Structure and Interpretation of Computer Programmers

I make it easier and faster for you to write high-quality software.

Monday, July 29, 2013

Programmer Values

A question and answer exchange over at reveals something interesting about how software is valued. The question asked whether there is any real-world data regarding costs and benefits of test-driven development.[*] One of the answers contained, at time of writing, the anthropologist’s money shot:

The first thing that needs to be stated is that TDD does not necessarily increase the quality of the software (from the user’s point of view). […] TDD is done primarily because it results in better code. More specifically, TDD results in code that is easier to change. [Emphasis original]

Programmers are contracted, via whatever means, by people who see quality in one way: presumably that quality is embodied in software that they can use to do some thing that they wanted to do. Maybe it’s safer to say that the person who provided this answer believes that their customers value quality in software in that way, rather than make an assumption on everybody’s behalf.

This answer demonstrates that (the author believed, and thought it uncontentious enough to post without defence in a popularity-driven forum) programmers value attributes of the code that are orthogonal to the values of the people who pay them. One could imagine programmers making changes to some software that either have no effect or even a negative effect as far as their customers are concerned, because the changes have a positive effect in the minds of the programmers. This issue is also mentioned in one of the other answers to the question:

The problem with developers is they tend to implement even things that are not required to make the software as generic as possible.

The obvious conclusion is that the quality of software is normative. There is no objectively good or bad software, and you cannot discuss quality independent of the value system that you bring to the evaluation.

The less-obvious conclusion is that some form of reconciliation is still necessary: that management has not become redundant despite the discussions of self-organised teams in the Agile development community. Someone needs to mediate between the desire of the people who need the software to get something that satisfies their norms regarding quality software, and the desire of the people who make the software to produce something that satisfies their norms instead. Whether this is by aligning the two value systems, by ignoring one of them or by ensuring that the project enables attributes from both value systems to be satisfied is left as an exercise for the reader.

[*] There is at least one relevant study. No, you might not think it relevant to your work: that’s fine.

posted by Graham at 09:21  

Friday, July 26, 2013

Know what counts

In Make it Count, Harry Roberts describes blacking out on stage at the end of a busy and sleepless week. Ironically, he was at the start of a talk in which he was to discuss being selective over side projects, choosing only those that you can actually “cash in” on and use to advance your career.

If you’re going to take on side projects and speaking and writing and open source and suchlike then please, make them fucking count. Do not run yourself into the ground working on ‘career moves’ if you’re not going to cash in on them. [emphasis original]

Obviously working until you collapse is not healthy. At that point, choosing which projects to accept is less important than just getting some damned sleep and putting your health back in order. In the 1950s, psychologist Abraham Maslow identified a “hierarchy of needs”, and sleep is at the base of the hierarchy meaning that, along with eating and drinking, you should take care of that before worrying about self-actualisation or esteem in the eyes of your peers.

Maslow's heirarchy, Wikipedia image

Here’s the little secret they don’t tell you in the hiring interview at Silicon Valley start-ups: you’re allowed to do things that aren’t career-centric. This includes, but is not limited to, sleeping, drinking enough water, eating non-pizza foodstuffs, having fun, seeing friends, taking breaks, and indulging in hobbies. It sometimes seems that programmers are locked in an arms race to see who can burn out first^W^W^Wdo more work than the others. That’s a short-term, economist-style view of work. I explained in APPropriate Behaviour that economists take things they don’t want to consider, or can’t work out a dollar value for, and call them “externalities” that lie outside the system.

Your health should not be an externality. Roberts attempted to internalise the “accounting” for all of his side projects by relating them in value to his career position. If you’re unhealthy, your career will suffer. So will the rest of your life. Don’t externalise your health. Worry not whether what you’re doing is good for your position in the developer community, but whether it’s good for you as a healthy individual. If you’ve got the basic things like food, shelter, sleep and safety, then validation in the eyes of yourself and your peers can follow.

posted by Graham at 14:56  

Friday, July 26, 2013

The future will be just like the past, right?

I’ve been having a bit of a retro programming session:

Z88 adventure game

The computer in the photo is a Cambridge Z88, and it won’t surprise you to know that I’ve owned it for years. However, it’s far from my first computer.

I was born less than a month before the broadcast of The Computer Programme, the television show that brought computers into many people’s living rooms for the first time. This was the dawn of the personal computer era. The Computer Programme was shown at the beginning of 1982: by the end of that year the Commodore VIC-20 had become the first computer platform ever to sell more than one million units.

