31. Building an ERC-20 token

An ERC-20 in Vyper combines everything so far: a `HashMap` for balances, a nested `HashMap` for allowances, `Transfer`/`Approval` events, and `transfer`/`approve`/`transferFrom` functions that enforce balance and allowance checks.

balanceOf: public(HashMap[address, uint256])
allowance: public(HashMap[address, HashMap[address, uint256]])

event Transfer:
    sender: indexed(address)
    receiver: indexed(address)
    value: uint256

@external
def transfer(to: address, amount: uint256) -> bool:
    assert self.balanceOf[msg.sender] >= amount, "insufficient balance"
    self.balanceOf[msg.sender] -= amount
    self.balanceOf[to] += amount
    log Transfer(sender=msg.sender, receiver=to, value=amount)
    return True

`transferFrom` additionally checks and decrements the caller's allowance before moving tokens on behalf of someone else.

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