TweetFollow Us on Twitter

September 90 - Assertions in MacApp

Assertions in MacApp

James Plamondon

Have you ever loved someone you couldn't have? Someone of such rare grace, beauty, and elegance that his or her charms were beyond the appreciation of many of your peers, whose tastes, shall we say, were not nearly as refined and discriminating as your own?

Such is my love affair with Bertrand Meyer's object-oriented language, Eiffel. Well described in his book, Object-oriented Software Construction (Prentice-Hall, 1988), Eiffel is a paragon of elegance, power, and simplicity. Every time I waste an hour fighting Pascal's USES clause, or stuff some routine into UObject to get around single inheritance, or write yet another list class (because TList only handles object references), or consider (out of desperation, not desire) turning to C++, I long for the simple solutions Eiffel would provide.

But my love is as hopeless as it is deep-there is, at the time of this writing, no implementation of Eiffel available for the Mac. There's a stripped-down A/UX version nearing completion, and a full Mac OS version planned for a January release-however, these are still in vapor form. Until they solidify-and mature into the complete implementations they almost certainly will not be, in their initial releases-my love must remain unrequited, its fires banked, but still glowing.

Until then, I will continue to console myself with Object Pascal. There are three reasons why I do so:

  • Object Pascal has a sterling virtue that even Eiffel can't claim: it's the lingua franca of MacApp.
  • If you can't be with the one you love, baby… love the one you're with!
  • MacApp's comprehensive utilities mask some of Object Pascal's deficiencies such that, if the bar's dark, the beer's cheap, and you squint a little bit, Object Pascal can look just enough like Eiffel to get you interested.

I'm going to address one aspect of that resemblance in this annotated compilation of MacApp.Tech$ links: assertions. An integral part of Eiffel, assertions are the means by which the semantics of an abstract data type are enforced. For example, Eiffel's preconditions (one kind of assertion) ensure that a method is never called with invalid arguments. Likewise, postconditions ensure that a routine has always done what it was supposed to have done before it returns. Loop variants and invariants ensure loop termination. While not allowing the direct production of formal, mathematical proofs directly from code, the Eiffel assertion mechanism is a powerful weapon in the commercial programmer's arsenal against bugs.

Neither Object Pascal nor MacApp directly address these issues. However, hidden away in the purdah of UMacAppUtilities, unmentioned, undocumented, and generally ignored, are the seeds of an assertion mechanism: the Assertion() global routine, and the gPrecondition global constant/variable. The use of these features are the topic of the discussion below.

Finally, I'm going to start this compilation out of sequence, with my response to a link from Mr. Cooley of After Hours Software. His link (which is, unfortunately, missing from the archives) requested an explanation of what assertions are and what they're good for. My response was actually the end of this thread, but in a zen-like manner not uncommon in programming, the end is really the beginning.

Please note that italicized comments in brackets [like these] have been added by me for the purpose of annotation.

5/2/90
POWERUP.ENG Power Up Software,PRT
Dear Mr. Cooley,

Pretty much everything I know about assertions I learned from Bertrand Meyer's book, Object-Oriented Software Construction, published in 1988 by Prentice-Hall.

The first four chapters of this book are the best discussion I've found anywhere of what makes some software "good" and other software "bad," and how OOP facilitates the production of "good" software.

A lot of the rest of the book is a discussion of other OOP issues - multiple inheritance, genericity (aka parameterized types), garbage collection, classes vs. types, exception handling, assertions, etc.. These issues are discussed in the context of Meyer's language, Eiffel, which I think is the greatest programming language ever, because of its elegant encapsulation of all of the ideas presented in the book. [Hear, hear.] (A number of companies are currently planning to produce Eiffel for the Mac; it already is available for A/UX.)

Assertions are statements of fact that are checked at runtime. If the asserted boolean condition evaluates to FALSE, then a run-time exception is raised - essentially a break into the debugger.

Assertions fall into three rough categories: class invariants, pre- and post- conditions, and other assertions.

The class invariant is checked after every method call, to ensure that the basic characteristics of the class are maintained (that an empty stack contains no elements, for example).

