Solidity PAtterns

@tomusdrw

Tomasz Drwięga

Parity Technologies

Title Text

contract SimpleToken { // TODO Use SafeMath!
    uint256 constant rate = 1000;
    event Transfer(address indexed _from, address indexed _to, uint _value);
    
    uint256 public totalSupply;
    mapping(address => uint256) public balanceOf;
    mapping(address => uint256) public lockTime;

    function buyTokens() payable {
        var tokens = msg.value * rate;
        var unlockAt = block.number + 256 * 24; // 24h

        totalSupply += tokens;
        balanceOf[msg.sender] += tokens;
        lockTime[msg.sender] = unlockAt;
    }
    function burnTokens(uint256 _value) {
        var tokens = _value * rate;
        require(block.number >= lockTime[msg.sender]);
        require(balanceOf[msg.sender] >= tokens);
        
        balanceOf[msg.sender] -= tokens;
        totalSupply -= tokens;
        delete lockTime[msg.sender];
        
        msg.sender.transfer(_value);
    }
    function transfer(address _to, uint _value) returns (bool success) {
        if (block.number < lockTime[msg.sender]) return false;
        if (balanceOf[msg.sender] < _value) return false;
        if (_value == 0 || balanceOf[_to] + _value <= balanceOf[_to]) return false;
        
        balanceOf[msg.sender] -= _value;
        balanceOf[_to] += _value;
        
        Transfer(msg.sender, _to, _value);
        
        return true;
    }
}

Token Contract

Token Standard (ERC20)

https://theethereum.wiki/w/index.php/ERC20_Token_Standard

// https://github.com/ethereum/EIPs/issues/20
contract ERC20 {
    function totalSupply()
        constant returns (uint totalSupply);
    function balanceOf(address _owner)
        constant returns (uint balance);
    function transfer(address _to, uint _value)
        returns (bool success);
    function transferFrom(address _from, address _to, uint _value)
        returns (bool success);
    function approve(address _spender, uint _value)
        returns (bool success);
    function allowance(address _owner, address _spender)
        constant returns (uint remaining);

    event Transfer(
        address indexed _from, address indexed _to, uint _value
    );
    event Approval(
        address indexed _owner, address indexed _spender, uint _value
    );
}

Questions to Discuss

How much is one token worth?

Does the lock period change anything?

What if you can do something ONLY if you own that token?

Some More Practical Examples?

Designing contracts is designing economic models around them.

"What are the incentives?"

Remote Purchase

Done wrong

contract Purchase {
    uint public value;
    address public seller;
    address public buyer;
   
    function Purchase(uint _value) {
        seller = msg.sender;
        value = _value;
    }

    /// Confirm the purchase as buyer.
    function confirmPurchase() payable {
        require(msg.value == value);
        buyer = msg.sender;
    }
    
    /// Confirm that you (the buyer) received the item.
    function confirmReceived() {
        require(msg.sender == buyer);
        seller.transfer(this.balance);
    }
}

Safe Remote Purchase

(Simplified)

contract Purchase {
    uint public value;
    address public seller;
    address public buyer;
   
    // Seller needs to deposit double the value of the item.
    function Purchase() payable {
        seller = msg.sender;
        value = msg.value / 2;
        require(value * 2 == msg.value);
    }
    
    function abort() {
        require(msg.sender == seller);
        require(buyer == 0);
        seller.transfer(this.balance);
    }

    /// Confirm the purchase as buyer. Deposit double the value of the item.
    function confirmPurchase() payable {
        require(msg.value == 2 * value);
        buyer = msg.sender;
    }
    
    /// Confirm that you (the buyer) received the item.
    function confirmReceived() {
        require(msg.sender == buyer);
        buyer.transfer(value); // transfer half of the deposit
        seller.transfer(this.balance); // transfer the entire deposit
        delete value;
    }
}

Simple Auction

Done slightly wrong

contract Auction {
    address public beneficiary;
    uint public endBlock;
    bool public ended;

    address public highestBidder;
    uint public highestBid;
    mapping(address => uint) pendingReturns;
   
    function Auction(uint _time) {
        beneficiary = msg.sender; endBlock = block.number + _time;
    }

    function bid() payable {
        require(block.number < endBlock);
        require(highestBid < msg.value);
        if (highestBidder != 0) { pendingReturns[highestBidder] += highestBid; }
        highestBidder = msg.sender;
        highestBid = msg.value;
    }
    
    function endAuction() {
        require(endBlock <= block.number);
        require(!ended);
        ended = true;
        beneficiary.transfer(highestBid);
    }
    
    function withdraw() {
        uint amount = pendingReturns[msg.sender];
        delete pendingReturns[msg.sender];
        msg.sender.transfer(amount);
    }
}

Why is it wrong?