My father being an early adopter (he’d already used a Commodore PET at work), we had a brand new Dragon 32 computer before I was a year old. There’s not much point doing the “hilarious” comparisons of its memory capacity and processor speed with today’s computers: the social systems into which micros were inserted and the applications to which they were put render most such comparisons meaningless.

In 1982, computers were seen by many people as the large cupboards in the back of “James Bond film” sets. They just didn’t exist for a majority of people in the UK, the US or anywhere else. The micros that supposedly revolutionised home life were, for the most part, mainly useful for hobbyists to find out how computers worked. Spreadsheets like VisiCalc might already have been somewhat popular in the business world, but anyone willing to spend $2000 on an Apple ][ and VisiCalc probably wasn’t the sort of person about to diligently organise their home finances.

Without being able to sell their computers on the world-changing applications, many manufacturers were concerned about price and designed their computers down to a level. The Register’s vintage hardware section has retrospectives on many of the microcomputer platforms from the early 1980s, many of which tell this tale. (Those that don’t tell the tale of focusing on time to market, and running out of money.) The microprocessors were all originally controllers for disk drives and other peripherals in “real” computers, repurposed as the CPUs of the micro platforms. Sinclair famously used faulty 64kB RAM chips to supply the 48kB RAM in the ZX Spectrum, to get a good price from the supplier.

So the manufacturers were able to make the hardware cheap enough that people would buy computers out of interest, but what would they then make of them? We can probably tell quite a lot by examining the media directed at home computer users. Start with The Computer Programme, as we’ve already seen that back at the beginning of the post. What you have is Ian “Mac” McNaught-Davies, positioned at the beginning of episode 1 as a “high priest” of the mainframe computer, acting as the Doctor to Chris Serle’s bemused and slightly apprehensive assistant. Serle is the perfectly ordinary man on the perfectly ordinary street, expressing (on behalf of us, the perfectly ordinary public) amazement at how a computer can turn a perfectly ordinary television set and a perfectly ordinary domestic cassette recorder into something that’s able to print poorly-defined characters onto perfectly ordinary paper.

During his perfectly ordinary tenure of ten episodes, Serle is taught to program in BBC BASIC by McNaught-Davis. In the first episode he demonstrates a fear of touching anything, confirming the spelling of every word (“list? L-I-S-T?”) he’s asked to type. If the computer requires him to press Return, he won’t do it until instructed by McNaught-Davis (thus making January 11, 1982 the first ever outing of The Return of the Mac). By the end of the series, Serle is able to get on a bit more autonomously, suggesting to Mac what the programs mean (“If temperature is more than 25, degrees I would assume…”).

Chris Serle suffered his way through nine weeks of BASIC tuition because there was no other choice for a freelance journalist to get any use out of a personal computer. Maybe as many as 8,000 hipster programmers would opt for a Jupiter Ace and the FORTH language, but for normal people it was BASIC or nothing. Even loading a game required typing the correct incantation into the BASIC prompt. Feedback was minimal because there wasn’t a lot of ROM in which to store the error messages: “Subscript at line 100” or even the Dragon’s “?BS ERROR” might be all you’re told about an error. If you didn’t have a handy McNaught-Davis around (perhaps the first user-friendly Mac in the computer field) you could easily lose ages working out what the computer thought was BS about your code.

Typing errors became manifold when using the common application distribution platform: the printed magazine. Much software was distributed as “type-ins”, often split over two (monthly) issues of a magazine: the program being presented in buggy form in one edition and an errata being supplied in the next. When you typed not one LOAD command, but a few hundred lines of BASIC in, only to find that your database program didn’t work as expected, you first had a tedious proof-reading task ahead to check that you’d typed it without error. If you had, and it still didn’t work, then out came the pencil and paper as you tried to work out what mistakes were in the listing.

Microcomputers represented seriously constrained hardware with limited application. The ability to get anything done was hampered by the primary interface being an error-prone, cryptic programming language. While the syntax of this language was hailed as simpler than many alternatives, it did nothing to smooth over or provide a soft landing for complex underlying concepts.

I’m willing to subject myself to those trials and terrors for the purpose of nostalgia. There are other people, though, who want to revert to this impression of computers as a way to get young people interested in programming. The TinyBASIC for Raspberry Pi announcement hails:

we’ve also had a really surprising number of emails from parents who haven’t done any programming since school, but who still have books on BASIC from when they were kids, remember enjoying computing lessons, and want to share some of what they used to do with their kids. It’s actually a great way to get kids started, especially if you have some enthusiasm of your own to share: enthusiasm’s contagious.