A method's preconditions are checked before entry into the method, to make sure that the method is being called in a valid fashion. For example, one cannot meaningfully pop from an empty stack, so the Pop() method would have a precondition stating that the stack was not empty. If it were empty when Pop() was called, then the assertion would be false, and an exception would be raised.

Likewise, a postcondition ensures that the method does what it is supposed to do. Pop() would include a postcondition stating that, after the method was done, the stack would contain one fewer element than before Pop() was called.

Other assertions include loop assertions (variants and invariants) and the check assertion, which is like the Assertion() routine in MacApp.

All of the assertions are present to ensure that the code does what it is supposed to do. They also enhance the code readability and maintainability, by making it crystal clear what each method is supposed to do, and under what circumstances it can be expected to do it.

That's a quick guide to assertions in Eiffel. They existed in other languages before Eiffel - as a number of people have pointed out to me - but the Eiffel implementation is pretty slick, nonetheless. For a more detailed discussion, I'd definitely see Meyer's book.

Looking forward to Mac Eiffel (later this year? Early in '91? That pesky memory manager sure causes problems!),

James Plamondon

[With that overview of assertions out of the way, we can return to the beginning of the thread.]

4/19/9
POWERUP.ENG Power Up Software,PRT
Dear Steve,

I was wandering around in MacApp 2.0B12 today, and I stumbled across a few interesting things that made me think that somebody has been taking a close look at Eiffel.

First, in UFailure, I found an the following procedure:

Assertion(condition: Boolean; description: StringPtr);

which is only ever called in TApplication.Idle(), as follows:

Assertion(gTarget <> NIL, AtStr('gTarget <> nil'));

Then, wandering further, I encountered two variables (or constants, depending on the state of qDebug): gPreCondition and gPostCondition, which are defined in UMacAppUtilities.p and initialized (to TRUE) in DoRealInitToolBox(), which is in UMacAppUilities.inc1.p. These entities are never referred to anywhere else in MacApp.

These look awfully useful - but the fact that they are not used anywhere in MacApp suggests that they are not safe to use in one's application. So, what's up?

James Plamondon

4/19/9
A14 Carnegie Mellon, Rob Chandok,PRA

James writes: I was wandering around in MacApp 2.0B12 today, and I stumbled across a few interesting things that made me think that somebody has been taking a close look at Eiffel.

I find it interesting that people think Eiffel invented assertions programming languages. While Eiffel is "way cool," it isn't the first language to think about assertions.

I find it more useful to think about good features in terms of the abstract concept, since that is the real importance. You can do assert(boolean,string) in any language. We have such a built-in procedure in our Pascal teaching environment. But we didn't get it from Eiffel.

There has been other interesting work in programming languages before Eiffel, although Eiffel is a well thought out language designed by a smart person. Besides, real assertions would and some way of saying "there exists" and "for all." :-)

Rob

4/19/9
ALGER Alger, Jeff,VCA
Rob,

Amen. There is too much "discovery" of old ideas out there, and assertion is one of them. All major languages are Turing machines, so there is no difference between what you can, in theory, do from one language to another. The differences are all in degree of difficulty to accomplish certain objectives versus execution time and space.

On the subject of "for all" and "there exists," that (sigh) would be a welcome addition to Object Pascal, but requires a good deal more infrastructure than OP provides. Specifically, OP does not have the concept of a set of objects, let alone placing all objects into "master" sets based on class or anything else.

I liked the term "syntactic sugar" applied to assertions in Eiffel, but would take that one step further. Objects in OP are, to me, syntactic sugar plus a little bit of dispatch glue, not objects in the same sense as Smalltalk, CLOS, Object Prolog, etc., etc., etc. "Even" C++ allows allocation and deallocation of storage of objects to be determined separately from the behavior of the objects themselves (a class behavior), class variables, and a number of other blue-blood OOP features. In C++, one can at least conveniently superimpose maintenance of class sets and, therefore, "for all" and "there exists," without changing the public interface to a class's objects. C++ has its problems, too, but the point here is that one can and should do MUCH better than OP.

Ah, nothing like stirring up a little controversy in the morning.

Jeff Alger, Exis

4/19/9
V0683 Amoco Tech, Eric Berdahl,VAR

Yes, the Assertion mechanism is nice to have, especially since OPascal doesn't have language support for the mechanism (I actually think that Eiffel's construct is more syntactic sugar than anything else).

