1. 1 Simple Lisp Programs References: http://www-itolab.ics.nitech.ac.jp/LISP/text-contents.html http://grimpeur.tamu.edu/~colin/lp/lp.html http://www-2.cs.cmu.edu/Groups/AI/html/cltl/clm/#R http://www.cs.utsa.edu/research/AI/cltl/clm/clm.html

2. 2 A data base called *db*. The data base is organized into a tree structure of nodes. Each node has three fields: the name of the object it represents, a node to go to if the answer is yes, and a node for when the answer is no. We traverse the nodes until we either get an “it” reply or have to give up. e.g. 3: A tree structure of nodes for data base (defstruct node name (yes nil) (no nil)) NODE (defvar *db* (make-node :name 'animal :yes (make-node :name 'mammal) :no (make-node :name 'vegetable :no (make-node :name 'mineral)))) *DB*

3. 3 (defun give-up () (format t "~&I give up - what is it?") (make-node :name (first (read)))) GIVE-UP (defun questions (&optional (node *db*)) (format t "~&Is it a ~a? " (node-name node)) (case (first (read)) ((y yes) (if (not (null (node-yes node))) (questions (node-yes node)) (setf (node-yes node) (give-up)))) ((n no) (if (not (null (node-no node))) (questions (node-no node)) (setf (node-no node) (give-up)))) (it 'aha!) (t (format t " reply with YES, NO, or IT if I have guessed it. ") (questions node)))) QUESTIONS

4. 4 (questions) Is it a ANIMAL? (yes) Is it a MAMMAL? (yes) I give up - what is it? (whale) #S( NODE :NAME WHALE :YES NIL :NO NIL ) (questions) Is it a ANIMAL? (no) Is it a VEGETABLE? (no) Is it a MINERAL? (no) I give up - what is it? (fruit) #S( NODE :NAME FRUIT :YES NIL :NO NIL ) (questions) Is it a ANIMAL? (no) Is it a VEGETABLE? (no) Is it a MINERAL? (no) Is it a FRUIT? (it) AHA! Execution results:

5. 5 (defun random-elt (choices) "Choose an element from a list at random" (elt choices (random (length choices)))) RANDOM-ELT (defun one-of (set) "pick one element of set and make a list of it" (list (random-elt set))) ONE-OF (defun Verb () (one-of '(hit took saw liked))) VERB (defun Noun () (one-of '(man ball woman table))) NOUN (defun Article () (one-of '(the a))) ARTICLE e.g. 4: A simple lisp program of generating English sentences

6. 6 (defun verb-phrase () (append (Verb) (noun-phrase) )) VERB-PHRASE (defun noun-phrase () (append (Article) (Noun))) NOUN-PHRASE (defun sentence () (append (noun-phrase ) (verb-phrase))) SENTENCE (sentence) (THE MAN SAW THE TABLE) (sentence) (A TABLE SAW THE BALL) (sentence) (A MAN SAW THE BALL)

7. 7 (defparameter *simple-grammar* '((sentence -> (noun-phrase verb-phrase)) (noun-phrase -> (ar Noun)) (verb-phrase -> (Verb noun-phrase)) (ar -> the a) (Noun -> man ball woman table) (Verb -> hit took saw liked)) "A grammar for a trivial subset of English.") *SIMPLE-GRAMMAR* (defvar *grammar* *simple-grammar* "The grammar used by generate. Initially, this is *simple-grammar*, but we can switch to other grammars.") *GRAMMAR* (defun rule-lhs (rule) "The left-hand side of a rule" (first rule)) RULE-LHS e.g. 5: A rule-based solution of generating English sentences

8. 8 (defun rule-rhs (rule) "The right-hand side of a rule" (rest (rest rule))) RULE-RHS (defun rewrite(category) "return a list of the possible rewritees for this category" (rule-rhs (assoc category *grammar*))) REWRITE (defun random-elt (choices) "choose an element from a list at random" (elt choices (random (length choices)))) RANDOM-ELT (defun mappend (fn the-list) "Apply fn to each element of list and append the result" (apply #'append (mapcar fn the-list))) MAPPEND

9. 9 (defun generate (phrase) "Generate a random sentence or a phrase" (cond ((listp phrase) (mappend #'generate phrase)) ((rewrite phrase) (generate (random-elt (rewrite phrase)))) (t (list phrase)))) GENERATE (generate 'sentence) (THE WOMAN LIKED A TABLE) (generate 'noun-phrase) (A WOMAN) (generate 'verb-phrase) (TOOK A WOMAN) (generate 'Verb) (HIT) (generate 'Noun) (BALL) (generate 'Article) (THE)

10. 10 Exercise Problems If you have time, try to do the following exercise problems. 1.Modify e.g.3 and make a data base of English words: verb (intransitive verb, transitive verb), noun, article, adjective, adverb, preposition, …, etc. 2.Write a version of generate function that explicitly differentiates between terminal symbols (those with no rewrite rules) and non terminal symbols. 3.Defining a new grammar that includes adjectives, prepositional phrases, proper names, and pronouns.