TweetFollow Us on Twitter

Lisp Objects
Volume Number:4
Issue Number:4
Column Tag:Learning Lisp

Lisp Objects: Ford vs Chevy

By Didier Guillon, Grenoble, France

Didier Guillon is a former restaurant manager, currently in the fourth year of a Computer Science program at Grenoble University in France. He is a member of AAAI and works in artificial intelligence as a consultant at the Medical University of Grenoble’s department of BioStatistics. He is also president of Mac Alpes, a club of Macintosh developers in Grenoble and has been using the Mac for two years.

Welcome to this new column. When I first started Macintosh programming, MacTutor and the contributing editors were there and helped me since issue number one. Now I think it’s time to share with you the knowledge I gained with the “oldest new language”. So welcome to symbolic computation... you have a problem? No, don’t be frightened, I have been so pleased when I discovered ABC’ s of C that I wanted to introduce you to the Lisp world in a gentle way. The technique I will use may sometimes be criticized by expert Lisp programmers...but do they remember their first steps?

Today I will present the object idea with a very dumb example and “how to” on the Macintosh. MacApp and MPW Pascal are there to let us remember that object programming becomes a reality day after day. But, since Smalltalk, no other language than Common Lisp did it with better efficiency.

My Kingdom for an Object

But what is an object ? To the outside world, an object is an entity. But for us programmers, an object is some private memory with a set of operations. An object can be a number, a square, a text editor. An object which is a square knows a lot of information about itself: how to calculate the area of the square, the center of the square... When we want the object ‘square’ to carry out such a calculation, we send him a message (we say: to invoke). But we just tell him which operation we want, not how to operate. (If he’s smart, he already knows that!)

When a set of objects represents the same kind of information, we implement a class with a data structure and the procedures that operate on this data structure. For example, a class implemented for a rectangle would know how a specific object, called an instance, would carry out the operation rectangular area. This instance has private variables called instance variables and the function that will operate on the instance of a particular class is call a method. We just defined in fact the fundamental concepts of object programming: data abstraction, information hiding (not to tell “how to”) and generic operation. But all this is just theory, so let’s start with our dumb example. We will use the ExperCommonLisp (ECL) from ExperTelligence™, who since the last review of this language in MacTutor, did a lot of improvement and removed many bugs.

First, let’s create a class for ‘cars’ (note: we wrote cars to avoid the “car” word which is a Lisp reserved word).

(defclass (cars object)
  (IVS (color ‘blue)
       (price 0)))

Defclass contains a list with the name of the call and the name of the superclass. In ECL all the classes come from a special class called object. In fact object is a root class from which all classes are derived. The IVS stands for instance variable and as we saw previously, will manage to store the IVS in each instance. You note the quote before blue? In Lisp every elementary object, called an atom, is evaluated. This evaluation returns a function name or the value attached to this specific atom. So, to prevent evaluation, we “quote” the atom which becomes itself a value and is attached to the instance variable color. This ‘quoting’ is not necessary for a numeric value. Then we define the method (using defmethod followed by the name of the class and the name of the method itself):

(defmethod (cars small) ()
 (setq price 800)
 (format T “A small car is just fun for ~a  dollars.~%” price))

This method is doing two things: setting price to 800 with setq, a Lisp word which will not evaluate the following word, price, and sets its value to 800 (for Pascal programmers it’s an assignment). Then it prints a message (good old Fortran world !!!). You are now able to trace by yourself what the second method does:

(defmethod (cars race) ()
 (setq color ‘red)
 (setq price 20000)
 (format T “A wonderful race car whose color is ~a.~%” color))

Now we link up what we define to this first class: we compile and define an instance (we will represent the return value by ->):

(setq my1car (cars ‘new))
-> #<CARS INSTANCE $5402341A>

“my1car” is the instance which holds a copy of the class ‘cars’. Let’s invoke the instance in various ways :

(my1car ‘race)
-> A wonderful race car whose color is RED.

(my1car ‘small)
-> A small car is just fun for 800 dollars.

