Ether Token

The first point of entry into the Computable ecosystem is the EtherToken contract. This contract is simply a wrapper for ETH that gives it an ERC-20 interface. You will need EtherToken for all your interactions with the Computable smart contracts.

To get started, you need to call the EtherToken.deposit() function and deposit some ETH

@public
@payable
def deposit():
  """
  @notice Facilitate a user purchasing EtherToken with Eth at a 1:1 ratio
  """
  self.balances[msg.sender] += msg.value
  self.supply += msg.value
  log.Deposited(msg.sender, msg.value)

This snippet is the first example in this book of Vyper code. Vyper is a smart contract language that runs on the Ethereum ecosystem. Vyper’s syntax is pretty similar to Python code so this code shouldn’t be too hard to follow. The basic idea is that the EtherToken contract maintains an internal ledger, self.balances. Here, msg.value is the ETH that you’re sending to the EtherToken contract. This function stores the ETH you give it in self.supply for safe keeping, and awards you an equivalent amount of EtherToken in its ledger. It ends by emitting an “event,” log.Deposited which signals to watchers than a deposit has happened on this contract.

There was some detail here, but the basic idea is simple. EtherToken.deposit() simply deposits the ETH you send it and mints you an equivalent amount of EtherToken in a 1-1 fashion.

Now let’s suppose that you’ve interacted with the Computable system to your hearts content and made some healthy capital that you’d like to take out as ETH. In that case, you simply call the EtherToken.withdraw() function:

@public
def withdraw(amount: wei_value):
  """
  @notice Allow msg.sender to withdraw an amount of ETH
  @dev The Vyper builtin `send` is used here
  @param amount An amount of ETH in wei to be withdrawn
  """
  self.balances[msg.sender] -= amount
  self.supply -= amount
  send(msg.sender, amount)
  log.Withdrawn(msg.sender, amount)

This function simply undoes the effect of EtherToken.deposit() which you just saw. It removes the requested amount from its internal ledger and sends back the ETH by calling the send function. Finally it issues an event with log.Withdrawn to signal that a withdrawal has occurred.

If you’re new to Vyper, you might be feeling confused here. What if amount is bigger than my balance? Can I steal money from the system? Luckily, Vyper protects against these types of attacks. self.balances stores values of type wei_value. This is an unsigned type. A subtraction that would cause a negative value will consequently throw an error and fail.

The ease of doing math in this fashion is one of the major advantages Vyper has over other languages such as Solidity, which require more verbose safe math libraries to achieve similar functionality.

Last Thoughts

Now that you have a grasp of EtherToken, let’s learn about some things we can do with it. To understand this better, you’ll learn about MarketToken in the next chapter.