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, the el clásico and the especial del chef.

To help Pedro open his dream taco shop, we'll 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 won't implement replenishing of the supply after it runs out.

Daily offer

kind	id	available_stock	max_price
el clásico	`1n`	`50n`	`50000000mtz`
especial del chef	`2n`	`20n`	`75000000mtz`

Calculating the current purchase price

Current purchase price is calculated with the following equation:

current_purchase_price = max_price / available_stock

El clásico

available_stock	max_price	current_purchase_price
`50n`	`50000000mtz`	`1tz`
`20n`	`50000000mtz`	`2.5tz`
`5n`	`50000000mtz`	`10tz`

Especial del chef

available_stock	max_price	current_purchase_price
`20n`	`75000000mtz`	`3.75tz`
`10n`	`75000000mtz`	`7.5tz`
`5n`	`75000000mtz`	`15tz`

Installing LIGO

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

Installing the next version of LIGO's CLI

Implementing our first entry point

From now on we'll 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 an entry point. We'll call it main and it'll specify our contract's storage (int) and input parameter (int). Of course this is not the final storage/parameter of our contract, but it's 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
  block {skip} with ((nil : list(operation)), contractStorage + parameter)

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

function main - definition of a function that serves as an entry point
(const parameter : int; const contractStorage : int) - parameters passed to the function
- const parameter : int - parameter provided by a transaction that invokes our contract
- const contractStorage : int - definition of our storage (int)
(list(operation) * int) - return type of our function, in our case a touple with a list of operations, and an int
block {skip} - our function has no body, so we instruct LIGO to skip it
with ((nil : list(operation)), contractStorage + parameter) - essentially a return statement
- (nil : list(operation)) - a nil value annotated as a list of operations, because that's 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've installed LIGO correctly, and that taco-shop.ligo is a valid contract, we'll 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 entrypoint function in the contract)
parameter (parameter to execute our contract with)
storage (starting storage before our contract's code is executed)

And outputs what's returned from our entrypoint - in our case a touple 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've used.

# Contract: taco-shop.ligo
# Entry point: 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 Taco shop's contract storage

We know that Pedro's Taco Shop serves two kinds of tacos, so we'll need to manage stock individually, per kind. Let's define a type, that will keep the stock & max_price per kind - in a record with two fields. Additionally, we'll 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;
end

type taco_shop_storage is map(nat, taco_supply);

Next step is to update the main entry point to include taco_shop_storage as its storage - while doing that let's 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;
end
type taco_shop_storage is map(nat, taco_supply);

function main (const parameter: unit ; const taco_shop_storage : taco_shop_storage) : (list(operation) * taco_shop_storage) is
  block {skip} with ((nil : list(operation)), taco_shop_storage)

Populating our storage in a dry-run

When dry-running a contract, it's 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 following:

Storage value

map
    1n -> record
        current_stock = 50n;
        max_price = 50000000mtz;
    end;
    2n -> record
        current_stock = 20n;
        max_price = 75000000mtz;
    end;
end

Storage value is a map, with two items in it, both items are records identified by natural numbers 1n & 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 = 50000000mtz;
    end;
    2n -> record
        current_stock = 20n;
        max_price = 75000000mtz;
    end;
end"

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 products we've defined based on the daily offer of Pedro's taco shop.

Providing an entrypoint for buying tacos

Now that we have our stock well defined in form of storage, we can move on to the actual sales. We'll replace the main entrypoint with buy_taco, that takes an id - effectively a key from our taco_shop_storage map. This will allow us to calculate pricing, and if the sale is successful - then we can reduce our stock - because we have sold a taco!

Selling the tacos for free

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

rename the entrypoint from main to buy_taco
rename parameter to taco_kind_index
change taco_shop_storage to a var instead of a const, because we'll want to modify it

taco-shop.ligo

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


function buy_taco (const taco_kind_index: nat ; var taco_shop_storage : taco_shop_storage) : (list(operation) * taco_shop_storage) is
  block { skip } with ((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 (nat) value of the subtraction above by using abs, otherwise we'd 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;
end
type taco_shop_storage is map(nat, taco_supply);

function buy_taco (const taco_kind_index: nat ; var taco_shop_storage : taco_shop_storage) : (list(operation) * taco_shop_storage) is
  begin
    // Retrieve the taco_kind from the contract's storage
    const taco_kind : taco_supply = get_force(taco_kind_index, taco_shop_storage);
    // Decrease the stock by 1n, because we've 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;
  end 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 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 and stop executing
- if yes, 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;
end
type taco_shop_storage is map(nat, taco_supply);

function buy_taco (const taco_kind_index: nat ; var taco_shop_storage : taco_shop_storage) : (list(operation) * taco_shop_storage) is
  begin
    // Retrieve the taco_kind from the contract's storage
    const taco_kind : taco_supply = get_force(taco_kind_index, taco_shop_storage);
    
    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 isn't correct
      fail("Sorry, the taco you're trying to purchase has a different price");
    else
      // Decrease the stock by 1n, because we've 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;
  end with ((nil : list(operation)), taco_shop_storage)

In order to test the amount sent, we'll 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 = 50000000mtz;
    end;
    2n -> record
        current_stock = 20n;
        max_price = 75000000mtz;
    end;
end"

Purchasing a taco with 1.0tz

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

Attempting to purchase a taco with 0.7tz

Stock does not decrease after a purchase attempt with a lower than required amount.

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

💰 Bonus: Accepting tips above the taco purchase price

If you'd like to accept tips in your contract as well, simply change the following line, depending on which behavior do you prefer.

Without tips

if amount =/= current_purchase_price then

With tips

if amount >= current_purchase_price then

Pricing

Daily offer

Calculating the current purchase price

El clásico

Especial del chef

Installing LIGO

Implementing our first entry point

taco-shop.ligo

Running LIGO for the first time

Designing Taco shop's contract storage

Populating our storage in a dry-run

Providing an entrypoint for buying tacos

Selling the tacos for free

Decreasing current_stock when a taco is sold

Making sure we get paid for our tacos

💰 Bonus: Accepting tips above the taco purchase price

`taco-shop.ligo`

Decreasing `current_stock` when a taco is sold