Because Auction Theory

https://en.wikipedia.org/wiki/Auction_theory

"There are many possible designs for an auction and typical issues studied by auction theorists include the efficiency of a given auction design, optimal and equilibrium bidding strategies, and revenue comparison."

Solution?

Blind Auction

https://en.wikipedia.org/wiki/Auction#Primary
https://en.wikipedia.org/wiki/Commitment_scheme

"In this type of auction all bidders simultaneously submit sealed bids so that no bidder knows the bid of any other participant. The highest bidder pays the price they submitted."

address public beneficiary;
uint public bidEndBlock;
uint public revealEndBlock;
bool public ended;

address public highestBidder;
uint public highestBid;
mapping(address => uint) pendingReturns;
mapping(address => bytes32) bids;
mapping(address => uint) deposits;
   
function Auction(uint _time) {
    beneficiary = msg.sender;
    bidEndBlock = block.number + _time;
    revealEndBlock = bidEndBlock + _time;
}
function bid(bytes32 _blindedBid) payable {
    require(block.number < bidEndBlock);
    bids[msg.sender] = _blindedBid;
    deposits[msg.sender] = msg.value;
}
function endAuction() {
    require(revealEndBlock <= block.number);
    require(!ended);
    ended = true;
    beneficiary.transfer(highestBid);
}
function withdraw() {
    uint amount = pendingReturns[msg.sender];
    delete pendingReturns[msg.sender];
    msg.sender.transfer(amount);
}

function reveal(bytes32 _secret, uint _value) {
    require(bidEndBlock <= block.number);
    require(block.number < revealEndBlock);
    require(deposits[msg.sender] > _value);
    require(
        bids[msg.sender] == keccak256(_value, _secret)
    );
    
    var isHighest = placeBid(msg.sender, _value);
    var deposit = deposits[msg.sender];
    
    delete bids[msg.sender];
    delete deposits[msg.sender];
    
    if (!isHighest) {
        msg.sender.transfer(deposit);
    } else {
        msg.sender.transfer(deposit - _value);
    }
}

function placeBid(address _bidder, uint _value) 
    internal returns (bool success)
{
    if (highestBid < _value) return false;
    if (highestBidder != 0) pendingReturns[highestBidder] += highestBid;
    highestBidder = _bidder;
    highestBid = _value;
    return true;
}

Blind auction (simplified)

Withdraw Pattern

pragma solidity ^0.4.11;

contract KingOfEthereum {
    address public richest;
    uint public mostSent;

    function KingOfEthereum() public payable {
        richest = msg.sender;
        mostSent = msg.value;
    }

    function becomeRichest() public payable returns (bool) {
        if (msg.value > mostSent) {
            // This line can cause problems
            richest.transfer(msg.value);
            richest = msg.sender;
            mostSent = msg.value;
            return true;
        } else {
            return false;
        }
    }
}

Withdraw Pattern

pragma solidity ^0.4.11;

contract KingOfEthereum {
    address public richest;
    uint public mostSent;

    mapping (address => uint) pendingWithdrawals;

    function KingOfEthereum() public payable {
        richest = msg.sender;
        mostSent = msg.value;
    }

    function becomeRichest() public payable returns (bool) {
        if (msg.value > mostSent) {
            pendingWithdrawals[richest] += msg.value;
            richest = msg.sender;
            mostSent = msg.value;
            return true;
        } else {
            return false;
        }
    }

    function withdraw() public {
        uint amount = pendingWithdrawals[msg.sender];
        // Remember to zero the pending refund before
        // sending to prevent re-entrancy attacks
        pendingWithdrawals[msg.sender] = 0;
        msg.sender.transfer(amount);
    }
}

State Machine

pragma solidity ^0.4.11;

contract StateMachine {
    enum Stages {
        AcceptingBlindedBids,
        RevealBids,
        AnotherStage,
        AreWeDoneYet,
        Finished
    }

    // This is the current stage.
    Stages public stage = Stages.AcceptingBlindedBids;

    uint public creationTime = now;

    modifier atStage(Stages _stage) {
        require(stage == _stage);
        _;
    }

    function nextStage() internal {
        stage = Stages(uint(stage) + 1);
    }

    // Perform timed transitions. Be sure to mention
    // this modifier first, otherwise the guards
    // will not take the new stage into account.
    modifier timedTransitions() {
        if (stage == Stages.AcceptingBlindedBids &&
                    now >= creationTime + 10 days)
            nextStage();
        if (stage == Stages.RevealBids &&
                now >= creationTime + 12 days)
            nextStage();
        // The other stages transition by transaction
        _;
    }


