The Taco Shop Smart Contract

Meet Pedro, our artisan taco chef, who has decided to open a Taco shop on the Tezos blockchain, using a smart contract. He sells two different kinds of tacos: el Clásico and the Especial del Chef.

To help Pedro open his dream taco shop, we will implement a smart contract that will manage supply, pricing & sales of his tacos to the consumers.


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Pricing

Pedro's tacos are a rare delicacy, so their price goes up as the stock for the day begins to deplete.

Each taco kind, has its own max_price that it sells for, and a finite supply for the current sales lifecycle.

For the sake of simplicity, we will not implement the replenishing of the supply after it has run out.

Daily Offer

kindidavailable_stockmax_price
Clásico1n50n50tez
Especial del Chef2n20n75tez

Calculating the Current Purchase Price

The current purchase price is calculated with the following formula:

current_purchase_price = max_price / available_stock

El Clásico

available_stockmax_pricecurrent_purchase_price
50n50tez1tez
20n50tez2.5tez
5n50tez10tez

Especial del chef

available_stockmax_pricecurrent_purchase_price
20n75tez3.75tez
10n75tez7.5tez
5n75tez15tez

Installing LIGO

In this tutorial, we will use LIGO's dockerized version, for the sake of simplicity. You can find the installation instructions here.

The best way to install the dockerized LIGO is as a global executable through the installation script, as shown in the screenshot below:

Installing the next version of LIGO's CLI

Implementing our First main Function

From now on we will get a bit more technical. If you run into something we have not covered yet - please try checking out the LIGO cheat sheet for some extra tips & tricks.

To begin implementing our smart contract, we need a main function, that is the first function being executed. We will call it main and it will specify our contract's storage (int) and input parameter (int). Of course this is not the final storage/parameter of our contract, but it is something to get us started and test our LIGO installation as well.

taco-shop.ligo

function main (const parameter : int; const contractStorage : int) :
list (operation) * int is
((nil : list (operation)), contractStorage + parameter)

Let us break down the contract above to make sure we understand each bit of the LIGO syntax:

  • function main - definition of the main function, which takes the parameter of the contract and the storage
  • (const parameter : int; const contractStorage : int) - parameters passed to the function: the first is called parameter because it denotes the parameter of a specific invocation of the contract, the second is the storage
  • (list (operation) * int) - return type of our function, in our case a tuple with a list of operations, and an int (new value for the storage after a succesful run of the contract)
  • ((nil : list (operation)), contractStorage + parameter) - essentially a return statement
  • (nil : list (operation)) - a nil value annotated as a list of operations, because that is required by our return type specified above
    • contractStorage + parameter - a new storage value for our contract, sum of previous storage and a transaction parameter

Running LIGO for the First Time

To test that we have installed LIGO correctly, and that taco-shop.ligo is a valid contract, we will dry-run it.

Dry-running is a simulated execution of the smart contract, based on a mock storage value and a parameter.

Our contract has a storage of int and accepts a parameter that is also an int.

The dry-run command requires a few parameters:

  • contract (file path)
  • entrypoint (name of the main function in the contract)
  • parameter (parameter to execute our contract with)
  • storage (starting storage before our contract's code is executed)

It outputs what is returned from our main function: in our case a tuple containing an empty list (of operations to apply) and the new storage value, which, in our case, is the sum of the previous storage and the parameter we have used for the invocation.

# Contract: taco-shop.ligo
# Main function: main
# Parameter: 4
# Storage: 3
ligo dry-run taco-shop.ligo --syntax pascaligo main 4 3
# tuple[ list[]
# 7
# ]
Simulating contract execution with the CLI

3 + 4 = 7 yay! Our CLI & contract work as expected, we can move onto fulfilling Pedro's on-chain dream.


Designing the Taco Shop's Contract Storage

We know that Pedro's Taco Shop serves two kinds of tacos, so we will need to manage stock individually, per kind. Let us define a type, that will keep the stock & max_price per kind in a record with two fields. Additionally, we will want to combine our taco_supply type into a map, consisting of the entire offer of Pedro's shop.

Taco shop's storage

type taco_supply is record [
current_stock : nat;
max_price : tez
]
type taco_shop_storage is map (nat, taco_supply)

Next step is to update the main function to include taco_shop_storage in its storage. In the meanwhile, let us set the parameter to unit as well to clear things up.

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 main (const parameter : unit; const taco_shop_storage : taco_shop_storage) : return is
((nil : list (operation)), taco_shop_storage)

Populating our Storage in a dry-run

When dry-running a contract, it is crucial to provide a correct initial storage value. In our case the storage is type-checked as taco_shop_storage. Reflecting Pedro's daily offer, our storage's value will be defined as follows:

Storage value

map [
1n -> record [
current_stock = 50n;
max_price = 50tez
];
2n -> record [
current_stock = 20n;
max_price = 75tez
]
]

The storage value is a map with two bindings (entries) distinguished by their keys 1n and 2n.

Dry run command with a multi-line storage value

ligo dry-run taco-shop.ligo --syntax pascaligo main unit "map [
1n -> record [
current_stock = 50n;
max_price = 50tez
];
2n -> record [
current_stock = 20n;
max_price = 75tez
]
]"
Dry-run with a complex storage value

If everything went as expected, the dry-run command will return an empty list of operations and the contract's current storage, which is the map of the products we have defined based on the daily offer of Pedro's taco shop.


Providing another Access Function for Buying Tacos

Now that we have our stock well defined in form of storage, we can move on to the actual sales. The main function will take a key id from our taco_shop_storage map and will be renamed buy_taco for more readability. This will allow us to calculate pricing, and if the sale is successful, we will be able to reduce our stock because we have sold a taco!

Selling the Tacos for Free

Let is start by customizing our contract a bit, we will:

  • rename parameter to taco_kind_index
  • change taco_shop_storage to a var instead of a const, because we will want to modify it

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
((nil : list (operation)), taco_shop_storage)

Decreasing current_stock when a Taco is Sold

In order to decrease the stock in our contract's storage for a specific taco kind, a few things needs to happen:

  • retrieve the taco_kind from our storage, based on the taco_kind_index provided;
  • subtract the taco_kind.current_stock by 1n;
  • we can find the absolute value of the subtraction above by calling abs (otherwise we would be left with an int);
  • update the storage, and return it.

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;
// 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)
Stock decreases after selling a taco

Making Sure We Get Paid for Our Tacos

In order to make Pedro's taco shop profitable, he needs to stop giving away tacos for free. When a contract is invoked via a transaction, an amount of tezzies to be sent can be specified as well. This amount is accessible within LIGO as Tezos.amount.

To make sure we get paid, we will:

  • calculate a current_purchase_price based on the equation specified earlier
  • check if the sent amount matches the current_purchase_price:
    • if not, then our contract will fail (failwith)
    • otherwise, stock for the given taco_kind will be decreased and the payment accepted

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 Tezos.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)

In order to test the amount sent, we will use the --amount option of dry-run:

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
]
]"

Purchasing a Taco with 1tez

Stock decreases after selling a taco, if the right amount of tezzies is provided

Attempting to Purchase a Taco with 0.7tez

Stock does not decrease after a purchase attempt with an insufficient payment.

That's it - Pedro can now sell tacos on-chain, thanks to Tezos & LIGO.


💰 Bonus: Accepting Tips above the Taco Purchase Price

If you would like to accept tips in your contract, simply change the following line, depending on your preference.

Without tips

if Tezos.amount =/= current_purchase_price then

With tips

if Tezos.amount >= current_purchase_price then