day_4

Daily guide : day 4 🐓

Welcome into the day 4 of the Motoko Bootcamp !
Today we will cover the following topics : Custom type and Linked list.

Prerequisites ✅

• Make sure you have dfx installed on your machine.

  dfx --version
  

• Start a new project called day_4 and turn on your local replica.

  dfx new day_4
  cd day_4
  dfx start
  

Custom type 🔧

During the previous days we have manipulated different types (Nat, Text, Bool, Array); today we are creating to create our own type ! Create a new file called person.mo in the day_4 folder (next to main.mo) and copy and paste the following declaration.

module {

}

This is a module declaration, contrary to all previous .mo files that were actor declarations.
A module is (just the like the base library modules we've been using) a piece of code that we can import and reuse.

This is how we can create a custom type. If you want your type to be accessible from the outside of the module you need to make it public.

public type Person = {
  name : Text;
  age : Nat;
};

The type Person is also called an object with different fields (name, age).

Now that we have declared our type we can create our first variable of type Person.

let tom : Person = {
  name = "Tom Cruise";
  age = 59;
};

You can access any field using the dot notation.

let tom_name = tom.name;
let tom_age = tom.age;

In your main.mo file you can import the type Person and use it as if it was part of Motoko native types.

import Person "person";
actor {
  public type Person = Person.Person;
  let penelope : Person = {
    name = "Penelope Cruz";
    age = 47;
  };

};

💡 Person.Person simply means : grab the type Person from the Person module.

Challenge 🎮

Take a break and try completing challenge 1 to 3.

Linked list 🔗

A linked list is a datastructure, that contrary to array has a dynamic size. It consists of many nodes each node contains :

• A value of type T.
• A pointer to the next node or to null.

A linked list of Nat

The first element of this linked list is a node where :

• Value of type Nat equals to 12.
• A pointer to next node.

💡 A pointer is just a memory location indicating where a data is stored. You can think of it as an arrow indicating where to find the next node.

Here's how one would define a linked list in Motoko.

public type List<T> = ?(T, List<T>);

This means that a List of type T is either :

• null
• A couple of two elements.
  • 1st element is a value of type T.
  • 2nd element is a List of type T.

💡 To acces an element in a linked list you need to start from the first node and follow all the nodes until you came accross the value you are looking for.
This is more complex than with arrays where we could just use array[i], but linked lists have other advantages that we will see.

Challenge 🎮

Take a break and try completing challenge 4 to 6.

Switch / Case

We have already seen the switch/case expression in previous lessons, but let's make a quick review. We can use the switch/case on optional expression. An element of type List can be of value null. We can check it that way.

public type List<T> = ?(T, List<T>);
public func is_list_null(list : List<Nat>) : async Text {
        switch(list){
            case(null) {
                return "The list is null... 😢"
            };
            case(?list){
                return "This list is not null 🥳"
            };
        }
    };

The switch/case can also be used for testing different values.

public func month_to_season (month : Text) : async Text {
  switch(month){
    case("January") return "Winter";
    case("February") return "Winter";
    case("March") return "Spring";
    case("April") return "Spring";
    case("May") return "Spring";
    case("June") return "Summer";
    case("July") return "Summer";
    case("August") return "Summer";
    case("September") return "Autumn";
    case("October") return "Autumn";
    case("November") return "Autumn";
    case("December") return "Winter";
    case(_) return "This is not a month";
  }
};

The last case statement is used to indicate a default value, in case the value in the switch (in our case the month) do not find in any of the previous cases. It will default to the case with the _ indicator :

case(_) return "This is not a month";

You can actually try this function by deploying it in the Motoko Playground and check what value you get back with "April" and then with "Hello".

Challenge 🎮

Take a break and try completing challenge 7 to 10.