Paying out profits from the Taco Shop

In the previous tutorial we have learnt how to setup & interact with the LIGO CLI. Followed an implementation of a simple Taco Shop smart contract for our entrepreneur Pedro.

In this tutorial we will make sure Pedro has access to tokens that people have spent at his shop when buying tacos.


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Analyzing the Current Contract

taco-shop.ligo

type taco_supply is record [
current_stock : nat;
max_price : tez
]
type taco_shop_storage is map (nat, taco_supply)
type return is list (operation) * taco_shop_storage
function buy_taco (const taco_kind_index : nat ; var taco_shop_storage : taco_shop_storage) : return is
block {
// Retrieve the taco_kind from the contract's storage or fail
const taco_kind : taco_supply =
case taco_shop_storage[taco_kind_index] of
Some (kind) -> kind
| None -> (failwith ("Unknown kind of taco.") : taco_supply)
end;
const current_purchase_price : tez =
taco_kind.max_price / taco_kind.current_stock;
if amount =/= current_purchase_price then
// We won't sell tacos if the amount is not correct
failwith ("Sorry, the taco you are trying to purchase has a different price");
else skip;
// Decrease the stock by 1n, because we have just sold one
taco_kind.current_stock := abs (taco_kind.current_stock - 1n);
// Update the storage with the refreshed taco_kind
taco_shop_storage[taco_kind_index] := taco_kind
} with ((nil : list (operation)), taco_shop_storage)

Purchase Price Formula

Pedro's Taco Shop contract currently enables customers to buy tacos, at a price based on a simple formula.

const current_purchase_price : tez =
taco_kind.max_price / taco_kind.current_stock

Replacing spendable Smart Contracts

However, due to the recent protocol upgrade of the Tezos mainnet, Pedro cannot access the tokens stored in his shop's contract directly. This was previously possible via spendable smart contracts, which are no longer available in the new protocol. We will have to implement a solution to access tokens from the contract programatically.


Designing a Payout Scheme

Pedro is a standalone bussines owner, and in our case, he does not have to split profits and earnings of the taco shop with anyone. So for the sake of simplicity, we will payout all the earned XTZ directly to Pedro right after a succesful purchase.

This means that after all the purchase conditions of our contract are met, e.g., the correct amount is sent to the contract, we will not only decrease the supply of the individual purchased taco kind, but we will also transfer this amount in a subsequent transaction to Pedro's personal address.

Forging a Payout Transaction

Defining the Recipient

In order to send tokens, we will need a receiver address, which, in our case, will be Pedro's personal account. Additionally we will wrap the given address as a contract (unit), which represents either a contract with no parameters, or an implicit account.

const ownerAddress : address = ("tz1TGu6TN5GSez2ndXXeDX6LgUDvLzPLqgYV" : address);
const receiver : contract (unit) =
case (Tezos.get_contract_opt (ownerAddress): option(contract(unit))) of
Some (contract) -> contract
| None -> (failwith ("Not a contract") : (contract(unit)))
end;

Would you like to learn more about addresses, contracts and operations in LIGO? Check out the LIGO cheat sheet

Adding the Transaction to the List of Output Operations

Now we can transfer the amount received by buy_taco to Pedro's ownerAddress. We will do so by forging a transaction (unit, amount, receiver) within a list of operations returned at the end of our contract.

const payoutOperation : operation = Tezos.transaction (unit, amount, receiver) ;
const operations : list (operation) = list [payoutOperation];

Finalizing the Contract

taco-shop.ligo

type taco_supply is record [
current_stock : nat;
max_price : tez
]
type taco_shop_storage is map (nat, taco_supply)
type return is list (operation) * taco_shop_storage
const ownerAddress : address =
("tz1TGu6TN5GSez2ndXXeDX6LgUDvLzPLqgYV" : address)
function buy_taco (const taco_kind_index : nat ; var taco_shop_storage : taco_shop_storage) : return is
block {
// Retrieve the taco_kind from the contract's storage or fail
const taco_kind : taco_supply =
case taco_shop_storage[taco_kind_index] of
Some (kind) -> kind
| None -> (failwith ("Unknown kind of taco.") : taco_supply)
end;
const current_purchase_price : tez =
taco_kind.max_price / taco_kind.current_stock;
if amount =/= current_purchase_price then
// We won't sell tacos if the amount is not correct
failwith ("Sorry, the taco you are trying to purchase has a different price");
else skip;
// Decrease the stock by 1n, because we have just sold one
taco_kind.current_stock := abs (taco_kind.current_stock - 1n);
// Update the storage with the refreshed taco_kind
taco_shop_storage[taco_kind_index] := taco_kind;
const receiver : contract (unit) =
case (Tezos.get_contract_opt (ownerAddress): option(contract(unit))) of
Some (contract) -> contract
| None -> (failwith ("Not a contract") : (contract(unit)))
end;
const payoutOperation : operation = Tezos.transaction (unit, amount, receiver);
const operations : list(operation) = list [payoutOperation]
} with ((operations : list (operation)), taco_shop_storage)

Dry-run the Contract

To confirm that our contract is valid, we can dry-run it. As a result, we see a new operation in the list of returned operations to be executed subsequently.

ligo dry-run taco-shop.ligo --syntax pascaligo --amount 1 buy_taco 1n "map [
1n -> record [
current_stock = 50n;
max_price = 50tez
];
2n -> record [
current_stock = 20n;
max_price = 75tez
];
]"
Operation(...bytes) included in the output

Done! Our tokens are no longer locked in the contract, and instead they are sent to Pedro's personal account/wallet.


👼 Bonus: Donating Part of the Profits

Because Pedro is a member of the Specialty Taco Association (STA), he has decided to donate 10% of the earnings to the STA. We will just add a donationAddress to the contract, and compute a 10% donation sum from each taco purchase.

const ownerAddress : address = ("tz1TGu6TN5GSez2ndXXeDX6LgUDvLzPLqgYV" : address);
const donationAddress : address = ("tz1KqTpEZ7Yob7QbPE4Hy4Wo8fHG8LhKxZSx" : address);
const receiver : contract (unit) =
case (Tezos.get_contract_opt (ownerAddress) : option(contract(unit))) of
Some (contract) -> contract
| None -> (failwith ("Not a contract") : contract (unit))
end;
const donationReceiver : contract (unit) =
case (Tezos.get_contract_opt (donationAddress) : option(contract(unit))) of
Some (contract) -> contract
| None -> (failwith ("Not a contract") : contract (unit))
end;
const donationAmount : tez = amount / 10n;
const operations : list (operation) = list [
// Pedro will get 90% of the amount
transaction (unit, amount - donationAmount, receiver);
transaction (unit, donationAmount, donationReceiver)
];

This will result into two operations being subsequently executed on the blockchain:

  • Donation transfer (10%)
  • Pedro's profits (90%)