Go Functions

Summary: in this tutorial, you will learn how to define Go functions to modulize your code and make it more reusable.

Functions are basic building blocks in Go. Functions allow you to encapsulate reusable logic, making your code more reusable.

Defining a function

To define a function, you use the func keyword, followed by the function name, parameters (optional), and the return type (if any):

func name(parameters) type {
   // body  
}

For example, the following defines a function sayHi:

func sayHi() {
    fmt.Println("Hi")
}

Calling a function

To call a function, you use the function name followed by the parentheses. For example, the following illustrates how to call the sayHi() function inside the main() function:

package main

import "fmt"

func sayHi() {
    fmt.Println("Hi")
}

func main() {
    sayHi()
}

Output:

Hi

Function parameters

Functions may accept one or more parameters, allowing you to pass values to them. To define parameters for a function, you specify them within the parentheses following the function name.

For example, the following modifies the sayHi() function that accepts a string parameter:

func sayHi(name string) {
    fmt.Println("Hi", name)
}

When calling the sayHi() function, you need to pass a value to the name parameter:

package main

import "fmt"

func sayHi(name string) {
    fmt.Println("Hi", name)
}

func main() {
    sayHi("Joe")
}

Output:

Hi Joe

The string "Joe" is called an argument of the sayHi() function.

Parameters are defined in the function declaration while arguments are values you pass to the function.

Return values

A function may return a value. In this case, you need to specify the function’s return type after the parameter list.

For example, the following defines the sayHi() function that returns a string:

func sayHi(name string) string {
	return "Hi " + name
}

Here’s the complete code:

package main

import "fmt"

func sayHi(name string) string {
    return "Hi " + name
}

func main() {
    fmt.Println(sayHi("Joe"))
}

Output:

Hi Joe

Named Return Value

Go allows you to assign a name to the return value. For example:

func sayHi(name string) (message string) {
	message = "Hi " + name
	return
}

In this example, we assign the message as the name of the return value of the sayHi function. Go treats the message as a variable defined at the top of the sayHi function.

Typically, you use the named return value to document the meaning of the return value.

A return statement that does not have any argument will return the named return value. It is known as a “naked” return:

return

In practice, you should use the naked return value in short functions only since it will make the code less readable.

Variadic Functions

A function can accept a variable number of arguments using the ... syntax. It means a function may accept one parameter, two parameters, and so on. This function is called a variadic function. For example:

package main

import "fmt"

func sayHi(names ...string)  {
	for _, name := range names {
		fmt.Println("Hi",name )
	}
}

func main() {
	sayHi("Joe","Jane", "Bob");
}

Output:

Hi Joe
Hi Jane
Hi Bob

In this example, the sayHi function has a variable number of string parameters. Inside the function, we iterate over each parameter using a for range loop and display the corresponding greeting message.

Function Literal

In Go, you can define functions without names. These functions are called functional literals. In other programming languages such as JavaScript, they are known as anonymous functions.

For example, the following shows how to define a function literal function and call it immediately:

package main

import "fmt"


func main() {
    func(names ...string){
        for _, name := range names {
            fmt.Println("Hi",name )
        }
    }("Joe","Jane", "Bob")
}

In this example:

First, define the function literal that accepts zero or more string parameters:

func(names ...string){
   for _, name := range names {
	fmt.Println("Hi",name )
   }
}

Second, immediately call the functional literal and pass three string arguments:

func(names ...string){
   for _, name := range names {
	fmt.Println("Hi",name )
   }
}("Joe","Jane", "Bob")

In practice, you often use function literals as inline functions.

Summary

Use functions to encapsulate reusable logic.
Use the func keyword to define a function.
A function can accept parameters and return values.
A variadic function can accept a variable number of arguments.
A functional literal is a function without a name.

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