Learning Go, 2nd Edition

Chapter 5. Functions

So far, your programs have been limited to a few lines in the main function. It’s time to get bigger. In this chapter, you’re going to learn how to write functions in Go and see all the interesting things you can do with them.

Declaring and Calling Functions

The basics of Go functions are familiar to anyone who has programmed in other languages with first-class functions, like C, Python, Ruby, or JavaScript. (Go also has methods, which I’ll cover in Chapter 7.) Just as with control structures, Go adds its own twist on function features. Some are improvements, and others are experiments that should be avoided. I’ll cover both in this chapter.

You’ve already seen functions being declared and used. Every Go program starts from a main function, and you’ve been calling the fmt.Println function to print to the screen. Since a main function doesn’t take in parameters or return values, let’s see what it looks like when a function does:

func div(num int, denom int) int {
    if denom == 0 {
        return 0
    }
    return num / denom
}

Let’s look at all the new things in this code sample. A function declaration has four parts: the keyword func, the name of the function, the input parameters, and the return type. The input parameters are listed in parentheses, separated by commas, with the parameter name first and the type second. Go is a typed language, so you must specify the types of the parameters. The return type is written between the input parameter’s closing parenthesis ...