Describe an Interface in Go?

An interface in Go is a custom type that is used to specify a set of one or more method signatures and the interface is abstract, so you are not allowed to create an instance of the interface. But, you are allowed to create a variable of an interface type and this variable can be assigned with a concrete type value that has the methods the interface requires. Or in other words, the interface is a collection of methods as well as it is a custom type.

Here is a step by step description of an interface in Go:

1. Declaration: An interface is declared using the type keyword, followed by the name of the interface and the keyword interface. Inside the curly braces {}, you define a list of methods (with specific signatures) that a type must have in order to implement the interface.

type VowelsFinder interface {
    FindVowels() []rune
}

2. Implementation: A type implements an interface by providing definitions for all the methods declared in the interface. In Go, this is done implicitly, meaning you don't have to explicitly state that a type implements an interface.

type MyString string

func (ms MyString) FindVowels() []rune {
    var vowels []rune
    for _, rune := range ms {
        if rune == 'a' || rune == 'e' || rune == 'i' || rune == 'o' || rune == 'u' {
            vowels = append(vowels, rune)
        }
    }
    return vowels
}

3. Usage: Once a type implements an interface, an instance of that type can be assigned to an interface variable. This interface variable can then be used to call the methods defined in the interface.

var v VowelsFinder
v = MyString("Hello World")
fmt.Printf("Vowels are %c", v.FindVowels())

In this example, MyString type implements the VowelsFinder interface by defining the FindVowels method. Then, an instance of MyString is assigned to the VowelsFinder variable v, and the FindVowels method is called on v.

Interfaces allow us to treat different types in the same way as long as they implement the same methods, which is a powerful tool for abstraction.