Programming Languages

Warning: This contract will compile on the /future/ Glow release.

How does the Simple Auction (auction.glow) contract work?

Imagine an auction where each one of the bidders tries to outbid the previous bidder. However, in a blockchain auction, each bidder has to declare the money first to become the top bidder. They get their money back when they are outbid.

Visualization

Glow code

Copy
    #lang glow
    data Action = Bid(TokenAmount) | BuyItNow | Close;

    @interaction([Seller])
    let simpleAuction = (goods : Assets, expirationTime : Timestamp, buyItNowPrice: TokenAmount) => {
    require! Assets.is_positive(goods);
    require! expirationTime > currentTime();
    deposit! Seller -> goods;

    @interaction([Seller, CurrentBidder])
    let rec auction = (currentBid) => {
    assert! @deposited == goods + currentBid;
    choice {
    | ForAllParticipant (NewBidder) {
    @NewBidder bid = Bid(input(["Enter next bid"], TokenAmout));
    publish! NewBidder -> bid ; deposit! NewBidder -> bid;
    @NewBidder assume! @value(goods) > @value(bid);
    require! currentTime() < expirationTime;
    require! bid > currentBid;
    require! bid < buyItNowPrice;
    withdraw! CurrentBidder <- currentBid;
    @interaction([Seller, NewBidder]) auction(bid);
    | ForAllParticipant (NewBidder) {
    publish! NewBidder -> BuyItNow ; deposit! NewBidder -> buyItNowPrice;
    require! currentTime() < expirationTime;
    withdraw! NewBidder <- goods;
    withdraw! CurrentBidder <- currentBid;
    withdraw! Seller <- buyItNowPrice;
    | @_ { publish! Close; } =>
    require! currentTime() >= expirationTime;
    withdraw! Seller <- currentBid;
    withdraw! CurrentBidder <- goods;
    };
    @interaction([Seller, Seller]) auction(0);
    }

• Declares the possible commands that could be performed on this contract
• Only the Seller can create an auction
• In order to create an auction, a Seller must provide:

The Goods that could be anything that can be represented as an asset, an expirationTime at which point the auction is over and a price at which to buy it immediately buyItNowPrice.

• The Seller must deposit the goods in the smart contract. This could, for example, a photograph.
• The real auction begins at 0, and the "first bidder" is the Seller himself.
• The real auction is an interaction between the Seller and the Current bidder.
• This function is recursive, so it can call itself.
• A sanity check, let's make sure that the amount deposited to the contract is the goods plus the current bidder.
• This is key in this contract; anyone that bids may become a participant.
• The contract as the new participant for their bid.
• The new bid should be public and it's deposited into the contract.
• The new bid must be bigger than the current one in order to replace it.
• The previous highest bidder gets its money back since it has been replaced by the new bid.
• Now the auction may continue, recursively, with a new highest bidder.
• If the new bidder, reaches the buy it now price. Then the auction is settled right now.
• If the expiration time is reached, then the auction is settled.

Lessons learned

• Recursive function definitions
• How to involve participants that are not known beforehand.
• How to use an expiration date