Hello, World!

Now that Pop is installed, it is time to write your first program. When learning a new language, it is traditional to begin with a small program that prints Hello, World! to the screen. We will do the same here.

This book uses terminal commands in its examples, but Pop does not require a particular editor. Use the editor or IDE you are most comfortable with.

Create a project directory

First, make a directory for the programs you will write while reading this book. Then create a directory for the Hello World program:

mkdir -p ~/pop-projects/hello
cd ~/pop-projects/hello

Pop source files use the .pop extension. Create a file named hello.pop, open it in your editor, and enter the program from Listing 1-1.

hello.pop

namespace Hello

function main()
    print("Hello, World!")
end

Listing 1-1: A program that prints Hello, World!

Save the file and return to your terminal. Make sure you are still inside the hello directory, then run:

pop run hello.pop

You should see:

Hello, World!

If you see that line, congratulations! You have written and run your first Pop program.

Anatomy of a Pop program

Let us look at the program one piece at a time. The first line is:

namespace Hello

A namespace gives the code in a file a name. Every Pop source file begins with one namespace declaration. We called this one Hello, but a larger program could use names such as Game, Store.Payments, or MyApp.Users.

Next comes:

function main()

end

These lines define a function named main. A function is a named group of instructions. main is special because Pop starts an executable program by running this function.

The parentheses after main are where a function receives values from its caller. This program does not need any values, so the parentheses are empty. The end keyword marks the end of the function.

Inside main, we wrote:

print("Hello, World!")

print displays a value in the terminal. Here we give it the text "Hello, World!". Text written between double quotation marks is called a string. The value between the parentheses is an argument passed to print.

Notice that Pop does not use semicolons at the ends of these lines. Indentation makes the program easier to read, while end tells Pop where the function finishes.

Do not worry about memorizing every term yet. We will return to functions, arguments, strings, and namespaces as we build larger programs.

Checking a program

The run command checks the source, builds it, and starts the resulting program. Sometimes you only want to check whether the source is valid. Use pop check for that:

pop check hello.pop

When the program is valid, this command finishes without printing anything. Try removing the closing end and running the command again. Pop will report where it expected the function to finish. Restore end before continuing.

Building an executable

You can also compile the program without running it:

pop build hello.pop --output hello

This creates an executable named hello. Run it directly:

./hello

It prints the same result:

Hello, World!

Pop is an ahead-of-time compiled language. The build command turns your source file into a native executable that can be run later. During the first chapters, we will normally use pop run because it keeps the edit-and-run cycle short.

Next, we will change this program so it can receive information from the command line.