evm-general.mdx

EVM (general)

Overview

The Ethereum Virtual Machine (EVM) is the runtime environment for smart contracts, enabling compatibility with Ethereum-based dApps. Sei is an EVM compatible blockchain. Sei's parallelized EVM ensures high performance and efficiency.

Here are some key points about the EVM:

1. Turing Completeness: The EVM is Turing complete, meaning it can execute any computable function. This allows developers to write complex smart contracts.
2. Gas: Transactions and contract executions on the EVM compatible network consume gas. Gas is a measure of computational work, and users pay for it in usei on Sei networks . Gas ensures that malicious or inefficient code doesn’t overload the network.
3. Bytecode Execution: Smart contracts are compiled into bytecode (low-level machine-readable instructions) and deployed to the EVM compatible network. The EVM executes this bytecode.

Smart contract languages

The two most popular languages for developing smart contracts on the EVM are Solidity and Vyper.

Solidity

• Object-oriented, high-level language for implementing smart contracts.
• Curly-bracket language that has been most profoundly influenced by C++.
• Statically typed (the type of a variable is known at compile time).
• Supports:
  • Inheritance (you can extend other contracts).
  • Libraries (you can create reusable code that you can call from different contracts – like static functions in a static class in other object oriented programming languages).
  • Complex user-defined types.

Example solidity contract

// SPDX-License-Identifier: GPL-3.0
pragma solidity >= 0.7.0;

contract Coin {
    // The keyword "public" makes variables
    // accessible from other contracts
    address public minter;
    mapping (address => uint) public balances;

    // Events allow clients to react to specific
    // contract changes you declare
    event Sent(address from, address to, uint amount);

    // Constructor code is only run when the contract
    // is created
    constructor() {
        minter = msg.sender;
    }

    // Sends an amount of newly created coins to an address
    // Can only be called by the contract creator
    function mint(address receiver, uint amount) public {
        require(msg.sender == minter);
        require(amount < 1e60);
        balances[receiver] += amount;
    }

    // Sends an amount of existing coins
    // from any caller to an address
    function send(address receiver, uint amount) public {
        require(amount <= balances[msg.sender], "Insufficient balance.");
        balances[msg.sender] -= amount;
        balances[receiver] += amount;
        emit Sent(msg.sender, receiver, amount);
    }
}

Vyper

• Pythonic programming language
• Strong typing
• Small and understandable compiler code
• Efficient bytecode generation
• Deliberately has less features than Solidity with the aim of making contracts more secure and easier to audit. Vyper does not support:
  • Modifiers
  • Inheritance
  • Inline assembly
  • Function overloading
  • Operator overloading
  • Recursive calling
  • Infinite-length loops
  • Binary fixed points

Example Vyper contract

# Open Auction

# Auction params
# Beneficiary receives money from the highest bidder
beneficiary: public(address)
auctionStart: public(uint256)
auctionEnd: public(uint256)

# Current state of auction
highestBidder: public(address)
highestBid: public(uint256)

# Set to true at the end, disallows any change
ended: public(bool)

# Keep track of refunded bids so we can follow the withdraw pattern
pendingReturns: public(HashMap[address, uint256])

# Create a simple auction with `_bidding_time`
# seconds bidding time on behalf of the
# beneficiary address `_beneficiary`.
@external
def __init__(_beneficiary: address, _bidding_time: uint256):
    self.beneficiary = _beneficiary
    self.auctionStart = block.timestamp
    self.auctionEnd = self.auctionStart + _bidding_time

# Bid on the auction with the value sent
# together with this transaction.
# The value will only be refunded if the
# auction is not won.
@external
@payable
def bid():
    # Check if bidding period is over.
    assert block.timestamp < self.auctionEnd
    # Check if bid is high enough
    assert msg.value > self.highestBid
    # Track the refund for the previous high bidder
    self.pendingReturns[self.highestBidder] += self.highestBid
    # Track new high bid
    self.highestBidder = msg.sender
    self.highestBid = msg.value

# Withdraw a previously refunded bid. The withdraw pattern is
# used here to avoid a security issue. If refunds were directly
# sent as part of bid(), a malicious bidding contract could block
# those refunds and thus block new higher bids from coming in.
@external
def withdraw():
    pending_amount: uint256 = self.pendingReturns[msg.sender]
    self.pendingReturns[msg.sender] = 0
    send(msg.sender, pending_amount)

# End the auction and send the highest bid
# to the beneficiary.
@external
def endAuction():
    # It is a good guideline to structure functions that interact
    # with other contracts (i.e. they call functions or send ether)
    # into three phases:
    # 1. checking conditions
    # 2. performing actions (potentially changing conditions)
    # 3. interacting with other contracts
    # If these phases are mixed up, the other contract could call
    # back into the current contract and modify the state or cause
    # effects (ether payout) to be performed multiple times.
    # If functions called internally include interaction with external
    # contracts, they also have to be considered interaction with
    # external contracts.

    # 1. Conditions
    # Check if auction endtime has been reached
    assert block.timestamp >= self.auctionEnd
    # Check if this function has already been called
    assert not self.ended

    # 2. Effects
    self.ended = True

    # 3. Interaction
    send(self.beneficiary, self.highestBid)