TweetFollow Us on Twitter

Learning Smalltalk
Volume Number:10
Issue Number:10
Column Tag:Smalltalk

Learning Smalltalk by Examples

Smalltalk - coming of age and offering an alternative to C and C++

By R. L. Peskin and S. S. Walther, Landgrove Associates, Flemington, New Jersey

Note: Source code files accompanying article are located on MacTech CD-ROM or source code disks.

This is the first in a series of articles aimed at those who want to learn something about programming in Smalltalk. Our overall objective is to convey the ease and flexibility of Smalltalk programming to those who may be used to programming in more traditional languages and environments. The approach will be practical; learning by example. We will be using the SmalltalkAgents™ system as the vehicle to illustrate our examples.

The Wire Project

The Wire example is a simple tool to determine the shortest length of wire running between an arbitrarily selected set of points. (If you aren’t into wire routing, the same sort of analysis applies to routing a salesperson between cities.) Kent Beck and Ward Cunningham first introduced us to this example at the Tektronix, Inc. Smalltalk classes in 1986. The Wire Project is a good illustration of “tool” building in Smalltalk; that is, integrating the behavior of a Model and the user interface to interact with that Model. The Model refers to your data, perhaps including the computations that generate them. The View is the interface to that data. Commonly this means a visual interface (list, graph, visualization), but it may well include sound, touch, smell, etc. in future platforms. The Controller refers to the set of messages and/or events that facilitate communication and control between the Model and View(s). Our Model is a list of Wire coordinates (list of points), together with the methods (behaviors) that do calculations relevant to those Wire coordinates. Our View is a dynamically changing graphical representation of the Wire points and the lines connecting them. Our Controller is not a formal object, but rather a group of behaviors that effect control; some of these are behaviors of the View and some of the Model.

The Wire tool accepts an arbitrary point from the user’s click in a window. The length of the Wire is computed and displayed as points are added. When the user clicks a “Shorten” button, the Wire rearranges its points to minimize the total length and the new arrangement is shown in the window. Here is a diagram of the Wire tool. We’ll focus on the implementation of the Model (WireList).

Wire Tool Environment

Smalltalk Programming

Smalltalk consist of an “image” which holds objects and their behaviors and other data in the form of interpretable code, and a “virtual machine” which acts as an interpreter of code. (This is an much oversimplified description, but sufficient for present purposes.) Unlike interpreters of old, these virtual machines can run full-out.

The Smalltalk environment is a basic set of tools. It makes Smalltalk a much more productive language to work in than traditional static languages. These tools allow immediate feedback for testing and debugging, as well as incremental development and alteration of code. Changes to individual algorithms (methods) as well as whole classes are easily accomplished interactively. This is in marked contrast to static object-oriented languages such as C++. The Smalltalk environment’s tools include classes that are available for the your immediate use. In addition, browsers give you access to the source code for these classes. Debugging tools and text editing fields in browsers and editors let you do code entry, compilation, and execution almost anywhere. Getting from the development process to a standalone application can be done directly within Smalltalk’s environment.

The main idea in Smalltalk programming is to program from examples. The primary tool is the Browser, an interactive tool which gives you access to the Smalltalk system code. It also lets you modify code and add your own classes and methods. Here’s a Browser view.

A Smalltalk Browser

In the upper left hand corner is the class being browsed (Rectangle). The main text edit section shows the method being browsed (intersects:). Comments are in quotes. A typical Smalltalk programming statement is: new := Rectangle basicNew. The object (in this case the class Rectangle) is sent a message (basicNew), and the resulting object assigned to the variable, new. Smalltalk syntax refers to the “receiver” (Rectangle in the above example) and the message “selector” (basicNew). Also note the line: «SectRect(self:Ptr; rect:Ptr; new:Ptr):Boolean», this is how a Mac ToolBox call (or other external function) is made in SmalltalkAgents™ (STA). Smalltalk programming is done by editing new or existing methods in the Browser’s edit view and saving the edit; saving automatically invokes the compiler. The result is a new or changed method in your “image” with associated byte code (or other binary) representation.

