An interface is a set of methods. We can define an interface by listing out the methods it's expected to support. For example, consider the following interface:

var a interface {
     check()
 }

Here we are defining a to be a variable that has the type interface{ check() }. What on earth does that mean?

It means that you can put any value into a, as long as the value has a type that has a method called check().

Why is this valuable? It's valuable when considering multiple types that do similar things. Consider the following:

 type complicatedEmail struct {...}
 
 func (e complicatedEmail) check() {...}
 func (e complicatedEmail) send(a string) {...}
 
 type simpleEmail string
 
 func (e simpleEmail) check() {...}
 func (e simpleEmail) send(a string) {...}

Now we want to write a function do, which does two things:

• Check that an email address is correct
• Send "Hello World" to the email

You would need two do functions:

func doC(a complicatedEmail) {
     a.check()
     a.send("Hello World")
 }
 
func doS(a simpleEmail) {
     a.check()
     a.send("Hello World")
 }

Instead, if that's all the bodies of the functions are, we may opt to do this:

func do(a interface{
     check()
     send(a string)
     }) {
         a.check()
         a.send("Hello World")
     }

This is quite hard to read. So let's give the interface a name:

type checkSender interface{
     check()
     send(a string)
 }

Then we can simply redefine do to be the following:

func do(a checkSender) {
     a.check()
     a.send("Hello World")
 }

A note on naming interfaces in Go. It is customary to name interfaces with a -er suffix. If a type implements check(), then the interface name should be called checker. This encourages the interfaces to be small. An interface should only define a small number of methods—larger interfaces are signs of poor program design.