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Introduction to CLOS and Data Abstraction in Common Lisp Programming

This lecture, led by Alok Mehta, provides an overview of the Common Lisp Object System (CLOS) and techniques for data abstraction in Lisp. The concepts of association lists, constructors, and accessors are discussed to illustrate how to efficiently manage and interact with data structures. The course emphasizes the use of lists for representing objects and records, and showcases comparison with C++ for better understanding of object-oriented principles in Lisp. Key functions and methodologies for accessing and updating data are also covered.

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Introduction to CLOS and Data Abstraction in Common Lisp Programming

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  1. 66-2210-01 Programming in Lisp Common Lisp Object System (CLOS) Alok Mehta - Programming in Lisp - Data Abstraction and Mapping

  2. Association Lists • Previous lecture: Association Lists • A common way to represent data • (setf course-1 '( • (course-number 66221001) • (credit-hours 1) • (name (Programming in Lisp)) • (instructor (Alok Mehta)) • (department (Computer Science)))) • Association • Pair: (<Association-Name> <Association Value>) • Example: (course-number 66221001) • Association List • A list of associations • Example: ( (course-number 66221001) (credit-hours 1) … ) Alok Mehta - Programming in Lisp - Data Abstraction and Mapping

  3. Assoc • Assoc • Lisp procedure • Searches for a given association in a list of associations • Example: • > (assoc 'credit-hours course-1) • (credit-hours 1) • Can be used to find the value of an association • > (second (assoc 'credit-hours course-1)) • 1 • List of Associations • Common way to represent a single object (structure/record) • Object = Set of Properties • Each property is stored as an association • The set of properties is stored as a list of associations Alok Mehta - Programming in Lisp - Data Abstraction and Mapping

  4. Simple Database • A database as a list of “records” • (setf course-database '( • ((course-number 66221001) • (credit-hours 1) • (name (Programming in Lisp)) • (instructor (Alok Mehta)) • (department (Computer Science))) • ((course-number 66220001) • (credit-hours 1) • (name (Programming in C++)) • (instructor (Louis Ziantz))) • )) • This is a List of a “List of Associations” Alok Mehta - Programming in Lisp - Data Abstraction and Mapping

  5. Constructors, Readers, Writers • Use readers, constructors, and writers • To hide data details • Example functions written for a single course • make-course • get-course-name, get-course-number, get-course-instructor • taught-by-alok-p • Example functions written for a list of courses • get-course-numbers • get-courses-taught-by-alok • count-courses-taught-by-alok • find-first-course-taught-by-alok Alok Mehta - Programming in Lisp - Data Abstraction and Mapping

  6. Comparison to C++ • Lisp: Free form • ((course-number 66221001) • (credit-hours 1) • (name (Programming in Lisp)) • (instructor (Alok Mehta)) • (department (Computer Science))) • C++: Well defined class • class Course { • private: • int credit_hours; • String name; • String instructor; • String department; • Course (…) // Instead of make-course • int get_credit_hours(); // Accessors • bool is_taught_by_alok (); // Other functions • }; Alok Mehta - Programming in Lisp - Data Abstraction and Mapping

  7. CLOS • CLOS = Common Lisp Object System • Brings object oriented programming concepts to Lisp • Declare class course • > (defclass course () • ((course-number :accessor course-number • :initarg :course-number • :initform NIL) • (credit-hours :accessor credit-hours • :initarg :credit-hours • :initform 3) • (instructor :accessor instructor • :initarg :instructor • :initform NIL) • (department :accessor department • :initarg :department • :initform NIL))) • Defclass automatically creates readers, constructors, writers • Can extend pre-written operations with your own methods Alok Mehta - Programming in Lisp - Data Abstraction and Mapping

  8. Predefined constructor • Make-instance: Predefined constructor • Example • > (setf c1 (make-instance 'course • :course-number 66221001 • :instructor '(Alok Mehta) • :department '(Computer Science))) • #<COURSE #xE7CA90> • Syntax • (make-instance '<class-name> • <initarg-1> <value-1> <initarg-2> <value-2> … ) • Unspecified initarg’s are set to default values • Example: credit-hours • Name of initarg is specified by “:initarg” parameter in defclass • Default value is specified by “:initform” parameter in defclass Alok Mehta - Programming in Lisp - Data Abstraction and Mapping

  9. Accessing/Updating variables • Readers • Accessors are predefined by defclass • Name is specified by “:accessor” parameter in defclass • > (department c1) • (Computer Science) • > (credit-hours c1) • 3 • Writers • Mechanism to update variables is to use setf with the accessor • > (setf (credit-hours c1) 1) • 1 Alok Mehta - Programming in Lisp - Data Abstraction and Mapping

