# Zoe

Zoe Alpha Release v0.8.1. Last updated 2020-9-15.

Zoe provides a framework for deploying and working with smart contracts. It is accessed as a long-lived and well-trusted service that enforces offer safety for the contracts that use it. Zoe has a single invitationIssuer for the entirety of its lifetime. By having a reference to Zoe, a user can get the invitationIssuer and thus validate any invitation they receive from someone else.

Zoe is accessed asynchronously

The Zoe service is accessed asynchronously, using a standards-track library extension to JavaScript that uses promises as remote references. In code, the Zoe service instance is referred to via zoe, which only supports asynchronous invocation. Operations are invoked asynchronously using the E helper for async messaging(opens new window) . All such operations immediately return a promise for their result. That may eventually fulfill to a local value, or to a Presence for another remote object (e.g. in another contract or service, running on another chain, etc.). Async messages can be sent using E with either promises or presences.

For more information about using E, see the section on it in Agoric's JavaScript Distributed Programming Guide(opens new window) .

# E(zoe).getBrands(instance)

  • instance {Instance}
  • Returns: {Promise<BrandKeywordRecord>}

Returns a BrandKeywordRecord containing all brands defined in the contract instance.

A BrandKeywordRecord is a record where the keys are keywords, and the values are the brands for particular issuers.

// Call example
const brandKeywordRecord = await E(zoe).getBrands(instance);
// Record example
const brandKeywordRecord = {
  Asset: quatloosBrand,
  Price: moolaBrand,
};

# E(zoe).getIssuers(instance)

  • instance {Instance}
  • Returns: {Promise<IssuerKeywordRecord>}

Returns a IssuerKeywordRecord containing all issuers defined in the instance argument.

An IssuerKeywordRecord is a record where the keys are keywords, and the values are issuers.

// Call example
const issuerKeywordRecord = await E(zoe).getIssuers(instance);
// Record example
const issuerKeywordRecord = {
  Asset: quatloosIssuer,
  Price: moolaIssuer,
};

# E(zoe).getTerms(instance)

  • instance {Instance}
  • Returns: {Object}

Returns the terms of the instance argument, including its issuers, brands and any custom terms.

const terms = await E(zoe).getTerms(instance);

# E(zoe).getPublicFacet(instance)

  • instance {Instance}
  • Returns: {Promise<PublicFacet>}

A contract instance's publicFacet is an object available via Zoe to anyone knowing that instance. You use it for general queries and actions, such as getting a current price or creating public invitations. Since a facet is defined just as any other object, the contract adds methods to the publicFacet just like you would any object.

Returns a publicFacet containing the public facet defined for instance.

const ticketSalesPublicFacet = await E(zoe).getPublicFacet(sellItemsInstance);

# E(zoe).getInvitationIssuer()

  • Returns {Issuer}

Zoe has a single invitationIssuer for its entire lifetime. By having a reference to Zoe, a user can get the invitationIssuer. This lets them claim any invitation they receive from someone else by calling E(invitationIssuer).claim() with the untrusted invitation as the argument. During the claiming process, the invitationIssuer validates the invitation.

The mint associated with the invitationIssuer creates invitations in the form of ERTP payments that represent the right to interact with a smart contract in particular ways.

The invitationIssuer has two methods, both of which take an invitation as an argument. Remember, an invitation is just a special case of an ERTP payment, so claim() and getAmountOf() are the same as for other issuers.

A successful call of invitationIssuer.claim(invitation) means you are assured the invitation is recognized as valid by the invitationIssuer. You are also assured the invitation is exclusively yours and no one else has access to it.

const invitationIssuer = await E(zoe).getInvitationIssuer();
// Here a user, Bob, has received an untrusted invitation from Alice.
// Bob uses the trusted `invitationIssuer` from Zoe to
// transform the untrusted invitation to a trusted one
const invitation = await invitationIssuer.claim(untrustedInvitation);
const { value: invitationValue } =
    await E(invitationIssuer).getAmountOf(invitation);

# E(zoe).getInvitationDetails(invitation)

  • invitation {Invitation}
  • Returns {Promise<Object>}

Takes an invitation as an argument and returns an object containing the following details about the invitation:

  • installation {Installation}: The contract's installation in Zoe.
  • instance {Instance}: The contract instance this invitation is for.
  • invitationHandle {Handle}: A handle used to refer to this invitation.
  • description {String}: describes the purpose of this invitation. Use it to match the invitation to the role it plays in the contract.
const invitation = await invitationIssuer.claim(untrustedInvitation);
const invitationValue = await E(zoe).getInvitationDetails(invitation);

# E(zoe).install(bundle)

  • bundle {SourceBundle}
  • Returns: {Promise<Installation>}

Takes bundled source code for a Zoe contract as an argument and installs the code on Zoe. Returns an installation object.

