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!!! [LISP]’s very basics

''In the examples, any bold stuff is the result of running the code above it.''

This is a list:

<verbatim>
(apple orange banana)
</verbatim>

Lists can be nested:

<verbatim>
((fruit (apple orange banana)) (colour (red green)))
</verbatim>

Lisp usually interprets lists as function calls:

<pre>
(+ 40 2)
<b>42</b>
</pre>

Lisp usually interprets symbols (i.e. identifiers) as variable references:

<pre>
(* pi 2)
<b>6.2831853071795862</b>
</pre>

A single-quote character "quotes" something, i.e. prevents Lisp from interpreting it. So the following is a list of symbols:

<verbatim>
'(elem1 elem2 elem3)
</verbatim>

<tt>let</tt> creates a list of variables that can be used in an expression:

<verbatim>
(let ((a 1) (b 2) (mylist '(a b))) expression)
</verbatim>

Hereafter, <tt>a</tt> will be 1, <tt>b</tt> = 2, <tt>mylist</tt> will contain the list <tt>'(a b)</tt>.

<tt>setq</tt> changes the value of a variable:

<verbatim>
(setq mylist '(a b c))
</verbatim>

The <tt>cons</tt> function makes a pair:

<verbatim>
(cons 'first 42)
</verbatim>

Cons pairs are usually used to make lists: the first element is the first element on the list; the second element is either a cons pair for the rest of the list or a <tt>nil</tt> to mark the end of the list. Consider the following list:

<verbatim>
'(1 2 3)
</verbatim>

This list is a shorthand for:

<verbatim>
(cons 1 (cons 2 (cons 3 nil)))
</verbatim>

<tt>car</tt> returns the first element of a list:

<pre>
(print (car mylist))
<b>a</b>
</pre>

<tt>cdr</tt> returns the rest of the list (ie all-but-first):

<pre>
(print (cdr mylist))
<b>(b c)</b>
</pre>

And that is basically [LISP].

!!! A somewhat realistic example

The following example is an inefficient factorial function:

<verbatim>
(defun fact (x)               ;a recursive function
  (if (> x 0)
    (* x (fact (- x 1)))
    1))
</verbatim>

(This stupid function has become the HelloWorld of functional ProgrammingLanguage~s for some reason.)

!!! Further reading

Look for the tutorial links on the [LISP] page.

!!! Aside: where do these weird names come from?

The names "<tt>car</tt>" and "<tt>cdr</tt>" came from the first machine that Lisp was written for: there were 2 registers and the commands "<tt>contents of address register</tt>" and "<tt>contents of decrement register</tt>". CommonLisp allows the more sensible names "<tt>first</tt>" and "<tt>rest</tt>".
One page links to QuickLispTutorial:
LISP
Last edited on Wednesday, April 4, 2007 10:47:35 am by AristotlePagaltzis
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