  10. Describe • Describe is used to … well, describe things • Works on almost any type of expression in Lisp • > (describe 3) ;; prints 3 is a fixnum • > (describe “Hello”) ;; Hello is a string • > (describe 'Hello2) ;; Hello2 is a symbol • > (describe '(1 2)) ;; (1 2) is a CONS • > (defun xyz () (print 'hi)) • > (describe #'xyz) ;; xyz is a code-object • > (describe c1) ;; ... is a COURSE, values… • > (describe (class-of c1)) ;; … is a STANDARD-CLASS … Alok Mehta - Programming in Lisp - Data Abstraction and Mapping

  11. Defmethod • Defmethod - Used to define methods • Example • > (defmethod is-taught-by-alok-p ((c course)) • (equal (instructor c) '(Alok Mehta))) • Syntax • > (defmethod <method-name> ( (var1 type1) (var2 type2) … ) • <definition>) • Each variable has a type specification • The variable “c” is of type “course” • Example call • > (is-taught-by-alok-p c1) • T • C1 is an object of type “course”. • Since this matches the prototype of the defmethod, the method is called • The method runs, evaluates to “T”, and returns Alok Mehta - Programming in Lisp - Data Abstraction and Mapping

  12. Case Study: The Throttle Class • Goal: store and manipulate the status of a simple throttle • Controls flow of fuel • Used often in small engines (e.g. lawn mower) • Functions needed • Function to set throttle to its “shutoff” position • > (shut-off t1) ;; where t1 is a throttle • Function to shift a throttle’s position by given amount • > (shift t1 -2) ;; Decrement position by 2 • Function that returns the fuel flow, as a proportion of maximum flow (current / MAX) • > (flow t1) • Function that tells whether the throttle is currently on. • > (on-p t1) ;; Is the throttle t1 on? 6 Fast 5 4 3 2 1 Slow OFF Alok Mehta - Programming in Lisp - Data Abstraction and Mapping

  13. Lisp Code • (defclass throttle () • ((pos :initform 0 :accessor pos))) • (defmethod shut-off ((the-throttle throttle)) • (setf (pos the-throttle) 0)) • (defmethod shift ((the-throttle throttle) (amount number)) • (let ((new-pos (+ (pos the-throttle) amount))) • (cond ((< new-pos 0) (setf new-pos 0)) • ((> new-pos 6) (setf new-pos 6))) • (setf (pos the-throttle) new-pos))) • (defmethod flow ((the-throttle throttle)) • (/ (pos the-throttle) 6)) • (defmethod on-p ((the-throttle throttle)) • (> (pos the-throttle) 0)) • ;; Example calls • (setf t1 (make-instance 'throttle)) • (shut-off t1) • (shift t1 2) • (flow t1) • (on-p t1) Alok Mehta - Programming in Lisp - Data Abstraction and Mapping

  14. Case Study: The Point Class • Store/manipulate two dimensional points • Coordinates (x,y) • Initialize point • Shift (Move) a point • Rotate a point • Display a point Alok Mehta - Programming in Lisp - Data Abstraction and Mapping

  15. C++ Point Class • C++ Class Declaration • class Point { • private: • double x; • double y; • public: • Point (); • Point (double initial_x, double initial_y); • void initialize (double new_x, double new_y); • void move (double dx, double dy); • void rotate90(); /* Rotate clockwise */ • double get_x () const { return x; } • double get_y () const { return y; } • }; Alok Mehta - Programming in Lisp - Data Abstraction and Mapping

  16. Methods of Point • Lisp code for handling points • (defclass point () • ((x :initform 0 :accessor x :initarg :x) • (y :initform 0 :accessor y :initarg :y))) • (defmethod initialize ((p point) (newx number) (newy number)) • (setf (x p) newx) (setf (y p) newy) p) • (defmethod move ((p point) (dx number) (dy number)) • (setf (x p) (+ (x p) dx)) (setf (y p) (+ (y p) dx)) p) • (defmethod rotate-90 ((p point)) • (let ( (newx (y p)) (newy (- (x p))) ) • (setf (x p) newx) • (setf (y p) newy)) • p) • Question: Why is p returned by each method? • How would you rotate 180? Alok Mehta - Programming in Lisp - Data Abstraction and Mapping

  17. Case Study: Line class • (defclass g-line () • ((p1 :accessor p1 • :initarg :p1 • :initform (make-instance 'point)) • (p2 :accessor p2 • :initarg :p2 • :initform (make-instance 'point)))) • (defmethod initialize-gline ((l g-line) • (x1 number) (y1 number) (x2 number) (y2 number)) • (initialize (p1 l) x1 y1) • (initialize (p2 l) x2 y2) • l) • (defmethod move ((l g-line) (dx number) (dy number)) • (move (p1 l) dx dy) • (move (p2 l) dx dy) • l) • (defmethod rotate-90 ((l g-line)) • (rotate-90 (p1 l)) • (rotate-90 (p2 l)) • l) Alok Mehta - Programming in Lisp - Data Abstraction and Mapping

