Introduction to Artificial Intelligence Lisp Ruth Bergman Fall 2002

LISP LISP (List Programming) –invented by John McCarthy, Stanford, 1958 –interpreted and compiled language –many firsts (interpreter, garbage collection, debugger....) Basic structure of LISP is the list –atoms quoted representation –cons –nil –car –cdr –null

LISP Functions Lisp expressions –quoted symbol –variable –functions The easiest way to understand how Lisp operates is to understand the interpreter: –(loop (print (eval (read))) read an expression (atom, quoted symbol, or list) evaluate that expression print the result

LISP Functions Lisp expressions –quoted symbol –variable –functions Functions are prefix notation –cons, cdr, cddr, cdddr... –arithmetic –list manipulations (append, list, cons) (append ‘(a b) ‘(c d)) (list ‘(a b) ‘(c d)) (cons ‘(a b) ‘(c d)) –other lists (subst, last, length)

LISP Variables Variables can be declared, or created dynamically –(set ‘a ‘b) –(setq a ‘b) –(setf a ‘b) We can even force evaluation –(setf a ‘b b ‘c) –(eval a)

More Functions/Predicate (defun fact (n) (if (= n 1) 1 (* n (fact (1- n))))) –values passed by value (copies) –variables are bound or free –variables are lexically scoped (dynamic is possible) numberp atom listp evenp, oddp and/or/not

Functions are Data Notice that a function is just a list; we can manipulate it like data –(setq fun ‘fact) –(funcall fun 3)  6 –(apply #’fact ‘(3)) We can define “anonymous” procedures –(funcall ‘(lambda (x) (list x)) 3)  (3) –(funcall (list ‘lambda ‘(x) ‘(list x)) 3) We can also define local variables –(defun foo (x) (let ((a 1) (b 3)) (setq b (* x b)) (list a b x))) There are lots of other “function functions” –map –reduce

Some exercises Define –reverse –flatten –member –union –intersection –difference