[Every expression we write, unless we're writing in machine language, is "syntactic sugar." Implementing assertions at the library level detracts from their utility (compared to implementing them at the language level), even if it can be done. As I tell my son at the dinner table, if you mix your food together on the plate, it loses something, even if it's going to get all mixed up by the time it gets to your stomach!]

If they [assertions] aren't safe to use, you couldn't prove it by me. I've been using assertions for a while with no consequences. Remember, though, that assertions are things that really are conceived of and placed during object design. If they were added anytime after a significant amount of MacApp design took place, they may not be very easily placed.

[Virginity, once lost, is rarely restored.]

Eric Berdahl

4/20/9
POWERUP.ENG Power Up Software,PRT
Dear Rob, Jeff, and Gentlepersons Everywhere,

Touché! You are correct both in that assertions are not new to Eiffel, and that "there is too much re-discovery of old ideas out there." I first encountered language-based assertions in Eiffel, though, and was not aware that they had been supported in any earlier languages. Thank you for your correction.

I am reminded of a girl whom I once overheard asking a friend, "Did you know that Paul McCartney was in a band before Wings?" [Yes, it's a true story, I swear!] [Ed: I must be slow or something. What band?]

James Plamondon

4/19/9
MACAPP.TEST MacApp SQA Team
James,

As one of the people trying to figure out how you ensure the correctness of a product like MacApp, I am intrigued by Eiffel's assertion mechanism. The routine you found (which was there in B9) is a small step, but it's worth noting that since Eiffel has language support for this stuff, they can add things like the "old" and "nochange" notations (p.115 of the good book), as in

ensure
Size = old fSize + 1

Just because MacApp doesn't use a routine doesn't mean you shouldn't (OK, so we forgot the VAR parameter in GetPortTextStyle and didn't catch it because we didn't use it, but anyway...). [So that's why one of my program's features didn't work! I just wrote it yesterday evening, using GetPortTextStyle, and I was wondering what I was doing wrong!]

Please share your suggestions for assertions with us. Do you think we should have a boolean function TObject.ClassInvariant? Homework: Write preconditions and postconditions for TApplication.TrackMouse.

Richard Rodseth, MacApp Quality

4/20/9
POWERUP.ENG Power Up Software,PRT
Dear Richard, et al.,

I would not know where to begin to write preconditions and postconditions for TApplication.TrackMouse()!

Assertions may work best for container classes (stacks, lists, tables, etc.), which have a mathematically definable nature. TList looks like the best place to start using it-especially given the wide use of lists in MacApp.

Now that I know that there are no impediments to the use of the routine Assertion(), I may very well start using Assertion() in my own code. […and so I have; the little buggers are sprinkled though my code like salt on popcorn.]

Of course, it would help if one could change the values of gPreCondition and gPostCondition from the Debug menu… but I'll leave that as an exercise for the reader. [Hint: use the UDebug routine DebugFlag().]

As to TObject.ClassInvariant(), it sounds like a great idea. Perhaps the following suggestion is possible:

  • Add the field fCheckInvariant (a boolean) to TObject (but only if qDebug is true).
  • Allow the user to set the state of TClassName.fCheckInvariant for each class, independently, in the MacApp debugger.
  • If TClassName.fCheckInvariant were true, then the MacApp debugger would call TClassName.ClassInvariant() (polymorphically, of course) after each call to a method of TClassName (other than ClassInvariant()). The MacApp programmer would NOT have to call TClassName.ClassInvariant() herself; that would defeat the purpose.

[Actually, the CheckInvariant flags would probably be better implemented using a table of boolean values, indexed by class ID.]

This might be exceedingly slow, but it would also be exceedingly nifty. The programmer would, of course, have to write her own ClassInvariant() overrides. Likewise, any precondition and postcondition assertions would have to be written by the programmer, and included in her code, since they would vary from method to method.

Now, if we can just add multiple inheritance and genericity to MacApp (while shovelling in some added simplicity, as Steve Friedrich would say), we can make MacApp and Object Pascal a real working system.