  18. Using Lines, Points • (setf p (make-instance 'point :x 3 :y 4)) • (initialize p 88 99) • (move p -5 -5) • (setf l1 (make-instance 'g-line)) • (initialize-gline l1 3 4 89 33) • (move l1 -1 -1) ;; Sets L1 coords to (2,3), (88,32) • ( x (p2 l1) ) ;; Get the x value of point p2 of line l1 • (setf (x (p2 l1)) 99) ;; Set l1.p2.x = 99 Alok Mehta - Programming in Lisp - Data Abstraction and Mapping

  19. Polymorphism - Display Method • Method to display a Point • > (defmethod display ((p point)) • (list (x p) (y p))) • Example Call • > (display p) • (83 94) • Method to display a Line • > (defmethod display ((l g-line)) • (list (display (p1 l)) (display (p2 l)))) • Example Call • > (display l1) • ( (2 3) (99 32) ) • The “display” method • Determines the appropriate definition to invoke • Based on the type of object being displayed (polymorphism) Alok Mehta - Programming in Lisp - Data Abstraction and Mapping

  20. Using Lines, Points • Create some points • (setf pA (make-instance 'point :x 0 :y 0)) • (setf pB (make-instance 'point :x 1 :y 1)) • (setf pC (make-instance 'point :x 2 :y 2)) • Create lines, initializing points explicitly • (setf l2 (make-instance 'g-line :p1 pA :p2 pB)) • (setf l3 (make-instance 'g-line :p1 pB :p2 pC)) • Move lines • (move l2 -1 -1) • (move l3 -1 -1) • Display line coordinates • (display l2) • (display l3) • This doesn’t work as expected! • pB is a SHARED object • Move operation is applied twice on pB Alok Mehta - Programming in Lisp - Data Abstraction and Mapping

  21. Calculate Distances, Midpoints • Distance between two points • (defmethod distance ((p1 point) (p2 point)) • (let ((a (- (x p1) (x p2))) • (b (- (y p1) (y p2)))) • (sqrt (+ (* a a) (* b b))))) • (setf p2 (make-instance 'point :x 2 :y 2)) • (setf p3 (make-instance 'point :x 4 :y 4)) • (distance p2 p3) ;; Example call • Distance between a point and a line • (defmethod distance ((p point) (l g-line)) • … • ) • Midpoint between two points • (defmethod midpoint ((p1 point) (p2 point)) • (make-instance 'point • :x (/ (+ (x p1) (x p2)) 2.0) • :y (/ (+ (y p1) (y p2)) 2.0))) Alok Mehta - Programming in Lisp - Data Abstraction and Mapping

  22. Inheritance • CLOS supports inheritance • Example • Document - DocumentName, Author, Editor, • AccessDocument(), Copy() OnLine ManPage - UnixCommandName HLPFile - FileName WWWPage - HttpAddress PrintedDoc - Publisher, nPage, ISBN Paperback Magazine Hardcover Alok Mehta - Programming in Lisp - Data Abstraction and Mapping

  23. CLOS class definition stubs • Definition of classes to set up example hierarchy • (defclass document () (documentName author editor)) • (defclass online (document) ()) • (defclass manpage (online) (commandName)) • (defclass HLPFile (online) (fileName)) • (defclass WWWPage (online) (httpAddress)) • (defclass printeddoc (document) (publisher nPage ISBN)) • (defclass paperback (printeddoc) ()) • (defclass magazine (printeddoc) ()) • (defclass hardcover (printeddoc) ()) • Note • CLOS supports multiple inheritance Alok Mehta - Programming in Lisp - Data Abstraction and Mapping

  24. Creating/manipulating instances • Creating and manipulating instances • (setf a (make-instance 'hardcover)) • (describe a) • #<HARDCOVER #xE79338> is a HARDCOVER: • DOCUMENTNAME: unbound • AUTHOR: unbound • EDITOR: unbound • PUBLISHER: unbound • NPAGE: unbound • ISBN: unbound • (setf (slot-value a 'isbn) 'ISBN-0-201-08319-1) • (slot-value a 'isbn) • ISBN-0-201-08319-1 • (slot-value a 'editor) • ** Error: Unbound slot ** • (slot-boundp a 'editor) • NIL • (slot-boundp a 'isbn) • T Alok Mehta - Programming in Lisp - Data Abstraction and Mapping

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