Overview of Functions

This lesson explains how to write a simple function in Go.

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Functions

The simplest function declaration has the format:

func functionName()

Between the mandatory parentheses ( ) no, one, or more parameters (separated by ,) can be given as input to the function. After the name of each parameter variable must come its type.

The main function as a starting point is required (usually the first function), otherwise the build-error: undefined: main.main occurs. The main function has no parameters and no return type (in contrary to the C-family) otherwise, you get the build-error: func main must have no arguments and no return values. When the program executes, after initializations the first function called (the entry-point of the application) will be the main.main() (like in C). The program exits immediately and successfully when main.main returns.

The code or body in functions is enclosed between braces { }. The first { must be on the same line as the declaration otherwise you get the error: syntax error: unexpected semicolon or newline before { ). The last } is positioned after the function-code in the column beneath the function. The syntax is as follows:

func func_Name(param1 type1, param2 type2, ...){
  ...
}

If the function is returning an object of type type1, we follow the syntax as:

func func_Name(param1 type1, param2 type2, ...) type1 {
  ...
}

or:

func func_Name(param1 type1, param2 type2, ...) ret1 type1 {
  ...
}

where ret1 is a variable of type type1 to be returned. So a general function returning multiple variables looks like:

func func_Name(param1 type1, param2 type2, ...) (ret1 type1, ret2 type2, ...) {
...
}

Smaller functions can be written on one line like:

func Sum(a, b int) int { return a + b }

Let’s create the main function now as an entry point.

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package main
import "fmt"
func main(){
}

The main structure of a Go program is ready. How about making it capable of doing something?

Hello World 🌍

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package main // making package for standalone executable
import "fmt" // importing a package   
func main() { // making an entry point 
    // printing using fmt functionality
    fmt.Println("Hello World")
} // exiting the program

BINGO! We just made our first Go program using the components above. At line 1, we make a package main and then import package fmt to get the functionality of formatted IO at line 2. At line 4, we make a main function to add an entry point, and at line 6, we print Hello World with fmt.Println("Hello World"). This function Println from the fmt package takes a parameter and prints it on the screen.

Comments

Explanation of source code added to a program as a text note is called a comment. Comments are un-compilable. They are just for the understanding of the user. In Go, a one-line comment starts with //. A multi-line or block-comment starts with /* and ends with */, where nesting is not allowed. You can see we had added comments (the sentences in green) to the above program.

Run the following program and see the magic.

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package main
import "fmt" // Package implementing formatted I/O.
func main() {
    fmt.Printf("Καλημέρα κόσμε; or こんにちは 世界\n")
}

This illustrates the international character of Go by printing Καλημέρα κόσμε; or こんにちは 世界.

Naming things in Go

Clean, readable code and simplicity are major goals of Go development. Therefore, the names of things in Go should be short, concise, and evocative. Long names with mixed caps and underscores which are often seen e.g., in Java or Python code, sometimes hinder readability. Names should not contain an indication of the package. A method or function which returns an object is named as a noun, no Get… is needed. To change an object, use SetName. If necessary, Go uses MixedCaps or mixedCaps rather than underscores to write multiword names.

Try it yourself

You’re tasked with implementing a simple function that takes a message as a parameter and prints it on the console using println. This function should be effective at later stages when you need to display various information throughout your application.

To implement a message printer function:

  • Create a function named PrintMessage that takes one parameter message of type string.

  • Inside the function body, use println to print the provided message parameter to the console.

Given below is a skeleton code to call the function in main:

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package main
import "fmt"
// Implement your PrintMessage function here
func main() {
    // Calling the PrintMessage function with a sample message
    PrintMessage("Hello, world!")
}

Note: Click the “Ask AI Mentor” button to receive guidance from the AI. If you’ve made changes to your code, simply click “Hide AI Mentor” and then click “Ask AI Mentor” again to ensure the AI provides feedback on the updated version.

You have finally run your first Go program. Now let’s get an overview of elementary types.