CS 63 LISP

1. CS 63LISP Philip Greenspun's Tenth* Rule of Programming: "Any sufficiently complicated C or Fortran program contains an ad-hoc, informally-specified bug-ridden slow implementation of half of Common Lisp.“ * Note: there aren’t actually nine other rules, just the tenth one.

2. Something I stumbled across… “…teaching Lisp by showing you how to write several complete Lisp-based games, including a text adventure, an evolution simulation, and a robot battle.” Disclaimer: I’m not endorsing this book, I just liked the cover.

3. 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

4. Great! How can I get started? • Run /usr/bin/clisp • From http://clisp.cons.org you can download CLISP for your own PC (Windows or Linux) • Great Lisp resource page: http://www.apl.jhu.edu/~hall/lisp.html

5. 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))))

6. 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!

7. 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)

8. 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

9. 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

10. 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))

11. 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

12. 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)

13. 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))))))

14. 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))))))

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