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Empty interface says nothing.

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So what is an empty interface?

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Well this is an empty interface.

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I'm not kidding.

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It doesn't have a method.

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It is completely empty.

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It says nothing.

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You don't need to satisfy it because it has no methods.

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Every type can satisfy it so it can represent any type of value.

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You can type it more concisely in a single line.

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We usually use it like this.

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This is the same thing.

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It's still an empty interface type.

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This is an empty interface value.

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You can declare it like so this variable holds an empty interface value.

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Let's say there is a Tetris value in it.

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The Tetris variable has two methods that you can use.

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Let's put it in the empty interface value.

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You can no longer use the methods of the game.

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The empty interface value wraps it in a hidden place so the empty interface says nothing.

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Let's take a look at an example.

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First I'm going to create a new directory for the next example with new main.go file.

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Next I'm going to declare an empty interface variable let's assign it a slice Let's print it.

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No problem.

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It can store a slice.

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Now let's assign it a map.
Again,

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no problem.

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Let's assign it's a string value.
No problem at all.

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Lastly let's assign it a number.

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There are no errors because you can store any type of value in an empty interface value.

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However you cannot directly use an empty interface value in operations.

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For example let's try to multiply it with a number.

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It didn't work because the any variable is not a number.

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It's an empty interface value.

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Remember an interface value has two parts a dynamic value and a dynamic type.

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So first you need to extract the dynamic value from the interface value using the dynamic type here.

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The dynamic type is int because lastly I assigned it an integer so let's use an assertion to extract

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the number

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nice.

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It works.

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Now as you can see it doesn't matter what kind of value you store in an interface value.

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However you want to use the dynamic value in operations.

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You must first extract it from the interface value here.

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The dynamic type is an integer.

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So first I get it from the interface value as an integer.

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Only then I can multiply it.

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Now I'm going to create another directory with a new file

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let's create an int slice let's create an empty interface variable.

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I'm going to assign it the integer slice as you can see.

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You can also assign a slice to an empty interface value.

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However as I said you cannot directly use the dynamic value of an interface value.

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It means you cannot use it here as a slice.

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For example let's try to find the length of the slice using the empty interface variable.

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Now I cannot get its length because any is an interface type.

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It's not a slice type so it has no length.

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However it stores an int slice.

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So first I need to get the slice back from it.

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Now I can get its length.

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The last statement works because I'm asserting that it's an int slice.

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If the dynamic type inside the interface was not an int slice my program would crash in the runtime.

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This time I'm going to create a slice of empty interface values.

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Why do you think? Can I assign it the nums slice?

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It didn't work.

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The error says you cannot assign an int slice to an empty interface slice.

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It's like trying to assign the nums slice to a string slice it is the same error a type mismatch.

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So what is the solution?

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Well I can use a loop then I need to add the next number.

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Let's print the result it works.

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But why?
I need to do so because the empty interface slice is not an empty interface value.

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It's an empty interface slice.

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That's why I need to add the numbers manually one by one to the empty interface slice.

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Let's talk about this a little bit more.

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The empty interface slice consists of empty interface values.
The int slice consists of integer values.

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The empty interface slice is a slice of empty interface values.

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But it's not an empty interface value so you cannot directly assign it another type of a slice.

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However, you can assign any type of value to one of its elements.

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It's because each one of its elements is an empty interface value.

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OK enough theory.

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Let's take a look at a practical example.

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First of all let me refactor this code a little bit.

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I'm going to put all the variables directly into the store.

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Now I can remove the store variable.

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Let's go to the book type.

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Let's say you need to store the publishing dates of the books to do that.

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I'm going to add a field with an empty interface.

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I'm going to tell you why in the main function first I'm going to add two more books.

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The first book uses an integer.

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The second book uses a string.

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The third book doesn't have a date at all.

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These are unix timestamps.

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You will see how you can convert them to actual dates in a minute.

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As you can see each book has a different type of publishing date.

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That's why I have declared the published field as an empty interface so I can store different type of

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values in it.

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Let's go back to the book type here.

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I'm going to print the published date.

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I'm going to run it.

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It doesn't look good.

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I'm going to make its prey to do that.

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I'm going to use a function.

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It will take an empty interface value and return back a string.

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I'm going to use it in the printf function.
OK. Let's create the format function it takes an empty interface

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and it returns a string.

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For now I will simply return a string.

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okay.

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It works.

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Let's format the dates now. As a reference,

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Let's copy the book values, here.

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Let's copy the book values, here.

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okay the last book has no date.

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So first let's check for it.

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The zero value of an interface value is nil.

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So if the publishing date is missing, it will be a nil value.

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Only then I will say it's unknown. Otherwise, I will return an empty string. Nice. Let's format the other

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dates as well.

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I'm going to start with Moby Dick.

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It uses an integer date so I'm going to declare an int variable and I'm going to extract the int value

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using a type assertion.

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If it succeeds the variable will be an integer.

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So I'm going to assign to the T variable.

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Now it's time to convert the integer to a time value.

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To do that I'm going to call the time Unix function its last parameter is not important but it doesn't

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work because the Unix function takes an int64 value.

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So I need to convert the variable like so.

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Now I've got a time value.

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It has a string method I can use it to convert the time value to a string.

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Now I can return it as well.

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Okay let's try it.

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It doesn't look good but it prints a more readable date.

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Now let's format the last book it uses a string date.

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So I'm going to extract the string value if it succeeds.

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The variable will be a string.

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So I need to convert it to an integer.

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Good.

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It works in summary.

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Go allows you to write type safe code.

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However the empty interface can cross that line.

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Your programs can become brittle and hard to maintain.

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Do not use the empty interface unless really necessary.

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See you in the next lecture.