The Category View shown at the bottom has four list views; Categories, Classes, Protocols, and Methods. Categories and Protocols are for reference and classification purposes only, they have no “programmatic” aspect; Categories group classes together and Protocols group methods. Classes list the classes in the Smalltalk class library (including any you have added) and Methods list the behaviors (methods) for the selected class.

How do we decide what are the objects? How do we decide which classes to subclass? These are common agonies in object-oriented programming. Frankly it may not make that much difference as long as the decisions are reasonable. (For example, a class for our Wires should probably not be a subclass of Rectangle; they have no common behavior.) Some useful rules are 1) If in doubt make your class a subclass of Object (the top level generic class); you can always relocate it later very easily. 2) Choose to subclass under a class (other than Object itself) only if there is a reason to do so. For example, a good reason is so that you can make use of existing methods (behaviors). 3) Don’t create a class that doesn’t have significant computational responsibilities; classes should have behaviors.

A class has a data structure that contains information (data) about its state. These data are objects called instance variables. A class also contains references to behaviors (methods) that let you change or simply read its instance variables. Typically an object is a concrete instantiation, that is, member of a class. An object is a specific instance of a class, and has its own set of instance variables. Sometimes classes have behavior and data sufficient to make them do concrete things on their own without instances. Normally a class functions as a template; an object belonging to that class has real values for the class data.

The Wire Model

Our wire is a collection of (geometric) points. One choice is to make the class associated with the Wire a subclass of one of the collection classes. We’ll choose to make it a subclass of List. An equally valid (and perhaps better) choice is to make Wire a subclass of Object, and then provide it with an instance variable which itself is a List. In the first case, the Wire will inherit all the behavior of a List; in the second case, the Wire’s list instance variable would be accessed when List behavior is needed. For simplicity, we’ll chose the former.

We create a subclass of List called “WireList”. This operation is usually done by filling in a template in the Browser. Below is the class browser view for WireList. The programmer supplies the name of the subclass as a symbol (#WireList); the superclass (#List), and a list of instance variable names #(first second). (We’ll ignore other information such as the paths (e.g. Environment@#List) for now.

WireList Browser

The category view shows three Protocols, accessing (adding point, relocating points, returning specific instance variables), calculating (computing wire length, shortening the wire), and drawing (supporting calculations for drawing the wire). (Note: You might decide that this last protocol is not appropriate for a List. You might want to create a new subclass for Wire directly under class Object, and have the list as an instance variable. The beauty of Smalltalk is that you can make these “relocations” easily as the program develops, a situation somewhat different than that in C++.)

Now let’s fill in the behavior in the first two protocols, starting with accessing. First we want to add points to our list. We’ll use the following notation: WireList>I>add: to express “WireList class, Instance method, add: aPoint”. (An instance method is a behavior for specific objects which are instances of a class; we’ll come back to class methods later.)

WireList > I> add:

The meaning of add: is that the message add with a parameter (aPoint) is sent to an object which is an instance WireList. The result is that a new point is added to the specific WireList object. The add: method shown here is sent to “super”; this starts the search for the implementation in WireList’s superclass, List, which has the add: method we need. WireList>I>add: is actually not needed, we include it for illustrative purposes. If WireList didn’t have a method, add:, the superclass would be tried automatically. If the method is not found there, the search continues until some class in the hierarchy either knew how to do add:, or we run out of classes to search and a failure message gets returned. It is instructive to use the Browser tools to see all the different implementations of add: found in the system. (List is an STA class; equivalents in other systems are classes like OrderedCollection.)

