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2 GlynWebster 1 !!! [LISP]’s very basics
2
3 ''In the examples, any bold stuff is the result of running the code above it.''
1 GlynWebster 4
5 This is a list:
7 AristotlePagaltzis 6
7 <verbatim>
8 (apple orange banana)
9 </verbatim>
1 GlynWebster 10
11 Lists can be nested:
7 AristotlePagaltzis 12
13 <verbatim>
14 ((fruit (apple orange banana)) (colour (red green)))
15 </verbatim>
1 GlynWebster 16
17 Lisp usually interprets lists as function calls:
7 AristotlePagaltzis 18
19 <pre>
20 (+ 40 2)
21 <b>42</b>
22 </pre>
1 GlynWebster 23
24 Lisp usually interprets symbols (i.e. identifiers) as variable references:
25
7 AristotlePagaltzis 26 <pre>
27 (* pi 2)
28 <b>6.2831853071795862</b>
29 </pre>
30
31 A single-quote character "quotes" something, i.e. prevents Lisp from interpreting it. So the following is a list of symbols:
32
33 <verbatim>
34 '(elem1 elem2 elem3)
35 </verbatim>
36
37 <tt>let</tt> creates a list of variables that can be used in an expression:
38
39 <verbatim>
40 (let ((a 1) (b 2) (mylist '(a b))) expression)
41 </verbatim>
42
43 Hereafter, <tt>a</tt> will be 1, <tt>b</tt> = 2, <tt>mylist</tt> will contain the list <tt>'(a b)</tt>.
44
45 <tt>setq</tt> changes the value of a variable:
46
47 <verbatim>
48 (setq mylist '(a b c))
49 </verbatim>
50
51 The <tt>cons</tt> function makes a pair:
52
53 <verbatim>
54 (cons 'first 42)
55 </verbatim>
56
57 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:
58
59 <verbatim>
60 '(1 2 3)
61 </verbatim>
62
63 This list is a shorthand for:
64
65 <verbatim>
66 (cons 1 (cons 2 (cons 3 nil)))
67 </verbatim>
68
69 <tt>car</tt> returns the first element of a list:
70
71 <pre>
72 (print (car mylist))
73 <b>a</b>
74 </pre>
75
76 <tt>cdr</tt> returns the rest of the list (ie all-but-first):
77
78 <pre>
79 (print (cdr mylist))
80 <b>(b c)</b>
81 </pre>
1 GlynWebster 82
7 AristotlePagaltzis 83 And that is basically [LISP].
1 GlynWebster 84
7 AristotlePagaltzis 85 !!! A somewhat realistic example
1 GlynWebster 86
7 AristotlePagaltzis 87 The following example is an inefficient factorial function:
1 GlynWebster 88
7 AristotlePagaltzis 89 <verbatim>
90 (defun fact (x) ;a recursive function
91 (if (> x 0)
92 (* x (fact (- x 1)))
93 1))
94 </verbatim>
1 GlynWebster 95
7 AristotlePagaltzis 96 (This stupid function has become the HelloWorld of functional ProgrammingLanguage~s for some reason.)
1 GlynWebster 97
2 GlynWebster 98 !!! Further reading
99
100 Look for the tutorial links on the [LISP] page.
1 GlynWebster 101
7 AristotlePagaltzis 102 !!! Aside: where do these weird names come from?
1 GlynWebster 103
7 AristotlePagaltzis 104 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>".

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