WEBVTT

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How can you make a type print itself automatically.

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Let me explain what I mean.

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Let me show you a quick example.

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Now I am on the list type and if I go and run this code it prints the products.

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But what if I want to print the products using the plain old printer function.

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So I'm going to add a printf here and print each product using its value so if I go and run this again

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What's this.

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These are the fields data of the product.

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Each one of these is a product value.

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So how can I print a product without calling its print method.

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Let's go ahead and talk about this.

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We're going to use an interface called Stringer to print a product automatically as a string.

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This interface customizes the way a type represents itself as a string.

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All of the printing functions in the fmt package can automatically call the string method.

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In other languages there are also similar ways to print a type as a string the stringer interface is

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not the only famous interface.

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There are a lot of other well-known interfaces in go standard library.

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We're going to talk about them in the later lectures.

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No worries.

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All right.

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Let's talk about how we can implement the stringer interface for the product type here.

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We were printing it using print.

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And it was printing some weird output here.

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So I'm going to go ahead to the product type here.

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I just need to change the name of this method to a string.

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It needs to return a String as well here.

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It prints itself instead of this.

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I'm going to call the sprinf function.

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Sorry and I'm going to return a String value from Sprintf. By the way the list already prints a new

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line here.

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So I'm going to remove this new line from here.

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All right let's try it again.

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Awesome.

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Right now we can print product values directly.

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We don't have to call print method it's because print f knows that the product has a string method and

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calls this method automatically.

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So it can print the product automatically.

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So of course we're not going to stop here and we're going to implement the string method the string

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or interface in our other types as well.

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You know this price field is a money type.

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And here I'm calling the string method to print a money value.

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Also in here the released field is a timestamp.

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And I also need to call the string method here.

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So let me remove these string methods and just to show you what happens when I do so let's save this

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file and let's retry it as you can see the money type cannot print itself as well as the timestamp

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type.

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So I'm going to go to the money type and make it a stringer it's easy here.

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It's because it already has a string method that returns a string.

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I just need to change the first letter to s uppercase as you know interfaces in go are case sensitive.

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That's why I need to do so.

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So if I go back and run this again you can see that

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Now the money type can print itself.

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So let's do the same thing for the timestamp type.

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I'm going to go to the time stamp and I'm going to change the first letter to s.

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Just like I did in the money type.

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All right.

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So if I go to the terminal and run this code again you can see that the timestamp can print itself.

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Awesome right.

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Nice.

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The money and timestamp types are now stringers.

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They know how to print themselves.

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What's left.

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Actually there is also a list type you know.

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And I want to make this list type also as Stringer.

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So how can I do that first.

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I'm going to change the method name to string and it's going to return a String value of course.

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And here it prints a message.

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But I want to return a string here.

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So I'm going to return a String.

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directly in here also.

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I'm going to add a new line character here.

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Now here I need to return these product values as a single string value.

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To do that I'm going to use a string builder.

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This is a buffer for string values.

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Actually I could have combined these string values myself but that will be very inefficient if I'm going

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to have multiple string values in the future.

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So it is better to be ready for it.

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That's why I'm using the string.

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builder type here.

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Now I need to write into the buffer using this string value.

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So I'm going to call string.

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I'm going to call the write string method.

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It's here.

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Right.

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It takes a string value but I don't care what it returns.

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So I'm just going to call the write string method and I'm going to pass it this string value.

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Next I need to print the product value.

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So I'm going to call the write string method again and I'm going to pass it in a product value but it

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doesn't work the product value is not a string value.

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So I just need to call the string method here.

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As you can see not every other type knows about the stringer interface so the string builder doesn't

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know anything about the stringer interface.

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So I just needed to call the string method here myself.

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And next I'm going to call the write rune method this time as you can see.

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This time it takes a rune value.

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So I'm doing this because I want to print a new line just like in here.

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So that's why I called this write rune method.

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So I'm going to pass it in a new line and that's it.

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Now I can delete these lines of code.

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All right finally I need to return a single string value so I'm going to say return and I'm going to

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say string here.

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I need to call the string method because it returns a string value.

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So you see the strings builder is also a stringer.

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So if you need too.

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You can also pass this builder to printing functions directly.

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There is only one problem left and it's in the main function here.

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It tries to call the print method but it doesn't exist anymore.

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So instead I'm going to print the list directly like this.

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So if I go back to my terminal and run this code it works as before.

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Nice isn't it.

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So how does the stringer work behind the scenes.

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Let me show you an example.

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I'm going to comment out this code and I'm going to declare a new money variable pocket money.

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With $10.

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So I'm going to print I have pocket.

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Okay.

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Now if I go to my terminal and run this code you can see that I'm not that rich.

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And I have only ten dollars.

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So how does the println function can print this money type this money value.

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Let's see how it works.

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First the program executes the print with a money value the money type has a string method the print

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function uses a type assertion and sees that the money is a stringer therefore it executes the string

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method of the money the string method of the money returns a string value.

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Finally the print function prints ten dollars.

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So basically it uses a type assertion behind the scenes.

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This is how it works so previously you learned how to use interfaces to group types by behavior.

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You learned that you don't always have to create interfaces by yourself.

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Instead you can satisfy existing interfaces existing interfaces can become bridges between your types

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and code that is written by others the most significant benefit of using common interfaces is reusability

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therefore most Go programmers prefer to implement methods of existing interfaces.

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That's all for now.

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You may pause the video here and take a look at the summary.