Looking forward to a Mac version of ANY LANGUAGE WHICH SUPPORTS ALL THE THINGS I LIKE ABOUT EIFFEL (but not necessarily Eiffel itself-one must keep an open mind, after all), I remain

James Plamondon

4/20/9
A14 Carnegie Mellon, Rob Chandok,PRA

Rich Rodseth's notes about corrrectness are interesting. Here are some more points about assertions.

While Eiffel can do things like

Size = old fSize + 1

doesn't it seem odd that you have to specify that and the code to add 1 to fSize? The difficulty to me is that you don't want two separate representations of your program–specification and implementation. In this case, you have to make sure that both your code and your invariant is correct. Seems like you might introduce bugs in the invariants as easily as in the code.

[…and that's a very good point. I have indeed written incorrect assertions and wondered why the heck they were (a) flagging errors that weren't errors (easily located and fixed), or (b) not flagging real errors (a much more subtle and insidious problem). I am open to suggestion on ways around this, which aren't themselves error-prone.]

But this is the real world, I know. I haven't seen any language that handle this, although I know of some promising research directions.

In addition, the idea of TObject.ClassInvariant is founded in the premise that inheritance requires the subclass to maintain the behavior of the superclass. This is contrary to many uses of inheritance. Let's see if I can rattle off some ideas off the top of my head.

There are two "kinds" of inheritance: protocols and base class. If you inherit in the protocol style, you are basically saying that "subclass S is just another kind of superclass T, with different internal details, and if you send message M to S, it will behave within the definition of T." I can fully define what the "sort" message will do, no matter what subclass of a sortable class I send the message to.

That doesn't jive with TApplication.TrackMouse, which is an example of inheriting in the "base class" style. No one could possible know what the message TrackMouse is going to do for any subclass of TApplication.

Bruce Horn and I have spent much time arguing over these issues. In addition, you might think about inheritance in this way: If you want to enforce pre/post conditions in the superclass, should the superclass control when the OVERRIDDEN methods get called? For example, just because you override DRAW, should that prevent your superclass's DRAW from being called ? What if DRAW is responsible for maintaining the superclass invariant? Imagine a world where a method call could either percolate up via calls to INHERITED (like object pascal) or down from the superclasses (which is similar to wrappers).

Well, that's enough for now. Suffice to say that invariants are tough. We can barely prove tiny programs correct, it's no wonder we don't know how to specify a language even for executing assertions.

[This is certainly true. Meyer has taken a lot of heat about Eiffel's assertions not being a "complete" formal system for automatic generation of proofs of correctness. But it was never intended nor presented to be such a mechanism-rather, it's just a means by which more robust code can be written. They're not perfect, but they're a big step in the right direction-and if you're dying of thirst, even muddy water looks good!]

I remain, as perplexed as ever,
Rob

Annotator's Summary

The UFailure routine Assertion() is there to be used. It is a powerful and convenient means of ensuring that some condition is true at runtime. I use it frequently to check handles vs. NIL, check values for correctness, and anything else that can be summed up in a BOOLEAN statement. If the condition is FALSE at runtime, the program drops into the MacApp debugger, and asks if you want to signal failure. If included in non-debug code, an Assertion() failure signals a silent failure. Using assertions liberally has saved me hours-days, by now-of debugging time.

The Boolean gPrecondition is a global variable (initialized to TRUE) if qDebug is TRUE, else it is a global constant (with the value FALSE). By adding a call to DebugFlag() in your application's initialization routine, you can toggle this flag on and off in the MacApp debugger. To use it, place the assertions which check a routine's arguments for validity inside an IF gPrecondition statement. In qDebug mode, these assertions will only be checked if the variable gPrecondition (as set by you in the debugger) is TRUE. In non-qDebug mode, gPrecondition is a constant with the value FALSE, so the assertions are stripped out by the linker.

Eiffel is not a perfect language; even blinded by infatuation as I am, I can see that. But Object Pascal-whose charms have long since begun to fade-could do far worse than to imitate Eiffel's standard of beauty.

[Ed: James can be contacted (for what would no doubt be a lengthy discussion] at AppleLink address POWERUP.ENG.]

