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Interfaces: the awesomesauce of Go

Let's recapitulate a little bit. We saw basic data types, and using them, we learned how to create composite data types. We also saw basic control structures. We then combined all of this to write functions.

We next discovered that functions are actually a kind of data, they are themselves values and have types. We went on to learn about methods. We used methods to write functions that act on data, and then moved on to data types that implement a functionality.

In this chapter, we will Go to the next realm. We will now learn to use the innate spiritual ability of objects to actually do something.

Enough spirituality, let's make this real.

What is an interface?

Stated simply, interfaces are sets of methods. We use an interface to specify a behavior of a given object.

For example, in the previous chapter, we saw that both Student and Employee can SayHi, they do it differently, but it doesn't matter. They both know how to say "Hi".

Let's extrapolate: Student and Employee implement another method Sing and Employee implements SpendSalary while Student implements BorrowMoney.

So: Student implements: SayHi, Sing and BorrowMoney and Employee implements: SayHi, Sing and SpendSalary.

These sets of methods are interfaces that Student and Employee satisfy. For example: both Student and Employee satisfy the interface that contains: SayHi and Sing. But Employee doesn't satisfy the interafeca that is composed of SayHi, Sing and BorrowMoney because Employee doesn't implement BorrowMoney.

The interface type

An interface type is the specification of a set of methods implemented by some other objects. We use the following syntax:

As you can see, an interface can be satisfied (or implemented) by an arbitrary number of types. Here, the interface Men is implemented by both Student and Employee.

Also, a type can implement an arbitrary number of interfaces, here, Student implements (or satisfies) both Men and YoungChap interfaces, and Employee satisfies Men and ElderlyGent.

And finally, every type implements the empty interface and that contains, you guessed it: no methods. We declare it as interface{} (Again, notice that there are no methods in it.)

You may find yourself now saying:

"Cool, but what's the point in defining interfaces types? Is it just to describe how types behave, and what they have in common?"

But wait! There's more! ---Of course there is more!

Oh, by the way, did I mention that the empty interface has no methods yet?

Interface values

Since interfaces are themselves types, you may find yourself wonderng what exactly are the values of an interface type.

Tada! Here comes the wonderful news: If you declare a variable of an interface, it may store any value type that implements the methods declared by the interface!

That is, if we declare m of interface Men, it may store a value of type Student or Employee, or even... (gasp) Human! This is because of the fact that they all implement methods specified by the Men interface.

Reason with me now: if m can store values of these different types, we can easily declare a slice of type Men that will contain heterogeneous values. This was not even possible with slices of classical types!

An example should help to clarify what we are saying here:


This is Mike, a Student:
Hi, I am Mike you can call me on 222-222-XXX
La la la la... November rain
This is Tom, an Employee:
Hi, I am Sam, I work at Things Ltd.. Call me on 444-222-XXX
La la la la... Born to be wild
Let's use a slice of Men and see what happens
Hi, I am Paul you can call me on 111-222-XXX
Hi, I am Sam, I work at Golang Inc.. Call me on 444-222-XXX
Hi, I am Mike you can call me on 222-222-XXX

As you may have noticed, interfaces types are abstract in that they don't themselves implement a given and precise data structure or method. They simply say: "if something can do this, it may be used here".

Notice that these types make no mention of any interface. Their implementation doesn't explicitly mention a given interface.

Similarly, an interface doesn't specify or even care which types implement it. Look how Men made no mention of types Student or Employee. All that matters for an interface is that if a type implements the methods it declares then it can reference values of that type.

The case of the empty interface

The empty interface interface{} doesn't contain even a single method, hence every type implements all 0 of its methods.

The empty interface, is not useful in describing a behavior (clearly, it is an entity of few words), but rather is extremely useful when we need to store any type value (since all types implement the empty interface).

A function that takes a parameter that is of type interface{} in actuality accepts any type. If a function returns a result of type interface{} then we expect it to be able to return values of any type.

How is this even useful? Read on, you'll see! (pixies)

Functions with interface parameters

The examples above showed us how an interface variable can store any value of any type which satisfies the interface, and gave us an idea of how we can construct containers of heterogeneous data (different types).

In the same train of thought, we may consider functions (including methods) that accept interfaces as parameters in order to be used with any type that satisfies the given interfaces.

For example: By now, you already know that fmt.Print is a variadic function, right? It accepts any number of parameters. But did you notice that sometimes, we used strings, and ints, and floats with it?

In fact, if you look into the fmt package, documentation you will find a definition of an interface type called Stringer

Every type that implements the Stringer interface can be passed to fmt.Print which will print out the output of the type's method String().

Let's try this out:


This Human is : ❰Bob - 39 years - ✆ 000-7777-XXX❱

Look at how we used fmt.Print, we gave it Bob, a variable of type Human, and it printed it so nice and purty like! All we had to do is make the Human type implement a simple method String() that returns a string, which means that we made Human satisfy the interface fmt.Stringer and thus were able to pass it to fmt.Print.

