The Structure of Offers

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

Making An offer

To make an offer, you use E(zoe).offer(...), which takes up to four arguments:

• An invitation to participate in this contract instance.
• A proposal stating your offer conditions.
• The payments escrowed for the offer, each corresponding with a give Keyword in proposal.
• offerArgs expressing additional arguments for the offerHandler associated with the invitation by zcf.makeInvitation(...).

Invitations

An Invitation is a special case of ERTP Payment. Each is linked to a specific contract Instance, and having one gives you the right to participate in that contract instance by using that invitation as the first argument to E(zoe).offer(...).

There are two ways for contract users to get an invitation:

• If you create the contract instance, the contract might supply a special creator invitation.
• Someone (possibly you) who holds the right objects has created an invitation for a contract instance and gives it to you in some way. This could've been by sending it to you, posting it on a public online location, etc. It doesn't matter (nor does Zoe specify or have any requirements) how or why it got to you, only that you have it.

Proposals

Proposals are records with give, want, and/or exit keys.

const myProposal = harden({
  give: { Asset: AmountMath.make(quatloosBrand, 4n) },
  want: { Price: AmountMath.make(moolaBrand, 15n) },
  exit: { onDemand: null },
});

give and want use Keywords defined by the contract. Keywords are unique identifiers per contract, that tie together proposals, payments to be escrowed, and payouts to users. In the example above, "Asset" and "Price" are the Keywords. However, in an auction contract, the Keywords might be "Asset" and "Bid".

Each AmountMath.make call above is just making an ERTP Amount, or description of digital assets. In this case, AmountMath.make(quatloosBrand, 4n) creates a description of 4 units of our imaginary Quatloos currency and AmountMath.make(moolaBrand, 15n) creates a description of 15 units of our imaginary Moola currency. (The "n" appended after each number indicates that it is represented as a BigInt rather than a Number)

Note

Amounts are just descriptions of assets, and have no intrinsic value of their own. In contrast, Payments hold actual digital assets.

exit specifies how the offer can be cancelled. It must conform to one of three shapes:

• { onDemand: null }: (Default) The offering party can cancel on demand.
• { waived: null }: The offering party can't cancel and relies entirely on the smart contract to complete (finish or fail) their offer.
• { afterDeadline: deadlineDetails }: The offer is automatically cancelled after a deadline, as determined by its timer and deadline properties.

For more details, see Proposals.

Escrowed Payments

Using the same Keywords as the give object in your proposal, you must specify a PaymentPKeywordRecord containing Payments of the corresponding digital assets. Zoe escrows these payments on behalf of your offer until it is completed or rejected or the assets are reassigned to another offer.

const payments = {
  Asset: quatloosPayment,
  Price: moolaPayment,
};

Offer Arguments

To pass additional arguments to the offerHandler contract code associated with the invitation, send them in an offerArgs CopyRecord. Each contract can define the properties it supports and which are required, and is responsible for handling any unexpected or missing arguments.

DANGER

Contract code should be careful interacting with offerArgs. These values need input validation before being used since they are coming directly from the caller and may have malicious behavior.

Returned Value

E(zoe).offer(...) returns a promise for a UserSeat object. Its name comes from the concept of "having a seat at the table" for the contract's execution.