WEBVTT

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Welcome back.

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In this lecture I'm going to show you what happens when you embed a type with methods to another type

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Here is a little warning before you continue.

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If you get confused at any moment of this lecture please check out the struct embedding lecture in the

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structs section.

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After that everything will be clear.

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Let's take a look at these types.

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They all have almost the same set of fields and methods they all use the title and price fields and

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they all use the print methods only the game and toy types have a discount method.

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I might want to make a discount for every type of product.

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So I'm going to add a discount to the book and puzzle types as well.

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Now all the types almost look alike.

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The only difference between them is the published field of the book type.

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But that's okay.

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It can stay.

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Let's invent another type with these common fields and methods.

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Okay great.

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Every product has a title and price fields.

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Also a print and discount methods.

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Let's embed the product type to each product type.

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Now all the types here embed a product type.

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So the book puzzle game and toy types can use the fields and methods of the product type as if they're

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on let's implement this in the coding editor.

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First, I'm going to open the toy type here.

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I'm going to copy everything then I'm going to create a new file for the product type here.

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I'm going to replace the toy word with the product word.

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Let's also change the receivers to P. Short for a product. When the other types embed this type,

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all these fields and methods will be available to them so they will be able to use them as if they're

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on.

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That's it.

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Now it's time to embed the product type.

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Let's start with the book type.

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The new product type already has a title and price so I'm going to delete them from the book type.

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Instead, I'm going to embed the product type. The product type has a print method as well.

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Should I remove it from here?

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No, I shouldn't because the book type needs to print a publishing date but the product type doesn't

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know how to print it.

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Instead, I'm going to call the print method of the product type as you can see you can run the method

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of an embedded type as well. The product type will print the title and the price of the book.

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It's because it's been embedded in this book type.

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So I'm going to remove these fields from here. For now, I am only going to add a tab character here.

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You will see what it looks like in a minute.

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One last thing I need to do here the product type uses pointer receivers.

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It's better to be consistent.

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So let's change the book type method to pointer receivers as well.

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Let's continue with the game type. The product type has all the fields and methods of this type.

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So let's remove all of them.

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Instead I'm going to embed the product type.

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That's all.

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Don't worry though.

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Thanks to the embedding the game type will have the fields and methods of the product type.

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Let's do the same thing for the puzzle type.

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Let's remove all the fields and methods.

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I'm going to embed the product type instead.

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Lastly let's go to the toy type.

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Let's do the same thing again.

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So I'm going to remove all the fields and methods and embed the product type.

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Okay we're almost ready.

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However as you can see there is a lot of errors in the main function.

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It's because since I embedded the product type I need to change these composite literals first.

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I'm going to remove all the field names this step wasn't necessary but I'm trying to keep the code

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concise.

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OK.

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Now I'm going to embed the fields using the product type.

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This second step was necessary because all the types embed the product type there is still an error.

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Let me tell you why.

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As you know the print method on the product type uses a pointer receiver.

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So I need to take the addresses of the book and puzzle values.

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So let's go to the main function here.

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I'm going to make the book and puzzle types pointer values like so there is only a single error left.

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Remember the third book doesn't have a publishing date. Since I'm not using the field names,

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I need to pass the published field value myself. So, I'm going to pass a nil value here.

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Nice.

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Errors are gone.

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Let's run the program.

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Good.

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It works.

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However, the publishing dates don't look cute.

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I will fix them afterward.

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Don't worry about them for now.

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OK.

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As an example let's create a toy variable as you can see the toy doesn't have any fields or methods.

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However, when I list its fields and methods, there happens to be many of them. It's because there are the

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product types' fields and methods.

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The toy type has embedded all of them automatically.

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Let me show it.

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I'm going to print it like this.

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As you can see this toy value only contains a product value the toy value doesn't have any fields but

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a product field. Every field and method comes from the product type.

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Let's take a look at a book value instead.

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It has a product field that contains title and price fields.

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However, the book also has a publishing date field. So, it takes only some part of its fields from the

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product the book type has a print method.

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It embeds a product type and the product has a print method as well.

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When you print a book, Go will run the book type's print method first.

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So the book type as the parent type takes priority.

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The book can also run the print method of the product.

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So the product print can use the title and price fields to print itself.

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Lastly, let's take a look at the list type.

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Since I embed the product type in all the product types, I can change 

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this discount method. I mean I don't have to check for the discount method all the product types already

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have a discount method.

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So I'm going to call the discount method directly like this but it doesn't work.

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It's because the list can only work with printer interface values but the printer interface doesn't

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have a discount method to solve this problem.

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I'm going to add a discount method to the printer OK.

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The error is gone but the printer interface not only prints,

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it also discounts. So it is better to change its name to item.

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Now the list type can store any type that has a print and a discount method.

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As I said, all the products have these methods already. So, they satisfy the item interface

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now.

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Let's run this program. As you can see it still works.

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This is one of the benefits of implicit interfaces.

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The book puzzle game and toy types don't know anything about the item interface.

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They satisfy it implicitly.

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That's why changing the interface from printer to item didn't break anything. Congrats!

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This was the interfaces.

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See you in the next lecture.