First and second are examples of “accessors” - methods to get instance variables. For example,

WireList > I> second

This translates to “send the message position: 1 (go to the first position in the list) to self”. Self is a reference to the object itself, in this case a specific WireList; self is equivalent to “this” in C++. The semicolon is shorthand for cascading messages. The message “next” to find the next element in the list is sent to the result of the position: 1 message. The result returned, indicated by “^”, is the second element in the list. Note that neither “first” nor “second” are actually needed in the Wire tool; they are there for testing purposes. It is good coding form to use accessor methods to get and set instance variables, rather than directly access data structures. The final method in this group does a simple exchange of two list elements.

WireList > I> exchange:and:

In this method, at:put: is a method of class Object. At: is an accessor method of class Object. Temporary variables (whose scope is limited to the method) are denoted by vertical bars, e.g. |temp1 temp2 temp3|. A period is used to denote the end of a statement, and := is the assignment operator. In creating the Wire tool, we figured out that we needed exchange:and: during development of one of the algorithms under the calculating protocol. Protocol groupings were done as a final step (more on this later).

At this point we can do some computation. We can create a new WireList, put some values in it, access the second element and exchange any two elements. Smalltalk’s analog to the “terminal” is a Console or Transcript window. You can also open a window called a “workspace” and do line by line computing. In fact, you can do this in any appropriate code edit field of a Smalltalk window, including the code edit field of the Browser.

A Console Window

We’ll show you how some of this interactive stuff works. In the first line of the Console window, we created a new instance (member) of class WireList. new is a “constructor” method and is an example of a class method; all classes know how to respond to new to create new instances. In the second Console line, we added some points, and the next line shows the list of points returned. Then we asked Q for its second member, and 5@6 was returned. Finally we exchanged the 2nd and 3rd elements, and the resulting new list is shown. (Code in a Console can be executed line-by-line or grouped; once a selection is made, you can issue some sort of “execute” or “doIt” command.)

We are now ready to develop the calculating protocols, that is, the algorithms to shorten the Wire by seeking the minimum length connecting its points. We’ll do this by exchanging point positions in the list until we get a minimum length. Right away, we know we’ll need a method to compute distance between points. (This sort of functionality is supplied as one of the methods in class Point in some Smalltalk versions.)

WireList > I> distance: indx: to: p

This method differs from the usual class Point distance method; here we compute distance between a Wire’s point referenced via an index and some other point, p. (For simplicity, error checking is not included.) Next we’ll look at the method to compute the length of the Wire.

WireList > I> length

There is a lot in this method. After initializing the length to 0, the List>I>reset method is used to set the WireList pointer to the beginning of the list. Smalltalk ordered collections start their indexing at 1, so we set a temporary variable index, ‘previous’, to the first position. Next we encounter an iteration (do:) over a block. Block contexts in Smalltalk are denoted by code contained inside brackets, [ ]. Blocks are a very important part of Smalltalk programming, and have no simple counterpart in C; the context of a Block is preserved even if evaluation is delayed. In the above code, the do: operation iterates over all elements in the list. The syntax [:p | ....code...] denotes that p is a temporary object that will take on different values, the do: assigns p to the points in the list starting at the beginning. Inside the block, we use the method distance to compute the total length, which is returned at the end of the iteration.

The real action algorithmically in the Wire tool is contained in the method, shorten. This is not the appropriate place to discuss the algorithmics employed. It is sufficient that a simplified form of “simulated annealing” is used. We randomly pick integers from the WireList indices and interchange them and check to see if the interchange shortens the Wire.

WireList > I> shorten

This method has some new features. 100 timesRepeat: [...] does the code in the outer block 100 times. self size is the size of the Wire list. Float random is how random numbers are generated in STA; other systems may use different methods for this. Note the use of length and exchange:and: that were previously developed. self length < minLength returns a Boolean. ifTrue:[] ifFalse:[] is a Smalltalk conditional test; if the Boolean is true, one code block is performed, else another code block is performed.

Now we can test out these new methods in a Console window.

