Simple Exchange Contract

Zoe v0.24.0. Last updated August 25, 2022.

View the code on Github (Last updated: Sep 12, 2020)

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The "simple exchange" is a very basic, un-optimized exchange. It has an order book for one asset, priced in a second asset. The order book is naively kept as an array that the contract iterates over for matches when a new order arrives.

Contract API

SimpleExchange is an exchange with a simple matching algorithm. This lets an unlimited number of parties create new orders or accept existing orders. A notifier lets callers find the current list of orders.

The SimpleExchange keywords are Asset and Price. The contract treats the two keywords symmetrically. New offers are created and existing offers accepted in either direction.

{ give: { Asset: simoleans(5) }, want: { Price: quatloos(3) } }
{ give: { Price: quatloos(8) }, want: { Asset: simoleans(3) } }

Note: Here we used a shorthand for assets whose values are 5 simoleons, 3 quatloos, 8 quatloos, and 3 simoleons. Elsewhere this might have been done by creating amounts inline (i.e. AmountMath.make(quatloosBrand, 8n)). Or by creating amounts outside the proposal and assigning them to variables. For example, const quatloos8 = AmountMath.make(quatloosBrand, 8n); and then using quatloos8 as the value for Price in the second clause above.

The want term is an exact amount to exchange, while the give term is a limit that may be improved on. This simple exchange does not partially fill orders.

The publicFacet is returned when the contract is started.

Instantiating SimpleExchange

const { publicFacet } = await E(zoe).startInstance(installation, {
   Asset: moolaIssuer,
   Price: simoleanIssuer,
   });
const simpleExchangeInvitation = await E(publicFacet).makeInvitation();
const { instance } = await E(zoe).getInvitationDetails(simpleExchangeInvitation);
const aliceInvitation = await E(publicFacet).makeInvitation();

Adding an Order

A user, Alice, escrows with Zoe to create a sell order. She wants to sell 3 moola and receive at least 4 simoleans in return:

const aliceSellOrderProposal = harden({
  give: { Asset: AmountMath.make(moolaBrand, 3n) },
  want: { Price: AmountMath.make(simoleanBrand, 4n) },
  exit: { onDemand: null },
});

const alicePayment = { Asset: aliceMoolaPayment };

Alice escrows her payment with Zoe to add her sell order to the exchange.

const aliceSeat = await E(zoe).offer(
  aliceInvitation,
  aliceSellOrderProposal,
  alicePayment,
);

Buying an Order

Bob knows about the simple exchange, and hears about Alice's offer. It sounds like a good deal to him, so he checks the installation with Zoe and sees the exchange is trading what he expects:

const bobInvitation = E(publicFacet).makeInvitation();
const invitationIssuer = E(zoe).getInvitationIssuer();
const bobExclusiveInvitation = E(invitationIssuer).claim(bobInvitation);
const {
  instance,
  installation,
} = await E(zoe).getInvitationDetails(bobExclusiveInvitation);
const bobIssuers = await E(zoe).getIssuers(instance);

Bob verifies the information is what he expects. He compares the installation he gets from the invitation with a canonical link he found in a public directory he trusts.

assert(installation === simpleExchangeInstallation, details`wrong installation`);
assert(bobIssuers.Asset === moolaIssuer, details`wrong Asset issuer`);
assert(bobIssuers.Price === simoleanIssuer, details`wrong Price issuer`);

Bob has checked that everything is in order, so he fulfills the buy order:

const bobBuyOrderProposal = harden({
  give: { Price: AmountMath.make(simoleanBrand, 7n) },
  want: { Asset: AmountMath.make(moolaBrand, 3n) },
  exit: { onDemand: null },
});

const bobSimPayment = await E(bobSimoleanPurse).withdraw(AmountMath(simoleanBrand, 7n));
const bobPayments = { Price: bobSimPayment };

const bobSeat = await E(zoe).offer(
  bobExclusiveInvitation,
  bobBuyOrderProposal,
  bobPayments,
);

Payout

When a match is made, the payout promise from a user's seat resolves to a promise for payment. For Bob:

const { Asset: bobAssetPayoutP, Price: bobPricePayoutP } = await bobSeat.getPayouts();
const bobAssetPayout = await bobAssetPayoutP;
const bobMoolaGainAmount = await E(bobMoolaPurse).deposit(bobAssetPayout);
const bobPricePayout = await bobPricePayoutP;
const bobSimGainAmount = await E(bobSimPurse).deposit(bobPricePayout);

Alice gets her payouts the same way. (The choice between getPayouts() and getPayout(keyword) is based on which is more convenient in each circumstance).

const aliceAssetPayout = await aliceSeat.getPayout('Asset');
const aliceMoolaGainAmount = aliceMoolaPurse.deposit(aliceAssetPayout);
const alicePricePayout = await aliceSeat.getPayout('Price');
const aliceSimGainAmount = aliceSimPurse.deposit(alicePricePayout);