No book is complete without some quintessential Hello World examples. So here it is:

printfn "Hello World";;

Yes, this is all you need. Notice the terseness, simplicity, and lack of clutter. Now let's run this in the F# interactive environment. In order to run it, you would need to have ";;" at the end of the statement. We will provide more details on this interactive environment setup later in Chapter 2, Now Lazily Get Over It, Again.

This is the response that you see when you run the preceding line of code. It is a minimal viable example; however these attributes of simplicity, terseness, and simplification extend beyond HelloWorld samples as you will see.

Let's look at a simple function, square. You can write a function in F# as follows:

let square = fun n -> n * n

Or you can write it in a simpler syntax like the next one. Notice the first-class citizenship in action here:

let square n = n * n

When this function is executed in F# interactive, you can immediately see the results upon invocation as in the following screenshot: