Reading a Go Program

Let’s understand the Hello World Go Program by reading it line by line.

package main

import "fmt"

func main() {
    fmt.Println("Hello! World")
}

The Go program starts with package keyword followed by package name in this case main.

packages are one of the building blocks of Go programming language.

In simple words a go package contains re-usable code or functionality.

For example Go has a built in package called math Package which provides basic constants and mathematical functions.

We can simply import the math package in our programs and we can use functions like math.abs() etc.

If you want more generic definition, a package contains set of .go files and may include other packages.

There are two types of packages in go

1. Executable package (main package)
2. Non Executable packages (non main packages) (You can think of them as libraries).

We can discuss more about this later as of now what you should understand is every go program must start with a package declaration

package main #

Package main defines a standalone executable program.

package main

Here we are declaring package main.

Let’s run the program.

go run hello_world.go
Hello! World

To understand further rename the main package to some other name

package main1

import "fmt"

func main() {
	fmt.Println("Hello! World")
}

Now let’s run the program

go run hello_world.go

You will get package command-line-arguments is not a main package error.

That means we can run go program which contains package main.

i.e., main package is executable package.

As we renamed it to main1 it’s acting like a library (just like math package, we can import it in another file).

import "fmt" #

fmt package is non executable package, it contains re-usable code or functions related to formatting input and output.

Inside main() we are calling fmt.Println("Hello! World") method.

Println is a function in fmt package which displays the message in command prompt (output).

To use Println function, we must include package fmt in our program.

Using import keyword, we can use the code from other packages in our go programs.

Let’s remove import "fmt" from our code run the program.

go run hello-world.go
.\hello-world.go:8:2: undefined: fmt

We will get undefined: fmt error.

So to use functions or code from other packages in go programs, we must include corresponding packages using import statement.

import "package name"

Package name should be included in double quotes.

And to call the package functions, we must prefix the function name with package name and dot.

fmt.Println("Hello! World")

To know more about Println function type go doc fmt.Printf command.

>go doc fmt.Printf

package fmt // import "fmt"

func Printf(format string, a ...any) (n int, err error)
    Printf formats according to a format specifier and writes to standard
    output. It returns the number of bytes written and any write error
    encountered.

And let’s remove or comment out the fmt.Println() statement.

package main

import "fmt"

func main() {
	//fmt.Println("Hello! World")
}

You will get imported and not used: "fmt" error.

The go compiler is saying that the fmt package is imported but not used.

That means every go program that imports a package must use that package in the code.

It helps in maintaining the go source code. If the package is not useful and there is no point in importing it in the source code.

func main() #

The func keyword declares a function in go.

In this program we are declaring a function named main().

Like other programming languages, the body of the go functions are enclosed in curly braces {}.

main() function is very important function inside package main.

It’s where the execution of the program starts.

Let’s rename main() function and see what happens.

go run hello-world.go

The go compilers throws runtime.main_main·f: function main is undeclared in the main package error.

When you type go run command, the compiler searches for the main() function inside the package main.

If there is no main() function the compiler does not know where to start the program, so it will throw an error.

We must declare main() function inside main package.

Summary #

1. main is an executable package.
2. func keyword used to declare the functions in go
3. To include other packages in the programs we should use import keyword.
4. If you import a package, then we must use it in some way in the code.
5. Package main must declare function main().
6. The go program execution starts from the main() function.