In the first line, a new WireList is created. (Smalltalk code is shown in bold; results are in plain.) Next its distance from the first element to the last point is computed; then the length is calculated. The Wire is told to shorten itself, and a new list results; in this simple case only the last two elements were interchanged. Finally the new length of the shortened wire is obtained. At this point in the development process, we have a fairly complete “model”. The next task is to create the tool per se so that the user can perform these operations on the model via mouse clicks and graphical viewing.

The Wire View and Control

We’ll have to leave a complete description of the Wire tool window (WireListWindow) and the control class (WireTool) to the next article, but here’s a quick overview. The steps are generally as follows:

First, we have to make a Window for the tool; in STA we create a subclass of a generic user interface window (UIWindow) which we call WireList Window. Usually this sort of thing is done with a GUI builder facility built into the Smalltalk system. Following is part of the window creation method, namely some code associated with the addition of a <shorten> button. The line ‘on: #buttonRelease send: #shorten to: self’ associates the event action (release of the Mouse button) with a method (shorten) that is sent to the WireListWindow when the Mouse is released.

Similar code sets up handling of Mouse action to add points to the Wire. A portion of the WireListWindow is shown below after the user has added some points:

The window looks as follows after pressing the “shorten” button two times (two iterations of the shorten algorithm):

Notice that the wire is shorter and the length has been reduced. How does the user action of clicking at points in the window result in a) drawing the wire, and b) adding said points to the Wire list? And how do we connect the pressing of the ‘shorten’ button in the window to invocation of the WireList > I> shorten method?

The answer to these questions rests on the fact that, in Smalltalk, views like the above WireListWindow maintain an instance variable (called module in STA) object which is assigned the model (in this case WireList) object instance. The point to be added is supplied by something like “p := Mouse localPosition”, which returns local window coordinates from the Mouse click. Code like “module add: p” adds a point to the list of the selected points. Pressing the ‘shorten’ button works the same way - the window’s shorten method simply asks the WireList to shorten; this invokes the wire shorten method we described previously.

How the actual drawing of the Wire is accomplished is an interesting point. The details are reasonably complex, and can be Smalltalk system dependent, but the programming is straightforward. When Smalltalk receives an update event, that update request is passed through to the WireListWindow, which handles it by invoking a rendering operation that includes the necessary call to draw the latest Wire based on the current contents of the WireList.

Conclusion

In this article we have tried to present an introduction to Smalltalk programming from a practical point of view. All of the code presented for the WireList can be used (with minor changes) in any Smalltalk version, including public domain ones. If a version of Smalltalk is available, we suggest typing in the WireList code and experimenting. The only way to get a feel for the simplicity and productivity of Smalltalk is to experiment with it. In the next article we will cover the programming support needed for the view and control process, as well as consider some additional tool features such as interactive point relocation and inspection tools.

For SmalltalkAgents™ users, the WireProject code can be found at ftp://qks.com/pub/sta/tutorials/WireList/. A text version of the WireProject suitable for implementation in other Smalltalk versions is available on request (email only) from R. Peskin. Internet: peskin@caip.rutgers.edu, Applelink: D6615, CompuServe: 70372,616. You can also get the project from the usual MacTech Magazine sites or source disk (see p. 2 for details).

