Introducing Functions

As you write more Lisp, you're going to find yourself writing a lot of functions. Before we write one, though, let's call some!

> (car objects)

The car function is one of the oldest functions in Lisp (some modern Lisps no longer define it) and it gets its name from manipulating memory registers (originally on the old IBM 704, which was used to create Lisp!). car gets the first element of a list. It has a complement, cdr:

> (cdr objects)
(bucket frog chain)

cdr gets the rest of a list. Let's do some more:

> (cdr (cdr objects))
(frog chain)
> (car (cdr (cdr objects)))

Ignoring the fact that there's a function which defines this exact behaviour, let's define our own which computes this playful exploration:

> (defun cccr (objs)
    (car (cdr (cdr objs))))

In Lisp, defun lets us define a function which we can call later. A function definition has several key parts:

  • the call to defun
  • a name for the function
  • the arguments that the function will accept when we call it later (in this case, we only have one argument, objs)
  • the function body

Notice that we defined what is called a local variable in that function. This means that the objs variable will only be accessible inside the function itself.

Let's try it out:

> (cccr objects)

That's much easier to type! (For the curious, Common Lisp and LFE define the function caddr which does the same as (car (cdr (cdr ...)))).)

Note that functions in Lisp are often more like functions in math than in other programming languages: Just like in math, this function doesn't print stuff for the user to read or pop up a message box. All it does is return a value as a result of the function.

In LFE, there are all sorts of ways to create functions, so be prepared -- you're going to see some different ways soon!