(my1car ‘price)
-> 800

(my1car ‘price 200)
-> 200

(my1car ‘price)
-> 200

We can see the graphic image of the object we define, including the next subclass we will create:

Here is the subclass Ferrari:

(defclass (Ferrari cars)
 (IVS a_speed))

(defmethod (cars buy_a_Ferrari) ()
 (setq a_speed 200)
 (self ‘race)
 (format T “But when you drive at ~a miles... Whoo !!!!~%”    a_speed 
) )

Same as the ‘cars’ object, but note the “self” word which is used to invoke the method race from the method buy_a_Ferrari. As previously, we can say:

(setq I_buy_a_Ferrari (Ferrari ‘new))
-> #<FERRARI INSTANCE $540233AA>

(I_buy_a_Ferrari ‘buy_a_Ferrari)
-> A wonderful race car whose color is RED.

But when you drive at 200 miles... Whoo !!!!

(I_buy_a_Ferrari ‘price)
-> 20000

(I_buy_a_Ferrari ‘color)
-> RED

We can see easily that I_buy_a_Ferrari inherits from its superclass ‘cars’ all the methods and instance variables. We will not cover here some other concepts like class variables, super....

So now that we have an idea on how objects live, we’ll have a look at ECL. In ECL, the object idea has been expanded to the Macintosh Toolbox.

Before going on to the Macintosh toolbox, we have a look at another object expression, metamethod. A metamethod is a procedure which is invoked by sending an appropriate message to a class. We explain metamethod because on the Macintosh we create a NEW metamethod which sets up the required data structure. When invoking a specific metamethod, the instance of a particular class will have instance variables which will correlate (and with the same name) to the equivalent fields in the Pascal data structure. An example from QuickDraw:

(setq yourRect (Rect ‘new 30 40 250 280))

We just created the instance call ‘yourRect’ with a set of parameters. If we send it a message, we can access these instance accessors by the same name as in the Pascal structure : top, left, bottom, right.

(yourRect ‘bottom)
-> 250

or change their value :

(yourRect ‘left 60)
-> 60

We can also draw, calling Toolbox procedures with the instance ‘yourRect’ :

(!PaintRect yourRect)

Note that trap names are prefixed by an exclamation mark. We are now ready to start programming our first minimum shell in ECL... but that’s for next month.

For those interested in following me, and starting Lisp, I recommend the very nice and easy book of Touretzky “A Gentle Introduction to Symbolic Computation”. For the others, obviously, Lisp 2e by Winston is a must. I intend to present the next coming months various aspects of Lisp programming in Artificial Intelligence, but also use some special languages as OPS 5, SmallTalk, Humble, Prolog (or why not Nexpert, just to let us dream...). If you have any suggestions, please contact me :

Didier Guillon

8 Impasse Saint Jean

38240 Meylan

France.

The complete program :

(defclass (cars object)
 (IVS (color ‘blue)
         (price 0)))

(defmethod (cars small) ()
 (setq price 800)
 (format T “A small car is just fun for ~a  dollars.~%” price))


(defmethod (cars race) ()
 (setq color ‘red)
 (setq price 20000)
 (format T “A wonderful race car whose color  is ~a.~%” color))

;**********Examples*****************

;(setq my1car (cars ‘new))
;#<CARS INSTANCE $5402341A>
;(my1car ‘race)
;A wonderful race car whose color is RED.
;(my1car ‘small)
;A small car is just fun for 800 dollars.
;(my1car ‘price)
;800
;(my1car ‘price 200)
;200
;(my1car ‘price)
;200

;**********Second class**************

(defclass (Ferrari cars)
  (IVS a_speed))

(defmethod (Ferrari buy_a_Ferrari) ()
   (setq a_speed 200)
   (self ‘race)
 (format T “But when you drive at ~a miles ...  Whoo !!!!~%”         
                              a_speed))

;***********Example****************

;(setq I_buy_a_Ferrari (Ferrari ‘new))
;#<FERRARI INSTANCE $540233AA>

;(I_buy_a_Ferrari ‘buy_a_Ferrari)
;A wonderful race car whose color is RED.
;But when you drive at 200 miles... Whoo !!!!

