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ZRC Title Status Type Author Created (yyyy-mm-dd) Updated (yyyy-mm-dd)
2 Standard for Fungible Tokens Implemented Standard Vaivaswatha Nagaraj vaivaswatha@zilliqa.com
Chua Han Wen hanwen@zilliqa.com		 2019-11-18 2020-11-12

I. What are Fungible Tokens?

The Fungible Token is an open standard for creating currencies. Fungibility is the property of goods or commodities whose individual units are essentially interchangeable, and each of its parts is indistinguishable from another part.

II. Abstract

ZRC-2 defines a minimum interface that a smart contract must implement to allow fungible tokens to be managed, tracked, owned, and traded peer-to-peer via wallets or exchanges.

III. Motivation

A standard for fungible tokens can serve as an interface for developers to create currencies, utility tokens or stable coins. Generally, fungible tokens can be used to represent interchangeable, identical and divisible assets as tokens.

IV. Specification

The fungible token contract specification describes:

  1. the global error codes to be declared in the library part of the contract;
  2. the names and types of the immutable and mutable variables (aka fields);
  3. the transitions that will allow the changing of values of the mutable variables;
  4. the events to be emitted by them.

A. Roles

Name Description
contract_owner The owner of the contract initialized by the creator of the contract.
token_owner A user (identified by an address) that owns tokens.
approved_spender A user (identified by an address) that can transfer tokens on behalf of the token_owner.
operator A user (identified by an address) that is approved to operate all tokens owned by another user (identified by another address). This role is optional.
default_operator A special user (identified by an address) that is approved to operate all tokens owned by all users (identified by addresses). This role is optional.

B. Error Codes

The fungible token contract must define the following constants for use as error codes for the Error exception.

Name Type Code Description
CodeIsSender Int32 -1 Emit when an address is same as is the sender.
CodeInsufficientFunds Int32 -2 Emit when there is insufficient balance to authorise transaction.
CodeInsufficientAllowance Int32 -3 Emit when there is insufficient allowance to authorise transaction.
CodeNotOwner Int32 -4 Emit when the sender is not contract_owner. This error code is optional.
CodeNotApprovedOperator Int32 -5 Emit when caller is not an approved operator or default_operator. This error code is optional.

C. Immutable Variables

Name Type Description
contract_owner ByStr20 The owner of the contract initialized by the creator of the contract.
name String The name of the fungible token.
symbol String The symbol of the fungible token.
decimals Uint32 The number of decimal places a token can be divided by.
init_supply Uint128 The initial supply of fungible tokens when contract is created.
default_operators List ByStr20 The list of default operators initialized by the creator of the contract. This immutable variable is optional.

D. Mutable Fields

Name Type Description
total_supply Uint128 Total amount of tokens available.
balances Map ByStr20 Uint128 Mapping between token owner to number of owned tokens.
allowances Map ByStr20 (Map ByStr20 Uint128) Mapping from token owner to approved spender address. Token owner can give an address an allowance of tokens to transfer tokens to other addresses.
operators Map ByStr20 (Map ByStr20 Unit) Mapping from token owner to designated operators. A token owner can approve an address as an operator (as per the definition of operator given above). This mapping is optional.
revoked_default_operators Map ByStr20 (Map ByStr20 Unit) Mapping from token owner to revoked default operators. Default operators are intialised by the contract owner. A token owner can revoked a default operator (as per the definition of default operator given above) at will. This mapping is optional.

E. Getter Transitions

1. IsOperatorFor() (Optional)

(* @dev: Check if an address is an operator or default operator of a token_owner. Throw if not. *)
(* @param operator:    Address of a potential operator.                                         *)
(* @param token_owner: Address of a token_owner.                                                *)
transition IsOperatorFor(token_owner: ByStr20, operator: ByStr20)

Arguments:

Name Type Description
@param token_owner ByStr20 An address of a particular token_owner.
@param operator ByStr20 An address of a particular operator of a token_owner.