Recall the :ref:`colored boxes example<colored-boxes-example>`? We had a type named Color which implemented a String() method. Let's again go back to that program, and instead of calling fmt.Println with the result of this method, we will call it directly with the color. You'll see it will work as expected.

Cool, isn't it?

Another example that will make you love interfaces is the sort package which provides functions to sort slices of type int, float, string and wild and wonderous things.

Let's first see a basic example, and then I'll show you the magic of this package.


The list is: [1 23 65 11 0 3 233 88 99]
The sorted list is: [0 1 3 11 23 65 88 99 233]

Simple and does the job. But what I wanted to show you is even more beautiful.

In fact, the sort package defines a interface simply called Interface that contains three methods:

From the sort Interface doc:

"A type, typically a collection, that satisfies sort. Interface can be sorted by the routines in this package. The methods require that the elements of the collection be enumerated by an integer index.

So all we need in order to sort slices of any type is to implement these three methods! Let's give it a try with a slice of Human that we want to sort based on their ages.


The unsorted group is:
(name: Bart - age: 24 years)
(name: Bob - age: 23 years)
(name: Gertrude - age: 104 years)
(name: Paul - age: 44 years)
(name: Sam - age: 34 years)
(name: Jack - age: 54 years)
(name: Martha - age: 74 years)
(name: Leo - age: 4 years)

The sorted group is:
(name: Leo - age: 4 years)
(name: Bob - age: 23 years)
(name: Bart - age: 24 years)
(name: Sam - age: 34 years)
(name: Paul - age: 44 years)
(name: Jack - age: 54 years)
(name: Martha - age: 74 years)
(name: Gertrude - age: 104 years)

Victory! It worked like promised. We didn't implement the sorting of HumanGroup, all we did is implement some methods (Len, Less, and Swap) that the sort.Sort function needed to use to sort the group for us.

I know that you are curious, and that you wonder how this kind of magic works. It's actually simple. The function Sort of the package sort has this signature: func Sort(data Interface)

It accepts any value of any type that implements the interface sort.Interface, and deep inside (specifically: in functions that sort.Sort calls) there are calls for the Len, Less, and Swap methods that you defined for your type.

Go and look into the sort package source code, you'll see that in plain Go.

We have seen different examples of functions that use interfaces as their parameters offering an abstract way to do this on variables of different types provided they implement the interface.

Let's do that ourselves! Let's write a function that accepts an interface as it's parameter and see how brilliant we can be.

Our own example

We've seen the Max(s []int) int function, do you remember? We also saw Older(s []Person) Person. They both have the same functionality. In fact, retrieving the Max of a slice of ints, a slice of floats, or the older person in a group comes down to the same thing: loop and compare values.

Let's do it.


The biggest integer in islice is : 222
The biggest float in fslice is : 24.8
The oldest person in the group is: (name: Gertrude - age: 104 years)

The MaxInterface interface contains three methods that must be implemented by types to satisfy it.

  • Len() int: must return the length of the collection.
  • Get(int i) interface{}: must return the element at index i in the collection. Notice how this method returns a result of type interface{} - We designed it this way, so that collections of any type may return a value their own type, which may then be stored in an empty interface result.
  • Bigger(i, j int) bool: This methods returns true if the element at index i of the collection is bigger than the one at index j, or false otherwise.

Why these three methods?

  • Len() int: Because no one said that collections must be slices. Imagine a complex data structure that has its own definition of length.
  • Get(i int) interface{}: again, no one said we're working with slices. Complex data structures may store and index their elements in a more complex fashion than slice_or_array[index].
  • Bigger(i, j int) bool: comparing numbers is obvious, we know which one is bigger, and programming languages come with numeric comparison operators such <, ==, > and so on... But this notion of bigger value can be subtile. Person A is older (They say bigger in many languages) than B, if his age is bigger (greater) than B's age.

The different implementations of these methods by our types are fairly simple and straightforward.

The heart of the program is the function: Max(data MaxInterface) (ok bool, max interface{}) which takes as a parameter data of type MaxInterface. data has the three methods we discussed above. Max() returns two results: a bool (true if there is a Max, false if the collection is empty) and a value of type interface{} which stores the Maximum value of the collection.

Notice how the function Max() is implemented: no mention is made of any specific collection type, everything it uses comes from the interface type. This abstraction is what makes Max() usable by any type that implements MaxInterface.

Each time we call Max() on a given data collection of a given type, it calls these methods as they are implemented by that type. This works like a charm. If we need to find out the max value of any collection, we only need implement the MaxInterface for that type and it will work, just as it worked for fmt.Print and sort.Sort.

That wrapps up this chapter. I'll stop here, take a break and have a good day. Next, we will see some little details about interfaces. Don't worry! It will be easier than this one, I promise.

C'mon, admit it! You've got a NERDON after this chapter!!!

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