;(I_buy_a_Ferrari ‘price)
;20000

;(I_buy_a_Ferrari ‘color)
;RED
 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Tunnelblick 3.6.8beta10 - GUI for OpenVP...
Tunnelblick is a free, open source graphic user interface for OpenVPN on OS X. It provides easy control of OpenVPN client and/or server connections. It comes as a ready-to-use application with all... Read more
FileZilla 3.22.0 - Fast and reliable FTP...
FileZilla (ported from Windows) is a fast and reliable FTP client and server with lots of useful features and an intuitive interface. Version 3.22.0: New Directory listing filters can now be... Read more
Postbox 5.0.3 - Powerful and flexible em...
Postbox is a new email application that helps you organize your work life and get stuff done. It has all the elegance and simplicity of Apple Mail, but with more power and flexibility to manage even... Read more
OnyX 3.2.0 - Maintenance and optimizatio...
OnyX is a multifunction utility that you can use to verify the startup disk and the structure of its system files, to run miscellaneous maintenance and cleaning tasks, to configure parameters in the... Read more
Macs Fan Control 1.4.4.0 - Monitor and c...
Macs Fan Control allows you to monitor and control almost any aspect of your computer's fans, with support for controlling fan speed, temperature sensors pane, menu-bar icon, and autostart with... Read more
calibre 2.69.0 - Complete e-book library...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital librarian... Read more
Evernote 6.9.1 - Create searchable notes...
Evernote allows you to easily capture information in any environment using whatever device or platform you find most convenient, and makes this information accessible and searchable at anytime, from... Read more
jAlbum Pro 13.5 - Organize your digital...
jAlbum Pro has all the features you love in jAlbum, but comes with a commercial license. You can create gorgeous custom photo galleries for the Web without writing a line of code! Beginner-friendly... Read more
jAlbum 13.5 - Create custom photo galler...
With jAlbum, you can create gorgeous custom photo galleries for the Web without writing a line of code! Beginner-friendly, with pro results - Simply drag and drop photos into groups, choose a design... Read more
Google Chrome 53.0.2785.143 - Modern and...
Google Chrome is a Web browser by Google, created to be a modern platform for Web pages and applications. It utilizes very fast loading of Web pages and has a V8 engine, which is a custom built... Read more

3 tips to aid your journey in Banner Sag...
Stoic Games brings us another viking epic in the guise of Banner Saga 2 this week. It’s a sweeping tale, richly animated and draped in melancholy and moments of beauty. The game’s been received as a much improved follow up to the first entry in the... | Read more »
Pumped BMX 3: Beginner tips and tricks
There’s a whole lot more to Pumped BMX 3 than meets the eye. Your goal is to perform a wide array of sweet flips and tricks, but that’s easier said than done. It takes well practiced timing and coordination, and the game doesn’t really explain that... | Read more »
Cybird’s latest release - BFB Champions...
Launched in the UK in early September, BFB Champions’ newest update is loaded with great new features, and looks set to outshine the original version by taking it out of soft launch and giving it a new lease of life. | Read more »
3 apps to boost your focus
As someone who works from home, my workspace is a minefield of distraction. Cats, tasty snacks, the wind blowing past my window, that cleaning that I suddenly can’t put off any longer. If I let distraction takes its course, I find that soon half... | Read more »
Pumped BMX 3 (Games)
Pumped BMX 3 1.0 Device: iOS Universal Category: Games Price: $3.99, Version: 1.0 (iTunes) Description: The final instalment of the smash hit #1 rated BMX game is here! Following on from the insane success of Pumped BMX 2, Pumped 3... | Read more »
4 games like Burly Men at Sea to inspire...
Burly Men at Sea is out today and it looks a treat. It tells the tale of three Scandinavian fishermen who leave the humdrum of their daily lives to go exploring. It’s a beautiful folksy story that unfurls as you interact with the environment... | Read more »
3 reasons you need to play Kingdom: New...
Developed by a tag team of indie developers - Thomas "Noio" van den Berg and Marco "Licorice" Bancale - Kingdom is a vibrant medieval fantasy adventure that casts players as a king or queen who must expand their empire by exploring the vasts lands... | Read more »
JoyCity have launched a brand new King o...
Great news for all of you Game of Dice fans out there - JoyCity have just released a brand new limited edition pack with a really cool twist. The premise of Game of Dice is fairly straightforward, asking you to roll dice to navigate your way around... | Read more »
Burly Men at Sea (Games)
Burly Men at Sea 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Burly Men at Sea is a folktale about a trio of large, bearded fishermen who step away from the ordinary to seek adventure. | Read more »
3 tips for catching the gnarliest waves...
Like a wave breaking on the shore, Tidal Rider swept its way onto the App Store charts this week settling firmly in the top 10. It’s a one-touch high score-chaser in which you pull surfing stunts while dodging seagulls and collecting coins. The... | Read more »