Messages sent:

Name Description Callback Parameters
_tag IsOperatorForCallBack Provide the sender a callback if specified address is indeed an approved operator of specified token_owner. token_owner : ByStr20, operator: ByStr20, where token_owner is the address of the token_owner, operator is the address of the approved operator.

F. Interface Transitions

1. Mint() (Optional)

(* @dev: Mint new tokens. Only contract_owner can mint.                      *)
(* @param recipient: Address of the recipient whose balance is to increase.  *)
(* @param amount:    Number of tokens to be minted.                          *)
transition Mint(recipient: ByStr20, amount: Uint128)

Arguments:

Name Type Description
@param recipient ByStr20 Address of the recipient whose balance is to increase.
@param amount Uint128 Number of tokens to be minted.

Messages sent:

Name Description Callback Parameters
_tag RecipientAcceptMint Dummy callback to prevent invalid recipient contract. minter : ByStr20, recipient: ByStr20, amount: Uint128, where minter is the address of the minter, recipient is the address of the recipient, and amount is the amount of fungible tokens minted.
_tag MintSuccessCallBack Provide the sender the status of the mint. minter : ByStr20, recipient: ByStr20, amount: Uint128, where minter is the address of the minter, recipient is the address of the recipient, and amount is the amount of fungible tokens minted.

Events/Errors:

Name Description Event Parameters
_eventname Minted Minting is successful. minter : ByStr20, recipient: ByStr20, amount: Uint128, where minter is the address of the minter, recipient is the address whose balance will be increased, and amount is the amount of fungible tokens minted.
_eventname Error Minting is not successful. - emit CodeNotOwner if the transition is not called by the contract_owner.

2. Burn() (Optional)

(* @dev: Burn existing tokens. Only contract_owner can burn.                      *)
(* @param burn_account: Address of the token_owner whose balance is to decrease.  *)
(* @param amount:       Number of tokens to be burned.                            *)
transition Burn(burn_account: ByStr20, amount: Uint128)

Arguments:

Name Type Description
@param burn_account ByStr20 Address of the token_owner whose balance is to decrease.
@param amount Uint128 Number of tokens to be burned.

Messages sent:

Name Description Callback Parameters
_tag BurnSuccessCallBack Provide the sender the status of the burn. burner : ByStr20, burn_account: ByStr20, amount: Uint128, where burner is the address of the burner, burn_account is the address whose balance will be decreased, and amount is the amount of fungible tokens burned.

Events/Errors:

Name Description Event Parameters
_eventname Burnt Burning is successful. burner : ByStr20, burn_account: ByStr20, amount: Uint128, where burner is the address of the burner, burn_account is the address whose balance will be decreased, and amount is the amount of fungible tokens burned.
_eventname Error Burning is not successful. - emit CodeNotOwner if the transition is not called by the contract_owner.
- emit CodeNoBalance if balance of token_owner does not exists.
- emit CodeInsufficientFunds if the amount to be burned is more than the balance of the token_owner.

3. AuthorizeOperator() (Optional)

(* @dev: Make an address an operator of the caller.             *)
(* @param operator: Address to be authorize as operator or      *)
(* Re-authorize as default_operator. Cannot be calling address. *)
transition AuthorizeOperator(operator: ByStr20)

Arguments:

Name Type Description
@param operator ByStr20 Address to be authorize as operator or re-authorize as default_operator. Cannot be calling address.

Events/Errors:

Name Description Event Parameters
_eventname AuthorizeOperatorSuccess Authorizing is successful. authorizer: ByStr20 which is the caller's address, and authorized_operator: ByStr20 which is the address to be authorized as an operator of the token_owner.
_eventname Error Authorizing is not successful. - emit CodeIsSender if the user is trying to authorize himself as an operator.

4. RevokeOperator() (Optional)

(* @dev: Revoke an address from being an operator or default_operator of the caller. *)
(* @param operator: Address to be removed as operator or default_operator.           *)
transition RevokeOperator(operator: ByStr20)

Arguments:

Name Type Description
@param operator ByStr20 Address to be unset as operator.