 
AAPL
$99.76
Apple Inc.
+2.09
MSFT
$44.08
Microsoft Corpora
+0.45
GOOG
$520.84
Google Inc.
+9.67

MacTech Search:
Community Search:

Software Updates via MacUpdate

TechTool Pro 7.0.5 - Hard drive and syst...
TechTool Pro is now 7, and this is the most advanced version of the acclaimed Macintosh troubleshooting utility created in its 20-year history. Micromat has redeveloped TechTool Pro 7 to be fully 64... Read more
PDFKey Pro 4.0.2 - Edit and print passwo...
PDFKey Pro can unlock PDF documents protected for printing and copying when you've forgotten your password. It can now also protect your PDF files with a password to prevent unauthorized access and/... Read more
Yasu 2.9.1 - System maintenance app; per...
Yasu was originally created with System Administrators who service large groups of workstations in mind, Yasu (Yet Another System Utility) was made to do a specific group of maintenance tasks... Read more
Hazel 3.3 - Create rules for organizing...
Hazel is your personal housekeeper, organizing and cleaning folders based on rules you define. Hazel can also manage your trash and uninstall your applications. Organize your files using a... Read more
Autopano Giga 3.7 - Stitch multiple imag...
Autopano Giga allows you to stitch 2, 20, or 2,000 images. Version 3.0 integrates impressive new features that will definitely make you adopt Autopano Pro or Autopano Giga: Choose between 9... Read more
MenuMeters 1.8 - CPU, memory, disk, and...
MenuMeters is a set of CPU, memory, disk, and network monitoring tools for Mac OS X. Although there are numerous other programs which do the same thing, none had quite the feature set I was looking... Read more
Coda 2.5 - One-window Web development su...
Coda is a powerful Web editor that puts everything in one place. An editor. Terminal. CSS. Files. With Coda 2, we went beyond expectations. With loads of new, much-requested features, a few... Read more
Arq 4.6.1 - Online backup to Google Driv...
Arq is super-easy online backup for the Mac. Back up to your own Google Drive storage (15GB free storage), your own Amazon Glacier ($.01/GB per month storage) or S3, or any SFTP server. Arq backs up... Read more
Airfoil 4.8.10 - Send audio from any app...
Airfoil allows you to send any audio to AirPort Express units, Apple TVs, and even other Macs and PCs, all in sync! It's your audio - everywhere. With Airfoil you can take audio from any... Read more
Apple iMovie 10.0.6 - Edit personal vide...
With an all-new design, Apple iMovie lets you enjoy your videos like never before. Browse your clips more easily, instantly share your favorite moments, and create beautiful HD movies and Hollywood-... Read more

Latest Forum Discussions

See All

This Week at 148Apps: October 13-17, 201...
Expert App Reviewers   So little time and so very many apps. What’s a poor iPhone/iPad lover to do? Fortunately, 148Apps is here to give you the rundown on the latest and greatest releases. And we even have a tremendous back catalog of reviews; just... | Read more »
Angry Birds Transformers Review
Angry Birds Transformers Review By Jennifer Allen on October 20th, 2014 Our Rating: :: TRANSFORMED BIRDSUniversal App - Designed for iPhone and iPad Transformed in a way you wouldn’t expect, Angry Birds Transformers is a quite... | Read more »
GAMEVIL Announces the Upcoming Launch of...
GAMEVIL Announces the Upcoming Launch of Mark of the Dragon Posted by Jessica Fisher on October 20th, 2014 [ permalink ] Mark of the Dragon, by GAMEVIL, put | Read more »
Interview With the Angry Birds Transform...
Angry Birds Transformers recently transformed and rolled out worldwide. This run-and-gun title is a hit with young Transformers fans, but the ample references to classic Transformers fandom has also earned it a place in the hearts of long-time... | Read more »
Find Free Food on Campus with Ypay
Find Free Food on Campus with Ypay Posted by Jessica Fisher on October 20th, 2014 [ permalink ] iPhone App - Designed for the iPhone, compatible with the iPad | Read more »
Strung Along Review
Strung Along Review By Jordan Minor on October 20th, 2014 Our Rating: :: GOT NO STRINGSUniversal App - Designed for iPhone and iPad A cool gimmick and a great art style keep Strung Along from completely falling apart.   | Read more »
P2P file transferring app Send Anywhere...
File sharing services like Dropbox have security issues. Email attachments can be problematic when it comes to sharing large files. USB dongles don’t fit into your phone. Send Anywhere, a peer-to-peer file transferring application, solves all of... | Read more »
Zero Age Review
Zero Age Review By Jordan Minor on October 20th, 2014 Our Rating: :: MORE THAN ZEROiPad Only App - Designed for the iPad With its mind-bending puzzles and spellbinding visuals, Zero Age has it all.   | Read more »
Hay Ewe Review
Hay Ewe Review By Campbell Bird on October 20th, 2014 Our Rating: :: SAVE YOUR SHEEPLEUniversal App - Designed for iPhone and iPad Pave the way for your flock in this line drawing puzzle game from the creators of Worms.   | Read more »
My Very Hungry Caterpillar (Education)
My Very Hungry Caterpillar 1.0.0 Device: iOS Universal Category: Education Price: $3.99, Version: 1.0.0 (iTunes) Description: Care for your very own Very Hungry Caterpillar! My Very Hungry Caterpillar will captivate you as he crawls... | Read more »

