The basics of LISP

(If this this interests you, there are some tutorial links on the LISP page.)

This is a list

(apple orange banana)

Lists can be nested

((fruit (apple orange banana)) (colour (red green)))

Lisp usually interprets lists as function calls

(+ 40 2) 42

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

(* pi 2) 6.2831853071795862

A ' sign "quotes" a something, i.e. prevents Lisp from interpreting it. So

'(elem1 elem2 elem3)

is a list of symbols.

let creates a list of variables that can be used in an expression

(let ((a 1) (b 2) (mylist '(a b))) expression)

in expression a = 1, b = 2, mylist = '(a b).

setq changes the value of a variable.

(setq mylist '(a b c))

The cons function makes a pair

(cons 'first 42)

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 nil to mark the end of the list.

'(1 2 3)

is shorthand for

(cons 1 (cons 2 (cons 3 nil)))

car returns the first element of a list

(print (car mylist))

a

cdr returns the rest of the list (ie all-but-first)

(print (cdr mylist)) (b c)

And that is basically LISP ;)

The names "car" and "cdr" came from the first machine that Lisp was written for: there were 2 registers and the commands "contents of address register" and "contents of decrement register". CommonLisp allows the more sensible names "first" and "rest".

The following example is an inefficient factorial function1?

(defun fact (x) ;a recursive function

(if (> x 0)

(* x (fact (- x 1))) 1

) )

1? this stupid function has become the HelloWorld of functional ProgrammingLanguages, for some reason.