Events/Errors:

Name Description Event Parameters
_eventname RevokeOperatorSuccess Revoking is successful. revoker: ByStr20 which is the caller's address, and revoked_operator: ByStr20 which is the address to be removed as an operator of the token_owner.
_eventname Error Revoking is not successful. - emit CodeNotApprovedOperator if the specified address is not an existing operator or default_operator of the token_owner.

5. IncreaseAllowance()

(* @dev: Increase the allowance of an approved_spender over the caller tokens. Only token_owner allowed to invoke.   *)
(* param spender:      Address of the designated approved_spender.                                                   *)
(* param amount:       Number of tokens to be increased as allowance for the approved_spender.                       *)
transition IncreaseAllowance(spender: ByStr20, amount: Uint128)

Arguments:

Name Type Description
@param spender ByStr20 Address of an approved_spender.
@param amount Uint128 Number of tokens to be increased as spending allowance of the approved_spender.

Events/Errors:

Name Description Event Parameters
_eventname IncreasedAllowance Increasing of allowance of an approved_spender is successful. token_owner: ByStr20 which is the address the token_owner, spender: ByStr20 which is the address of a approved_spender of the token_owner, and new_allowance is the new spending allowance of the approved_spender for the token_owner.
_eventname Error Increasing of allowance is not successful. - emit CodeIsSelf if the user is trying to authorize himself as an approved_spender.

6. DecreaseAllowance()

(* @dev: Decrease the allowance of an approved_spender over the caller tokens. Only token_owner allowed to invoke. *)
(* param spender:      Address of the designated approved_spender.                                                 *)
(* param amount:       Number of tokens to be decreased as allowance for the approved_spender.                     *)
transition DecreaseAllowance(spender: ByStr20, amount: Uint128)

Arguments:

Name Type Description
@param spender ByStr20 Address of an approved_spender.
@param amount Uint128 Number of tokens to be decreased as spending allowance of the approved_spender.

Events/Errors:

Name Description Event Parameters
_eventname DecreasedAllowance Decreasing of allowance of an approved_spender is successful. token_owner: ByStr20 which is the address the token_owner, spender: ByStr20 which is the address of a approved_spender of the token_owner, and new_allowance is the new spending allowance of the approved_spender for the token_owner.
_eventname Error Decreasing of allowance is not successful. - emit CodeIsSelf if the user is trying to authorize himself as an approved_spender.

7. Transfer()

(* @dev: Moves an amount tokens from _sender to the recipient. Used by token_owner. *)
(* @dev: Balance of recipient will increase. Balance of _sender will decrease.      *)
(* @param to:  Address of the recipient whose balance is increased.                 *)
(* @param amount:     Amount of tokens to be sent.                                  *)
transition Transfer(to: ByStr20, amount: Uint128)

Arguments:

Name Type Description
@param to ByStr20 Address of the recipient whose balance is to increase.
@param amount Uint128 Amount of tokens to be sent.

Messages sent:

Name Description Callback Parameters
_tag RecipientAcceptTransfer Dummy callback to prevent invalid recipient contract. sender : ByStr20, recipient: ByStr20, amount: Uint128, where sender is the address of the sender, recipient is the address of the recipient, and amount is the amount of fungible tokens to be transferred.
_tag TransferSuccessCallBack Provide the sender the status of the transfer. sender : ByStr20, recipient: ByStr20, amount: Uint128, where sender is the address of the sender, recipient is the address of the recipient, and amount is the amount of fungible tokens to be transferred.

Events/Errors:

Name Description Event Parameters
_eventname TransferSuccess Sending is successful. sender: ByStr20 which is the sender's address, recipient: ByStr20 which is the recipient's address, and amount: Uint128 which is the amount of fungible tokens transferred.
_eventname Error Sending is not successful. - emit CodeInsufficientFunds if the balance of the token_owner lesser than the specified amount that is to be transferred.

