information (if any) provided to the send or the burn process. -
operatorData
MUST contain the extra information provided by the address which triggered the decrease of the balance (if any). -
The holder MAY block a send or burn process by
revert
ing. (I.e., reject the withdrawal of tokens from its account.)
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, thekeccak256
hash ofERC777TokensRecipient
(0xb281fc8c12954d22544db45de3159a39272895b169a852b314f9cc762e44c53b
) as the interface hash, and the address of the contract implementing theERC777TokensRecipient
as the implementer.
If the recipient is a contract, which has not registered an ERC777TokensRecipient
implementation;
then the token contract:
-
MUST
revert
if thetokensReceived
hook is called from a mint or send call. -
SHOULD continue processing the transaction if the
tokensReceived
hook is called from an ERC-20transfer
ortransferFrom
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.
tokensReceived
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 (or0x0
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:
-
The
tokensReceived
hook MUST be called for every send and mint processes. -
The
tokensReceived
hook MUST be called after the state is updated—i.e. after the balance is incremented. -
operator
MUST be the address which triggered the send or mint process. -
from
MUST be the address of the holder whose tokens are sent for a send. -
from
MUST be0x0
for a mint. -
to
MUST be the address of the recipient which receives the tokens. -
amount
MUST be the number of tokens the recipient sent or minted. -
data
MUST contain the extra information (if any) provided to the send or the mint process. -
operatorData
MUST contain the extra information provided by the address which triggered the increase of the balance (if any). -
The holder MAY block a send or mint process by
revert
ing. (I.e., reject the reception of tokens.)
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 | |||||
---|---|---|---|---|---|
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 /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 MAYrevert
.
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.