An installation is an object with one property:

  • bundle: The contract source code, accessible via bundle.source, and other info.
// bundleSource takes source code files and
// bundles them together in the format install expects.
import bundleSource from '@agoric/bundle-source';
const bundle = await bundleSource(pathResolve(`./src/contract.js`));
const installationP = await E(zoe).install(bundle);

# E(zoe).getInstance(invitation)

  • invitation {Invitation}
  • Returns: {Promise<Instance>}

Returns a Promise for the contract instance the invitation is part of.

While instances are opaque objects, you can get information about them via these methods:

  • getBrands()
  • getTerms()
  • getIssuers()
  • getPublicFacet()
const instance = await E(zoe).getInstance(invitation);

# E(zoe).getInstallation(invitation)

  • invitation {Invitation}
  • Returns: {Promise<Installation>}

Returns a Promise for the contract installation the invitation's contract instance uses.

An installation is an object with one property:

  • bundle: The contract source code, accessible via bundle.source, and other info.
const installation = await E(zoe).getInstallation(invitation);

# E(zoe).startInstance(installation, issuerKeywordRecord, terms)

  • installation {Installation}
  • issuerKeywordRecord {IssuerKeywordRecord}
  • terms {Object}
  • Returns: {Promise<StartInstanceResult>}

Create an instance of the installed smart contract (specified by the installation argument). You must also specify the instance's issuerKeywordRecord (as key-value pairs) and terms for the contract.

The issuerKeywordRecord is a record mapping string names (keywords) to issuers, such as { Asset: quatlooIssuer}. Keywords must begin with a capital letter and must be ASCII. Parties to the contract will use the keywords to index their proposal and their payments.

The terms are values used by this contract instance, such as the number of bids an auction will wait for before closing. These values may be different for different instances of the same contract, but the contract defines what variables need their values passed in as terms.

It returns a promise for a StartInstanceResult object. The object consists of:

  • creatorFacet {any}
  • publicFacet {any}
  • instance {Instance}
  • creatorInvitation {Payment | undefined}

A publicFacet is an object available via Zoe to anyone knowing the instance they are associated with. The publicFacet is used for general queries and actions, such as getting a current price or creating public invitations. Since a facet is defined just as any other object, the contract developer can add methods to them just like they would any object.

The creatorFacet is only available in this return value (i.e. only when starting a contract instance). The contract designer should use it to encapsulate things that the contract runner might not want to share, or might want to control the distribution of. The party who starts the contract should carefully consider the impact before sharing access to the creatorFacet.

creatorInvitation is an invitation that the contract instance creator can use. It is usually used in contracts where the creator immediately sells something (auctions, swaps, etc.), so it's helpful for the creator to have an invitation to escrow and sell goods. Remember that Zoe invitations are represented as a Payment.

const issuerKeywordRecord = {
  'Asset': moolaIssuer,
  'Price': quatlooIssuer
};
const terms = { numBids: 3 };
const { creatorFacet, publicFacet, creatorInvitation } = await E(zoe).startInstance(
  installation, issuerKeywordRecord, terms);

# E(Zoe).offer(invitation, proposal, paymentKeywordRecord)

  • invitation {Invitation|Promise<Invitation>}
  • proposal {Proposal}
  • paymentKeywordRecord {PaymentKeywordRecord}
  • Returns: {Promise<UserSeat>}

Used to make an offer to the contract that created the invitation that is provided as the first argument.

The invocation normally includes a proposal (the rules under which they want to exercise the offer) and payments that correspond to the give property of the proposal. The payments will be escrowed by Zoe. If either the proposalor payments are empty, indicate this by omitting that argument or passing undefined, instead of passing an empty record.