8. TransferFrom()

(* @dev: Move a given amount of tokens from one address to another using the allowance mechanism. The caller must be an approved_spender. *)
(* @dev: Balance of recipient will increase. Balance of token_owner will decrease.                                                        *)
(* @param from:    Address of the token_owner whose balance is decreased.                                                                 *)
(* @param to:      Address of the recipient whose balance is increased.                                                                   *)
(* @param amount:  Amount of tokens to be transferred.                                                                                    *)
transition TransferFrom(from: ByStr20, to: ByStr20, amount: Uint128)

Arguments:

Name Type Description
@param from ByStr20 Address of the token_owner whose balance is to decrease.
@param to ByStr20 Address of the recipient whose balance is to increase.
@param amount Uint128 Number of tokens to be transferred.

Messages sent:

Name Description Callback Parameters
_tag RecipientAcceptTransferFrom Dummy callback to prevent invalid recipient contract. initiator: ByStr20, sender : ByStr20, recipient: ByStr20, amount: Uint128, where initiator is the address of an approved_spender,sender is the address of the token_owner, recipient is the address of the recipient, and amount is the amount of fungible tokens to be transferred.
_tag TransferFromSuccessCallBack Provide the initiator the status of the transfer. initiator: ByStr20, sender : ByStr20, recipient: ByStr20, amount: Uint128, where initiator is the address of an approved_spender,sender is the address of the token_owner, recipient is the address of the recipient, and amount is the amount of fungible tokens to be transferred.

Events:

Name Description Event Parameters
_eventname TransferFromSuccess Sending is successful. initiator: ByStr20, sender : ByStr20, recipient: ByStr20, amount: Uint128, where initiator is the address of an approved_spender,sender is the address of the token_owner, recipient is the address of the recipient, and amount is the amount of fungible tokens to be transferred.
_eventname Error Sending is not successful. - emit CodeInsufficientAllowance if the allowance of approved_spender is lesser than the specified amount that is to be transferred.
- emit CodeInsufficientFunds if the balance of the token_owner lesser than the specified amount that is to be transferred.

9. OperatorSend() (Optional)

(* @dev: Moves amount tokens from token_owner to recipient. _sender must be an operator of token_owner. *)
(* @dev: Balance of recipient will increase. Balance of token_owner will decrease.                      *)
(* @param from:        Address of the token_owner whose balance is decreased.                           *)
(* @param to:          Address of the recipient whose balance is increased.                             *)
(* @param amount:      Amount of tokens to be sent.                                                     *)
transition OperatorSend(from: ByStr20, to: ByStr20, amount: Uint128)

Arguments:

Name Type Description
@param from ByStr20 Address of the token_owner whose balance is to decrease.
@param to ByStr20 Address of the recipient whose balance is to increase.
@param amount Uint128 Amount of tokens to be sent.

Messages sent:

Name Description Callback Parameters
_tag RecipientAcceptOperatorSend Dummy callback to prevent invalid recipient contract. initiator: ByStr20, sender : ByStr20, recipient: ByStr20, amount: Uint128, where initiator is the address of an operator,sender is the address of the token_owner, recipient is the address of the recipient, and amount is the amount of fungible tokens to be transferred.
_tag OperatorSendSuccessCallBack Provide the operator the status of the transfer. initiator: ByStr20, sender : ByStr20, recipient: ByStr20, amount: Uint128, where initiator is the address of an operator,sender is the address of the token_owner, recipient is the address of the recipient, and amount is the amount of fungible tokens to be transferred.

Events/Errors:

Name Description Event Parameters
_eventname OperatorSendSuccess Sending is successful. initiator: ByStr20 which is the operator's address, sender: ByStr20 which is the token_owner's address, recipient: ByStr20 which is the recipient's address, and amount: Uint128 which is the amount of fungible tokens to be transferred.
_eventname Error Sending is not successful. - emit CodeNotApprovedOperator if sender is not an approved operator for the token_owner
- emit CodeInsufficientFunds if the balance of the token_owner is lesser than the specified amount that is to be transferred.

V. Existing Implementation(s)

To test the reference contract, simply go to the example folder and run one of the JS scripts. For example, to deploy the contract, run:

yarn deploy.js

NOTE: Please change the privkey in the script to your own private key. You can generate a testnet wallet and request for testnet $ZIL at the Nucleus Faucet.

VI. Copyright

Copyright and related rights waived via CC0.