Documentation

# Purse Object

Purses hold digital assets. They are created to hold assets of a single Brand, and the Brand that the Purse holds cannot be changed. For example, if you were to create a Purse that holds Quatloos, that Purse wouldn't ever be able to hold Moola, theater tickets, or any other non-Quatloo asset type.

Digital assets in Purses can be any of:

  • Currency-like, such as our imaginary Quatloos currency.
  • Goods-like digital assets, such as theater tickets or magic weapons used in a game.
  • Other kinds of rights, such as the right to participate in a particular contract.

While each Purse can only hold assets of one Brand, any number of Purses can be created that hold that Brand. So you could have, say, three Quatloos Purses, your friend Alice could have eight Quatloos Purses, and so on.

Purses are created by calling the anIssuer.makeEmptyPurse() method on the Issuer associated with the Brand of assets you want the new Purse to hold.

const { issuer: quatloosIssuer } = makeIssuerKit('quatloos');
const quatloosPurse = quatloosIssuer.makeEmptyPurse();

You change a Purse's balance by calling either aPurse.deposit() or aPurse.withdraw() on it. A Purse can be empty, which if it holds a fungible currency means it has a value of 0. If it holds a non-fungible asset (e.g., theater tickets), then the Purse simply doesn't contain any assets if it's empty.

Unlike Payments, Purses are not meant to be sent to others. To transfer digital assets, you should withdraw a Payment from a Purse and send the Payment to another party.

# aPurse.getCurrentAmount()

Returns the Purse's current balance as an Amount. The returned Amount AmountValue might be empty, and might be different the next time you call aPurse.getCurrentAmount() on the same Purse if assets have been deposited or withdrawn in the interim.

const { issuer: quatloosIssuer } = makeIssuerKit('quatloos');
const quatloosPurse = quatloosIssuer.makeEmptyPurse();
// quatloos5 is a payment with balance of 5 quatloos
const quatloosPurse.deposit(quatloos5);
// Returns an amount with value = 5 and brand = quatloos
const currentBalance = quatloosPurse.getCurrentAmount();

# aPurse.getCurrentAmountNotifier()

Returns a lossy notifier for changes to this Purse's balance. For more details, see Notifiers.

const notifier = purse.getCurrentAmountNotifier();
let nextUpdate = notifier.getUpdateSince();

const checkNotifier = async () => {
  const { value: balance, updateCount } = await nextUpdate;
  nextUpdate = notifier.getUpdateSince(updateCount);
};

# aPurse.deposit(payment, optAmount?)

Deposit all the contents of payment into the Purse, returning an Amount describing the Payment's digital assets (i.e. the deposited amount). If the optional argument optAmount does not equal the balance of payment, or if payment is a promise, this method throws an error.

While the above applies to local and remote Purses, for remote Purses there may be effects on this operation due to the use of promises and asynchronicity. You have to have a non-promise Payment before calling aPurse.deposit(). When you call aPurse.deposit() you get a response back (after waiting for the round trip) telling you if it succeeded. All later arriving calls see the value has been transferred into the Purse, and the Payment is no longer valid.

If any withdrawals are waiting for promises to resolve, a deposit operation may pass them by. This is safe, as even if all the assets are withdrawn, the deposit still works on an empty Purse.

const { issuer: quatloosIssuer, mint: quatloosMint, brand: quatloosBrand } =
  makeIssuerKit('quatloos');
const quatloosPurse = quatloosIssuer.makeEmptyPurse();
const payment = quatloosMint.mintPayment(AmountMath.make(quatloosBrand, 123n));
const quatloos123 = AmountMath.make(quatloosBrand, 123n);

// Deposit a payment for 123 Quatloos into the Purse. 
const depositAmountA = quatloosPurse.deposit(payment, quatloos123);

const secondPayment = quatloosMint.mintPayment(AmountMath.make(quatloosBrand, 100n));
// Throws error
const depositAmountB = quatloosPurse.deposit(secondPayment, quatloos123);

# aPurse.withdraw(amount)

Withdraws the specified Amount of digital assets from the Purse into a new Payment object.

If the call succeeds, it immediately extracts the value into a new Payment. The caller won't get the new Payment until a later turn, since the call is (nearly always) remote. But as soon as the message is processed, the value is gone from the Purse.

// Create a Purse and give it a balance of 10 Quatloos
const { issuer, mint, brand } = makeIssuerKit('quatloos');
const purse = issuer.makeEmptyPurse();
const payment = mint.mintPayment(AmountMath.make(brand, 10n));
const quatloos10 = AmountMath.make(brand, 10n);
purse.deposit(payment, quatloos10);

// Withdraw an amount of 3 from the Purse
const quatloos3 = AmountMath.make(brand, 3n);
const withdrawalPayment = purse.withdraw(quatloos3);

// The balance of the withdrawal payment is 3 Quatloos
issuer.getAmountOf(withdrawalPayment);

// The new balance of the Purse is 7 Quatloos
purse.getCurrentAmount();

# aPurse.getAllegedBrand()

Returns an alleged brand (Note: a Brand, not a String as allegedName() methods do), indicating what kind of digital asset the Purse purports to hold. This can identify the Purse's Brand if the Purse was made by a trusted Issuer using anIssuer.makeEmptyPurse().

const purseBrand = quatloosPurse.getAllegedBrand();

# aPurse.getDepositFacet()

  • Returns: DepositFacet

Creates and returns a DepositFacet, a new deposit-only facet of the Purse that allows other parties to deposit Payments into the Purse without the ability to check the Purse's balance or withdraw from it. This makes it a safe way to let other people send you Payments.

You can only deposit a Payment into a DepositFacet that's the same Brand as the original Purse.

const depositOnlyFacet = purse.getDepositFacet();

// Give depositOnlyFacet to someone else. Anyone with a deposit facet reference can tell it to receive
// a payment, thus depositing the payment assets in the Purse associated with the deposit facet.
depositOnlyFacet.receive(payment);

Once you have created a DepositFacet, there is one method you can call on it, aDepositFacet.receive(). The DepositFacet takes a Payment and adds it to the balance of the DepositFacet's associated Purse. The Payment must be of the same Brand as what the Purse holds.

Note the difference in method names for adding assets between a Purse and its DepositFacet. To add assets to a Purse directly, you use aPurse.deposit(). To add assets to a Purse via its DepositFacet, you use aDepositFacet.receive().

# aDepositFacet.receive(payment, optAmount?)

The DepositFacet takes the Payment and adds it to the balance of the DepositFacet's associated Purse.

If the optional argument optAmount does not equal the balance of payment, or if payment is an unresolved promise, this method throws an error.

const depositOnlyFacet = purse.getDepositFacet();

// Give depositOnlyFacet to someone else. Anyone with a deposit facet reference can tell it to receive
// a payment, thus depositing the payment assets in the Purse associated with the deposit facet.
depositOnlyFacet.receive(payment);

← Payment Object →