The optional exit's value should be an exitRule, an object with three possible keys for key:value pairs:

  • onDemand:null: (Default) The user can cancel on demand.
  • waived:null: The user can't cancel and relies entirely on the smart contract to promptly finish their offer.
  • afterDeadline: The offer is automatically cancelled after a deadline, as determined by its timer and deadline properties. The timer is a timer, and the deadline is with respect to the timer. Some example timers use Unix epoch time, while others count block height.
const myProposal = harden({
  give: { Asset: quatloos(4)},
  want: { Price: moola(15) },
  exit: { afterDeadline: {
    timer,
    deadline: 100,
  }}
})

paymentKeywordRecord is a record with keywords as keys, with values of the actual payments to be escrowed. A payment is expected for every entry under give.

offer() returns a promise for a UserSeat.

const paymentKeywordRecord = {
  'Asset' : quatloosPayment,
  'Price' : moolaPayment
};

# UserSeat Object

Zoe uses seats to access or manipulate offers. They let contracts and users interact with them. Zoe has two kinds of seats. ZCFSeats are used within contracts and with zcf methods. UserSeats represent offers external to Zoe and the contract. The party who exercises an invitation and sends the offer() message to Zoe gets a UserSeat that can check payouts' status or retrieve the result of processing the offer in the contract. This varies, but examples are a string and an invitation for another seat.

Also, a UserSeat can be handed to an agent outside Zoe and the contract, letting them query or monitor the current state, access the payouts and result, and, if it's allowed for this seat, call tryExit().

Since anyone can attempt to exit the seat if they have a reference to it, you should only share a UserSeat with trusted parties.

UserSeat includes queries for the associated offer's current state and an operation to request that the offer exit, as follows:

  • getCurrentAllocation()
    • Returns: { Promise<Allocation> }

    • An Allocation is an AmountKeywordRecord of key-value pairs where the key is a keyword such as Asset or Price applicable to the contract. The value is an amount.

      Allocations represent the amounts to be paid out to each seat on exit. Normal reasons for exiting are the user requesting to exit or the contract explicitly choosing to close out the seat. The guarantees also hold if the contract encounters an error or misbehaves. There are several methods for finding out what amount a current allocation is.

      An Allocation example:

      • {
          Asset: quatloosAmountMath.make(5),
          Price: moolaAmountMath.make(9)
        }
        
  • getProposal()
    • Returns: { Promise<ProposalRecord> }

    • A Proposal is represented by a ProposalRecord. It is the rules accompanying the escrow of payments dictating what the user expects to get back from Zoe. It has keys give, want, and exit. give and want are records with keywords as keys and amounts as values. If it is compatible with the contract, the contract tries to satisfy it. If not, the contract kicks the seat out.

      Offer safety is always enforced; if kicked out, the user gets back what they put in. If the contract attempts to satisfy it, they either get what they asked for or Zoe ensures they get back their deposit.

    • Example:

      const { want, give, exit } = sellerSeat.getProposal();
      
  • getPayouts()
    • Returns: { Promise<PaymentPKeywordRecord> }
    • A payout is a payment that goes to a party in a successful transaction, redirecting escrowed assets in accordance with the result of the transaction. Returns a record containing all the payout payments associated with the seat's offers.
  • getPayout(keyword)
    • Returns: { Promise<Payment> }
    • A payout is a payment that goes to a party in a successful transaction, redirecting escrowed assets in accordance with the result of the transaction. Returns the payout payment associated with the keyword argument.
  • getOfferResult()
    • Returns: { Promise<OfferResult> }
    • The returned OfferResult can be literally anything. For example, in tests for the Automated Refund Dapp, it's the string "The offer was accepted". In the Covered Call example, it's a call option, which is an assayable invitation to buy the underlying asset. Strings and invitations are the most common things returned. The value is set by the returned result of the offerHandlers function passed as an argument to zcf.makeInvitation().
  • getNotifier()
    • Returns: { Promise<Notifier> }
    • You use a notifier wherever some piece of code has changing state that other code wants updates on. The updates can be anything the contract wants to publish. For example, you could notify about price changes, new currency pools, etc. For more about notifiers, see our Distributed Programming Guide(opens new window)
  • hasExited()
    • Returns: { Promise<Boolean> }
    • Returns true if the seat has exited, false if it is still active.
  • tryExit()
    • Returns { Void }
    • Note: Only works if the seat's proposal has an OnDemand exit clause. Zoe's offer-safety guarantee applies no matter how a seat's interaction with a contract ends. Under normal circumstances, the participant might be able to call tryExit(), or the contract might do something explicitly. On exiting, the seat holder gets its current allocation and the seat can no longer interact with the contract.