# Contract Requirements

Zoe v0.5.0. Last updated 4/20/2020.

When writing a smart contract that will be run on Zoe, it's important to know the proper format and other expectations.

A Zoe contract is a JavaScript file that can import other JavaScript code, including:

  • @agoric/harden: a package for recursively deep-freezing
  • @agoric/nat: a package for testing whether something is a natural number (natural numbers are recommended for currency-related programming in order to better deal with rounding) and throwing if not.
  • @agoric/notifier: A package that provides updates through smartly resolving promises rather than polling
  • @agoric/zoe: Zoe has helpers that contracts can use by importing @agoric/zoe/src/contractSupport/zoeHelpers

A Zoe contract will be bundled up, so you should feel free to divide your contract into multiple files (perhaps putting helper functions in a separate file, for example) and import them.

A Zoe contract needs to be able to run under Agoric's SES. Some legacy JavaScript code is incompatible with SES, because SES freezes the JavaScript objects you start out with (the primordials, such as Object), and some legacy code tries to mutate these.

The "main" contract file that potentially imports other files and packages needs to have the following structure:

export const makeContract = harden(zcf => {
  return harden({
    invite, // required
    publicAPI, // optional
  • zcf is the Zoe Contract Facet.
  • invite must be a Zoe invite that will be provided to the user who instantiates the contract.
  • publicAPI is an optional object whose methods will be available to anyone who knows the instanceHandle of the contract instance. publicAPI is a good place to put public queries (i.e. getCurrentPrice of Autoswap) and other requests that shouldn't require making an offer or having an invite.

# Creating an invite

To create an invite in the contract, you can use the Zoe Contract Facet method directly (zcf.makeInvitation) or you can use the zoeHelpers function (inviteAnOffer) which has some additional functionality, like being able to specify what kind of proposal the offer should have.