Price Scanner via MacPrices.net

CAZE Annouces New Zero 5 Case for Jet Black i...
Hong Kong basd CAZE has announced Zero 5 case for iPhone 7/ 7 Plus, one of the world’s thinnest clear hard cases, measuring just 0.5 millimeters. CAZE has been producing and improving the Zero 5... Read more
Nest Egg Inventory App for iOS Offers Conven...
Campbell, California based Winprogger LLC has announced the release and immediate availability of Nest Egg – Inventory 4.1.22, an important update to their easy-to-use, yet comprehensive inventory... Read more
Factor4, LLC Launches Apple iOS and Android G...
Factor4, LLC, which offers gift and loyalty services to the SMB marketplace, has released free mobile applications that enable merchants to process via all Apple and Android devices. The Apple and... Read more
15-inch Retina MacBook Pros on sale for $200...
B&H Photo has 15″ Retina Apple MacBook Pros on sale for $200 off MSRP. Shipping is free, and B&H charges NY tax only: - 15″ 2.2GHz Retina MacBook Pro: $1799 $200 off MSRP - 15″ 2.5GHz Retina... Read more
Apple refurbished iMacs available for up to $...
Apple has Certified Refurbished 2015 21″ & 27″ iMacs available for up to $350 off MSRP. Apple’s one-year warranty is standard, and shipping is free. The following models are available: - 21″ 3.... Read more
Check Apple prices on any device with the iTr...
MacPrices is proud to offer readers a free iOS app (iPhones, iPads, & iPod touch) and Android app (Google Play and Amazon App Store) called iTracx, which allows you to glance at today’s lowest... Read more
Apple price trackers, updated continuously
Scan our Apple Price Trackers for the latest information on sales, bundles, and availability on systems from Apple’s authorized internet/catalog resellers. We update the trackers continuously: - 15″... Read more
Apple refurbished 2016 13-inch MacBook Airs a...
Apple has Certified Refurbished 2016 13″ MacBook Airs available starting at $849. An Apple one-year warranty is included with each MacBook, and shipping is free: - 2016 13″ 1.6GHz/8GB/128GB MacBook... Read more
1.4GHz Mac mini on sale for $449, save $50
Adorama has the 1.4GHz Mac mini on sale for $50 off MSRP including free shipping plus NY & NJ sales tax only: - 1.4GHz Mac mini (Apple sku# MGEM2LL/A): $449 $50 off MSRP To purchase a mini at... Read more
Apple refurbished 2015 13-inch MacBook Airs a...
Apple has Certified Refurbished 2015 13″ MacBook Airs available starting at $759. An Apple one-year warranty is included with each MacBook, and shipping is free: - 2015 13″ 1.6GHz/4GB/128GB MacBook... Read more

Jobs Board

Systems Architecture Prototyping - *Apple*...
Changing the world is all in a day's work at Apple . If you love innovation, here's your chance to make a career of it. You'll work hard. But the job comes with more Read more
*Apple* Retail - Multiple Positions- Akron,...
Job Description:SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Hardware Design Validation Engineer - *Apple...
Changing the world is all in a day's work at Apple . If you love innovation, here's your chance to make a career of it. You'll work hard. But the job comes with more Read more
Systems Architecture Prototyping - *Apple*...
Changing the world is all in a day's work at Apple . If you love innovation, here's your chance to make a career of it. You'll work hard. But the job comes with more Read more
*Apple* Retail - Multiple Positions- South B...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.