CS 63 LISP

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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 arent actually nine other rules, just the
tenth one.
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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 Im not endorsing this book, I just
  liked the cover.

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Why Lisp?

• Because its the most widely used AI programming
  language
• Because its good for writing production software
  (Graham article)
• Because its got lots of features other languages
  dont
• Because you can write new programs and extend old
  programs really, really quickly in Lisp

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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

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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 its 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))))

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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!

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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)

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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

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Built-in functions (contd)

• 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

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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))

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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

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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)

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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))))))

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Complete Version with Error Checking and Comments

• (defun fib (n)
• "Computes the nth Fibonacci number."
• (cond ((or (not (integerp n)) (lt n 0)) error
  case
• (error "s must be an integer gt 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))))))

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Now youve been enlightened!well, sort of
Cartoon from xkcd.com