This demo is roughly based on Reddit's r/Place, but has a number of additional features that showcase the unique affordances of the Agoric platform, including: higher-order contracts, easy creation of new assets, and safe code reusability.
|Reddit's r/place as a social experiment in cooperation|
|The testnet pixel demo. Slightly fewer pixels.|
The pixel demo runs on our private testnet. For instructions on how to run a local, off-chain version for yourself, please see Scenario 3. The testnet documentation also includes a short walkthrough of the demo with sample commands. If you would rather view the code and learn more about how the demo works, feel free to read on.
ERTP: Electronic Rights Transfer Protocol
The pixel demo uses our smart contract framework, called ERTP, or the Electronic Rights Transfer Protocol. Throw away all you know about public key hashes mapped to account balances, because ERTP doesn't use 'em. In fact, ERTP itself doesn't have any concept of cryptography. Instead, it uses object capabilities to enforce access control. Instead of having to prove ownership of a corresponding private key, in the world of object capabilities, if your program has a reference to an object, it can call methods on that object. If it doesn't have a reference, it can't. For more on object capabilities, see Chip Morningstar's post. For more on ERTP, see a quick tutorial and the ERTP interface descriptions for assay.js, unitOps.js and contractHost.js.
A preemption hierarchy of rights
All of the pixels (individual squares) on the demo webpage
are owned by the gallery, the code that administrates the pixels. The
gallery has the power to revoke the pixels from any user at any time. When a user
gallery!tapFaucet(), the gallery takes the least recently
colored pixel from its current holder (if anyone holds it) and gives
it to the user in the form of an ERTP payment.
|The preemption hierarchy of rights in the pixel demo|
The gallery is able to revoke the pixels held by users because the pixel demo creates a customized version of ERTP in which rights are hierarchical. Hierarchical rights are familiar in property. For instance, a property owner might lease an apartment to a tenant, who might in turn, lease it to a subtenant. Anyone higher in the hierarchy can take away access from anyone lower in the hierarchy, and give it to someone else.
ERTP provides a
makeMint function that takes in a configuration
function. The pixel demo configuration implements the hierarchical
rights and allows us to add additional methods to purses and payments.
To be able to color a pixel, the user must first get a "use object" from a purse or payment that contains pixels:
const useObj = E(payment).getUse()
with the hex color of their choice.
Transferring revocable rights
When a user wants to give another user the ability to color a pixel,
but wants to be able to revoke that ability later, the user can call
claimChild and pass the childPayment to the other user. The
other user can tell that they aren't getting a payment on the same
level as one that the gallery would give out, and it could be revoked
at any point if
claimChild() is called again on the original
Buying and selling pixels
The users can buy and sell pixels with the gallery at any time by
buyFromGallery(pixelUnits). They must pass in a
describes exactly what they want to sell or buy. If successful, they
will get an invitation from the gallery for a simple, secure escrow
exchange of the pixels for "dust", the currency that the gallery
transacts in. Users do not start out with any dust - they only start
out with access to the faucet.
In order to amass the pixels that they want in order to draw their masterpiece, the user will need to sell some pixels to get dust. The gallery will always buy pixels back, but it values pixels near the center much more than pixels on the periphery.