Hello, World!

Now that you have Carp installed, lets start with the tradition of writing a hello world program. It's a simple program that output in the terminal Hello, World! and that is it.

Creating a Project Directory

Start by making a directory to store your Carp code. It doesn't matter where, but for this book exercises we do recommend to crate a directory in your home directory just to put all book projects.

For UNIX/UNIX-like:

$ mkdir ~/book_projects
$ cd  ~/book_projects
$ mkdir hello_world
$ cd hello_world

For Windows:

> mkdir "%USERPROFILE%\book_projects"
> cd /d "%USERPROFILE%\book_projects"
> mkdir hello_world
> cd hello_world

Writing and Running a Carp Program

The next step is to create a new file called Main.carp.

Note that Carp files end with .carp. If you are using more than one word in your filename, write the first letter of each word as upper case, for example, use HelloWorld.carp instead of Helloworld.carp.

Now open the Main.carp file you just created and enter the code in Listing 1-1

(defn main []
    (println* "Hello, World!"))

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

Save the file and go to the terminal window and enter the following command:

carp -x Main.carp

The -x flag tells the compiler to compile and run the program.

Anatomy of a Carp Program

Let's understand what is going on this program.

(defn main []
                              )

Here we have tree things to look at. The first is the defn, it is a keyword that let us define a new function, it is followed by a function name, in this case it is main, then followed by the parameter list (the [] part), the list is empty because this function does not receive a parameter, if the parameter list was not empty it would have something inside the square brackets.

The main function is special, because it is the first part of the code that runs in every executable Carp program.

Inside of the main function we have:

    (println* "Hello, World!")

This line prints text to the screen. There is some details in here:

The println* here is a Carp macro. Although it is something different than a function, the call syntax is the same, you cannot tell the difference until you look into the function/macro implementation. And everything between e double quotes (") is a string literal.