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

VirtualBox 5.1.20 - x86 virtualization s...
VirtualBox is a family of powerful x86 virtualization products for enterprise as well as home use. Not only is VirtualBox an extremely feature rich, high performance product for enterprise customers... Read more
Arq 5.7.9 - Online backup to Google Driv...
Arq is super-easy online backup for Mac and Windows computers. Back up to your own cloud account (Amazon Cloud Drive, Google Drive, Dropbox, OneDrive, Google Cloud Storage, any S3-compatible server... Read more
Vienna 3.1.10 :d05d7a5d: - RSS and Atom...
Vienna is a freeware and Open-Source RSS/Atom newsreader with article storage and management via a SQLite database, written in Objective-C and Cocoa, for the OS X operating system. It provides... Read more
WhiteCap 6.7 - Visual plug-in for iTunes...
WhiteCap is a sleek and sophisticated music visualizer and screensaver that features futuristic, wireframe mesh visuals with dynamic backgrounds and colors. WhiteCap contains thousands of visual... Read more
Dropbox 24.4.16 - Cloud backup and synch...
Dropbox is an application that creates a special Finder folder that automatically syncs online and between your computers. It allows you to both backup files and keep them up-to-date between systems... Read more
Amazon Chime 4.2.5645 - Amazon-based com...
Amazon Chime is a communications service that transforms online meetings with a secure, easy-to-use application that you can trust. Amazon Chime works seamlessly across your devices so that you can... Read more
Notion 0.1.8 - A unified workspace for m...
Notion is the unified workspace for modern teams. Features: Integration with Slack Documents Wikis Tasks Note: This application contains in-app and/or external module purchases. Version 0.1.8:... Read more
Google Chrome 58.0.3029.81 - 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
Notion 0.1.8 - A unified workspace for m...
Notion is the unified workspace for modern teams. Features: Integration with Slack Documents Wikis Tasks Note: This application contains in-app and/or external module purchases. Version 0.1.8:... Read more
WhiteCap 6.7 - Visual plug-in for iTunes...
WhiteCap is a sleek and sophisticated music visualizer and screensaver that features futuristic, wireframe mesh visuals with dynamic backgrounds and colors. WhiteCap contains thousands of visual... Read more

Latest Forum Discussions

See All

ChordFlow (Music)
ChordFlow 1.0.0 Device: iOS Universal Category: Music Price: $6.99, Version: 1.0.0 (iTunes) Description: ChordFlow is a chord sequencer with a unique 4-track polyphonic arpeggiator, extensive chord library, MIDI out and Ableton Link... | Read more »
The Walking Dead: A New Frontier is out...
The newest season of Telltale Games'The Walking Dead is well underway. After the release of the third episode, "Above the Law" about a month ago, episode four, "Thicker Than Water" is hot and ready for more zombies and gut-wrenching emotional... | Read more »
Best games we played this week
Another week, another new wave of mobile games do dive into. We've dug through the list of apps that came out this week to tell you which apps are worth your sweet time. And while there weren't too many games this week, there were some big ones.... | Read more »
Vignettes (Games)
Vignettes 1.0.1 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.1 (iTunes) Description: Vignettes is a casual but unique exploration game without text or characters, where objects shapeshift as you spin them around... | Read more »
Get Me Outta Here is an 80s retro shoote...
Are you ready to fight some aliens? Because Crescent Moon Games has released the retro shooter Get Me Outta Here on iOS devices today. [Read more] | Read more »
Get a bunch of Apple productivity apps f...
If you're an Apple Mac owner, you're probably aware of the host of Apple productivity apps the company includes in all new Mac purchases. Apps like iMovie, Keynote, and of course, GarageBand. While you used to be able to also buy these apps... | Read more »
Terra Mystica (Games)
Terra Mystica 1.03 Device: iOS Universal Category: Games Price: $9.99, Version: 1.03 (iTunes) Description: Short Summary:≈≈≈≈≈≈≈≈≈≈≈≈≈ | Read more »
Ms. Spell (Games)
Ms. Spell 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: Cast spells and battle monsters in this turn based game, that has you delving into ever the changing Dreadwood to retrieve the lost... | Read more »
Invert - A Minimal Puzzle Game (Games)
Invert - A Minimal Puzzle Game 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Invert is a minimalist puzzle game for fans of brain teasers, logic puzzles, and Rubik's Cube fiddlers. | Read more »
Evergrow: Paper Forest (Games)
Evergrow: Paper Forest 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Solve puzzles in the handcrafted forests of Evergrow through the eyes of an imaginative child and his parents. Discover... | Read more »