    // Order of the modifiers matters here!
    function bid()
        public
        payable
        timedTransitions
        atStage(Stages.AcceptingBlindedBids)
    {
        // We will not implement that here
    }

    function reveal()
        public
        timedTransitions
        atStage(Stages.RevealBids)
    {}

    // This modifier goes to the next stage
    // after the function is done.
    modifier transitionNext()
    {
        _;
        nextStage();
    }

    function g()
        public
        timedTransitions
        atStage(Stages.AnotherStage)
        transitionNext
    {
    }
}

Modifier Anti-patterns?

State Machine

pragma solidity ^0.4.11;

contract StateMachine {
    enum Stages {
        AcceptingBlindedBids,
        RevealBids,
        AnotherStage,
        AreWeDoneYet,
        Finished
    }

    // This is the current stage.
    Stages public stage = Stages.AcceptingBlindedBids;

    uint public creationTime = now;

    modifier atStage(Stages _stage) {
        require(stage == _stage);
        _;
    }

    function nextStage() internal {
        stage = Stages(uint(stage) + 1);
    }

    // Perform timed transitions.
    // Anyone can call this function.
    function timedTransition() public {
        if (stage == Stages.AcceptingBlindedBids &&
                    now >= creationTime + 10 days)
            nextStage();
        if (stage == Stages.RevealBids &&
                now >= creationTime + 12 days)
            nextStage();
    }


    // Order of the modifiers DOES NOT matter!
    function bid()
        public
        payable
        atStage(Stages.AcceptingBlindedBids)
    {
        // We will not implement that here
    }

    function reveal()
        public
        atStage(Stages.RevealBids)
    {}

    function g()
        public
        atStage(Stages.AnotherStage)
    {
         // just manually progress to next stage
         nextStage();
    }
}

Modifier fixed.

Inspired to write some contracts?

Learn Game Theory basics first.

And please read this:

"Building a Fun Ethereum Game" by Conrad Barski

http://www.cointagion.com/2016/08/24/part-i/

References

Dan Finlay's Intro to how Ethereum works https://youtu.be/-SMliFtoPn8
Solidity docs https://solidity.readthedocs.io
ERC20 https://theethereum.wiki/w/index.php/ERC20_Token_Standard
Browser Solidity / Remix https://ethereum.github.io/browser-solidity/
Parity https://github.com/paritytech/parity
Safe Remote Purchase http://solidity.readthedocs.io/en/develop/solidity-by-example.html#safe-remote-purchase
Auction Theory https://en.wikipedia.org/wiki/Auction_theory
Game Theory https://en.wikipedia.org/wiki/Game_theory
Commitment Scheme https://en.wikipedia.org/wiki/Commitment_scheme
Building a Fun Ethereum Game http://www.cointagion.com/2016/08/24/part-i/

some other features of Solidity

That were not covered here

Libraries
Inheritance
Calling other contracts
Memory
Encapsulation (visibility modifiers)

https://www.meetup.com/Wroclaw-Blockchain-Meetup/

Solidity PAtterns

Title Text

Token Contract

Token Standard (ERC20)

Questions to Discuss

How much is one token worth?

Does the lock period change anything?

What if you can do something ONLY if you own that token?

Some More Practical Examples?

Remote Purchase

Done wrong

Safe Remote Purchase

(Simplified)

Simple Auction

Done slightly wrong

Why is it wrong?

Because Auction Theory

Solution?

Blind Auction

Blind auction (simplified)

Withdraw Pattern

Withdraw Pattern

State Machine

State Machine

Inspired to write some contracts?

References

some other features of Solidity

That were not covered here

Smart Contracts (part 2)

Smart Contracts (part 2)

Tomasz Drwięga

Solidity PAtterns

Title Text

Token Contract

Token Standard (ERC20)

Questions to Discuss

How much is one token worth?

Does the lock period change anything?

What if you can do something ONLY if you own that token?

Some More Practical Examples?

Remote Purchase

Done wrong

Safe Remote Purchase

(Simplified)

Simple Auction

Done slightly wrong

Why is it wrong?

Because Auction Theory

Solution?

Blind Auction

Blind auction (simplified)

Withdraw Pattern

Withdraw Pattern

State Machine

State Machine

Inspired to write some contracts?

References

some other features of Solidity

That were not covered here

Smart Contracts (part 2)

More from Tomasz Drwięga