Price Scanner via MacPrices.net

2013 15-inch 2.0GHz Retina MacBook Pro availa...
B&H Photo has leftover previous-generation 15″ 2.0GHz Retina MacBook Pros now available for $1599 including free shipping plus NY sales tax only. Their price is $400 off original MSRP. B&H... Read more
Updated iPad Prices
We’ve updated our iPad Air Price Tracker and our iPad mini Price Tracker with the latest information on prices and availability from Apple and other resellers, including the new iPad Air 2 and the... Read more
Apple Pay Available to Millions of Visa Cardh...
Visa Inc. brings secure, convenient payments to iPad Air 2 and iPad mini 3as well as iPhone 6 and 6 Plus. Starting October 20th, eligible Visa cardholders in the U.S. will be able to use Apple Pay,... Read more
Textkraft Pocket – the missing TextEdit for i...
infovole GmbH has announced the release and immediate availability of Textkraft Pocket 1.0, a professional text editor and note taking app for Apple’s iPhone. In March 2014 rumors were all about... Read more
C Spire to offer iPad Air 2 and iPad mini 3,...
C Spire on Friday announced that it will offer iPad Air 2 and iPad mini 3, both with Wi-Fi + Cellular, on its 4G+ LTE network in the coming weeks. C Spire will offer the new iPads with a range of... Read more
Belkin Announces Full Line of Keyboards and C...
Belkin International has unveiled a new lineup of keyboard cases and accessories for Apple’s newest iPads, featuring three QODE keyboards and a collection of thin, lightweight folios for both the... Read more
Verizon offers new iPad Air 2 preorders for $...
Verizon Wireless is accepting preorders for the new iPad Air 2, cellular models, for $100 off MSRP with a 2-year service agreement: - 16GB iPad Air 2 WiFi + Cellular: $529.99 - 64GB iPad Air 2 WiFi... Read more
Price drops on refurbished Mac minis, now ava...
The Apple Store has dropped prices on Apple Certified Refurbished previous-generation Mac minis, with models now available starting at $419. Apple’s one-year warranty is included with each mini, and... Read more
Apple refurbished 2014 MacBook Airs available...
The Apple Store has Apple Certified Refurbished 2014 MacBook Airs available for up to $180 off the cost of new models. An Apple one-year warranty is included with each MacBook, and shipping is free.... Read more
Refurbished 2013 MacBook Pros available for u...
The Apple Store has Apple Certified Refurbished 13″ and 15″ MacBook Pros available starting at $929. Apple’s one-year warranty is standard, and shipping is free: - 13″ 2.5GHz MacBook Pros (4GB RAM/... Read more

Jobs Board

*Apple* Retail - Multiple Positions (US) - A...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Position Opening at *Apple* - Apple (United...
…customers purchase our products, you're the one who helps them get more out of their new Apple technology. Your day in the Apple Store is filled with a range of Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** At the Apple Store, you connect business professionals and entrepreneurs with the tools they need in order to put Apple solutions to work in their Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** The Apple Store is a retail environment like no other - uniquely focused on delivering amazing customer experiences. As an Expert, you introduce people Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** As businesses discover the power of Apple computers and mobile devices, it's your job - as a Solutions Engineer - to show them how to introduce these Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.