Price Scanner via MacPrices.net

15-inch 2.7GHz Touch Bar MacBook Pros on sale...
Amazon has 2016 15″ 2.7GHz Apple Touch Bar MacBook Pros in stock today and on sale for $150-$200 off MSRP. Shipping is free: - 15″ 2.7GHz Touch Bar MacBook Pro Space Gray (sku MLH42LL/A): $2599 $200... Read more
Apple now offering Certified Refurbished 13-i...
Apple is now offering Certified Refurbished 2016 13″ Touch Bar MacBook Pros for $270-$300 off original MSRP. An Apple one-year warranty is included with each model, and shipping is free: - 13″ 2.9GHz... Read more
MyGiHealth Digestive Symptom Tracker Version...
My Total Health, Inc. has announced the release of MyGiHealth 2.1, an important update to their digestive symptom tracker developed exclusively for iPhone, iPad and iPod touch devices. MyGiHealth is... Read more
Galaxy S8 Materials Costs Highest by Far Comp...
The new Samsung Galaxy S8 equipped with 64 gigabytes (GB) of NAND flash memory carries a bill of materials (BOM) cost that comes out to US$301.60, much higher than for previous versions of the... Read more
iCarMode 4.0 Car Dashboard App For iOS Integr...
Indie developer Diego Resnik has announced the release of iCarMode 4.0, an update to his productivity app developed for iOS devices. iCarMode has positioned itself as a true car dashboard app,... Read more
How to save $150+ on Apple’s 13-inch 2.0GHz n...
Apple Authorized Reseller B&H Photo has non-Touch Bar 13″ 2.0GHz MacBook Pros on sale for $150 off MSRP for a limited time. Shipping is free, and B&H charges NY sales tax only: - 13″ 2.0GHz... Read more
15-inch 2.2GHz Retina MacBook Pro, Apple refu...
Apple has Certified Refurbished 2015 15″ 2.2GHz Retina MacBook Pros available for $1699. That’s $300 off MSRP, and it’s the lowest price available for a 15″ MacBook Pro. An Apple one-year warranty is... Read more
Apple Certified Refurbished iMacs available f...
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
Save up to $160 with Apple refurbished 9-inch...
Apple has Certified Refurbished 9″ and 12″ Apple iPad Pros available for up to $160 off the cost of new iPads. An Apple one-year warranty is included with each model, and shipping is free: - 32GB 9″... Read more
27-inch Apple iMacs on sale for $200 off MSRP
Amazon has 27″ iMacs on sale for $200-$201 off MSRP, each including free shipping: - 27″ 3.3GHz iMac 5K: $2099.99 $200 off MSRP - 27″ 3.2GHz/1TB Fusion iMac 5K: $1798 $201 off MSRP - 27″ 3.2GHz/1TB... Read more

Jobs Board

*Apple* Media Products - Commerce Engineerin...
Apple Media Products - Commerce Engineering Manager Job Number: 57037480 Santa Clara Valley, California, United States Posted: Apr. 18, 2017 Weekly Hours: 40.00 Job Read more
*Apple* Mac Computer Technician - GeekHampto...
…complex computer issues over the phone and in person? GeekHampton, Long Island's Apple Premium Service Provider, is looking for you! Come work with our crew Read more
*Apple* Mobile Master - Best Buy (United Sta...
**493714BR** **Job Title:** Apple Mobile Master **Location Number:** 001024-Weatherford-Store **Job Description:** **What does a Best Buy Apple Mobile Master Read more
Best Buy *Apple* Computing Master - Best Bu...
**496963BR** **Job Title:** Best Buy Apple Computing Master **Location Number:** 001061-Marina-Store **Job Description:** **What does a Best Buy Apple Computing Read more
*Apple* Mobile Master - Best Buy (United Sta...
What does a Best Buy Apple Mobile Master do? At Best Buy, our mission is to leverage the unique talents and passions of our employees to inspire, delight, and enrich Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.