1. CMSC 471 LISP

2. Why Lisp? Because it’s the most widely used AI programming language Because it’s good for writing production software (Graham article) Because it’s got lots of features other languages don’t Because you can write new programs and extend old programs really, really quickly in Lisp

3. Great! How can I get started? On sunserver (CS) and gl machines, run /usr/local/bin/clisp From http://clisp.cons.org you can download CLISP for your own PC (Windows or Linux)http://clisp.cons.org Great Lisp resource page: http://www.apl.jhu.edu/~hall/lisp.html http://www.apl.jhu.edu/~hall/lisp.html

4. Why all those parentheses? Surprisingly readable if you indent properly (use built-in Lisp editor in emacs!) Makes prefix notation manageable An expression is an expression is an expression, whether it’s inside another one or not (+ 1 2) (* (+ 1 2) 3) (list (* 3 5) ‘atom ‘(list inside a list) (list 3 4) ‘(((very) (very) (very) (nested list))))

5. Lisp basics Lisp syntax: parenthesized prefix notation Lisp interpreter: read-eval-print loop Nested evaluation Preventing evaluation ( quote and other special forms) Forcing evaluation ( eval ) –Allows us to evaluate code contained in a Lisp variable!

6. Basic Lisp types Numbers (integers, floating-point, complex) –27 -2 7.519 Characters, strings (arrays of chars) –#\x #\- #\B –“This is a string!” Symbols, which have property lists –‘a ‘x ‘jon Lists (linked cells) –Empty list: nil –‘(a b c) ‘(2 3 jon) –cons structure has car (first) and cdr (rest)

7. Built-in functions For numbers –+ - * / incf decf A diversion: destructive functions –(setf x 1) –(setf y (+ x 1)) vs. (setf y (incf x)) For lists –car (first) cdr (rest) second third fourth –length nth –cons append nconc list –mapcar mapcan –find remove remove-if

8. Built-in functions (cont’d) Printing: print, format –(print “string”)  print output –(format …)  formatted output Advanced list processing: assoc, mapcar Predicates: listp, numberp, stringp, atom, null, equal, eql, and, or, not Special forms: setq/setf, quote, defun, defparameter, defconstant, if, cond, case, progn, loop

9. More Lisp types Arrays (with zero or more dimensions) Hash tables Streams (for reading and writing) Structures Functions, including lambda functions –(defun incBy10 (n) (+ n 10)) –(mapcar #’(lambda (n) (+ n 10)) ‘(1 2 3 4 5))

10. Useful help facilities (apropos ‘str)  list of symbols whose name contains ‘str (describe ‘symbol)  description of symbol (describe #’fn)  description of function (trace fn)  print a trace of fn as it runs :a  abort one level out of debugger

11. A Lisp example Writing a function to compute the nth Fibonacci number –Fibonacci sequence: 0, 1, 1, 2, 3, 5, 8, 13, … fib(0) = 0 fib(1) = 1 fib(n) = fib(n-2) + fib(n-1)

12. Complete Version (defun fib (n) (cond ((eql n 0) 0) ; base case ((eql n 1) 1) ; base case (t (+ (fib (- n 1)) ; recursively compute fib(n) (fib (- n 2))))))

13. Complete Version with Error Checking and Comments (defun fib (n) "Computes the nth Fibonacci number." (cond ((or (not (integerp n)) (< n 0)) ; error case (error "~s must be an integer >= 0.~&" n)) ((eql n 0) 0) ; base case ((eql n 1) 1) ; base case (t (+ (fib (- n 1)) ; recursively compute fib(n) (fib (- n 2))))))

14. Complete Version with Error Checking and Comments (defun fib (n) "Computes the nth Fibonacci number." (cond ((or (not (numberp n)) (< n 0)) ;error case (error "~s must be a number >= 0.~&" n)) ((eql n 0) 0) ;base case ((eql n 1) 1) ;base case (t (+ (fib (- n 1)) ; recursively compute fib(n) (fib (- n 2))))))

15. Now you’ve been enlightened! …well, sort of… Cartoon from xkcd.com