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Vara Fungible Token Standard (VFT)

The Vara Fungible Token Standard provides a unified API for smart contracts to implement token functionalities. It encompasses critical operations like token transfer and approvals for third-party spending on the blockchain. Below, we detail the contract state, its interface, and key methods to facilitate these operations.

An example implementation of the VFT standard is available on GitHub.

Functions​

  Approve(spender, value)
  Transfer(to, value)
  TransferFrom(from, to, value)
  Allowance(owner, spender)
  BalanceOf(owner) 
  Decimals()
  Name()
  Symbol()
  TotalSupply()

Events​

    Approval(owner, spender, value);
    Transfer(from, to, value);

Key methods​

Approve​

pub fn approve(&mut self, spender: ActorId, value: U256) -> bool

This function allows a designated spender (spender) to withdraw up to an value of tokens from your account, multiple times up to the amount limit. Resets allowance to value with a subsequent call. Returns a boolean value indicating whether the operation succeeded.

Upon successful execution, triggers the event:

Approval {
    owner: ActorId,
    spender: ActorId,
    value: U256,
}

Transfer​

pub fn transfer(&mut self, to: ActorId, value: U256) -> bool

Transfers the specified value of tokens to the account to. Returns a boolean value indicating whether the operation

Upon successful execution generates the event:

Transfer {
    from: ActorId,
    to: ActorId,
    value: U256,
}

TransferFrom​

pub fn transfer_from(&mut self, from: ActorId, to: ActorId, value: U256) -> bool

Transfers a specified value of tokens from one account to another, using the allowance mechanism. Value is then deducted from the caller’s allowance. Returns a boolean value indicating whether the operation succeeded.

Upon successful execution generates the event:

Transfer {
    from: ActorId,
    to: ActorId,
    value: U256,
}

Query methods​

name​

Returns the name of the token.

pub fn name(&self) -> String

symbol​

Returns the symbol of the token.

pub fn symbol(&self) -> String

decimals​

Returns the decimals of the token.

pub fn decimals(&self) -> u8

total_supply​

Returns the total supply of the token.

pub fn total_supply(&self) -> U256

balance_of​

Returns the token balance of the owner address.

pub fn balance_of(&self, account: ActorId) -> U256

allowance​

Returns the number of tokens the spender account is authorized to spend on behalf of the owner.

pub fn allowance(&self, owner: ActorId, spender: ActorId) -> U256

Contract Interface​

The contract interface defines how users interact with the token contract, including token transfer, approval, and queries about balances, allowances, and other token information.

constructor {
  New : (name: str, symbol: str, decimals: u8);
};

service Vft {
  Approve : (spender: actor_id, value: u256) -> bool;
  Transfer : (to: actor_id, value: u256) -> bool;
  TransferFrom : (from: actor_id, to: actor_id, value: u256) -> bool;
  query Allowance : (owner: actor_id, spender: actor_id) -> u256;
  query BalanceOf : (account: actor_id) -> u256;
  query Decimals : () -> u8;
  query Name : () -> str;
  query Symbol : () -> str;
  query TotalSupply : () -> u256;

  events {
    Approval: struct { owner: actor_id, spender: actor_id, value: u256 };
    Transfer: struct { from: actor_id, to: actor_id, value: u256 };
  }
};

Conclusion​

By adhering to this standard, smart contracts can ensure interoperability and provide a consistent user experience across different platforms and applications within the blockchain ecosystem.

Using this standard, dApp developers can ensure that their in-app tokens, built on the basis of this standard, will be natively displayed in user wallets without requiring additional integration efforts, assuming that wallet applications also support this standard.