Undoubtedly there are some genuine, remembered benefits to programming on these platforms, which modern computer tuition could learn from. There was, as discussed above, no hurdle to jump to get into the programming environment. Try teaching any programming language on widely-available computing platforms today, and you’ve got to spend a while discussing what versions of what software are needed, differences between platforms, installation and so on. Almost anyone on a microcomputer could turn on, and start typing in BASIC code that would, if restricted to a limited subset of commands, work whatever they’d bought.

The cost of a “tyre-kicking” setup was modest, particularly as you could use your own TV and cassette deck (assuming you had them). Unlike many modern platforms, there was no need to have two computers tethered to program on one and run on the other, and no developer tithe to pay to the platform vendors. Where they were error-free and well documented, the type-ins gave you actually working applications that you could tweak and investigate. Such starting points are better for some learners than a blank screen and a blinking prompt.

Complete applications though these type-ins may have been, they would not satisfy the expectations of modern computer-using learners. There’s an important difference: people today have already used computers. They’re no longer magical wonder-boxes that can make a TV screen flash blue and yellow if you get the numbers correct in a PAPER command. People know what to expect from a laptop, tablet or smartphone: being able to print an endless march of RUMBELOWS IS SHIT to the screen is no longer sufficient to retain interest.

It’s not just the users of computers, nor the uses of computers, that have moved on in the last three decades. Teaching has evolved, too. There should probably be a name for the fallacy that assumes that however I was taught things is however everybody else should be taught them. A modern curriculum for novice programmers should reflect not only the technological and social changes in computing in the last thirty years, but also the educational changes. It should borrow from the positives of microcomputer programming courses, but not at the expense of throwing out a generation of evolution.

There are certainly things we can learn from the way microcomputers inspired a generation of programmers. There’s a place for ultra-cheap computers like the Raspberry Pi in modern computing pedagogy. But it would be a mistake to assume that if I gave a child my copy of “Super-Charge Your Spectrum”, that child would learn as much and be as enthused about programming as my rose-tinted model of my younger self apparently was.

posted by Graham at 00:42  

Monday, July 15, 2013

On what makes a “good” comment

I have previously discussed the readability of code:

The author must decide who will read the code, and how to convey the important information to those readers. The reader must analyse the code in terms of how it satisfies this goal of conveyance, not whether they enjoyed the indentation strategy or dislike dots on principle.

Source code is not software written in a human-readable notation. It’s an essay, written in executable notation.

Now how does that relate to comments? Comments are a feature of programming languages that allow all other text-based languages—executable or otherwise—to be injected into the program. The comment feature has no effect on the computer’s interpretation of the software, but wildly varying effects on the reader’s interpretation. From APPropriate Behaviour:

[There are] problems with using source code as your only source of information about the software. It does indeed tell you exactly what the product does. Given a bit of time studying, you can discover how it does it, too. But will the programming language instructions tell you why the software does what it does? Is that weird if statement there to fix a bug reported by a customer? Maybe it’s there to workaround a problem in the APIs? Maybe the original developer just couldn’t work out a different way to solve the problem.

So good documentation should tell you why the code does what it does, and also let you quickly discover how.

We need to combine these two quotes. Yes, the documentation—comments included—needs to express the why and the how, but different readers will have different needs and will not necessarily want these questions answered at the same level.

Take the usual canonical example of a bad comment, also given in APPropriate Behaviour and used for a very similar discussion:

//add one to i

To practiced developers, this comment is just noise. It says the same thing as the line below it.

The fact is that to novice developers too it says the same thing as the line below it, but they have not yet learned to read the notation fluently. This means that they cannot necessarily readily tell that they say the same thing: therefore the comment adds value.

Where someone familiar with the (programming) language might say that the comment only reiterates what the software does, and therefore adds no value, a neophyte might look at the function name to decide what it does and look to comments like this to help them comprehend how it does it.

Outside of very limited contexts, I would avoid comments like that though. I usually assume that a reader will be about as comfortable with the (computer) language used as I am, and either knows the API functions or (like me) knows where to find documentation on them. I use comments sparingly, to discuss trade-offs being made, information relied on that isn’t evident in the code itself or discussions of why what’s being done is there, if it might seem odd without explanation.

Have I ever written a good comment?

As examples, here are some real comments I’ve written on real code, with all the context removed and with reviews added. Of course, as with the rest of the universe “good” and “bad” are subjective, and really represent conformance with the ideas of comment quality described above and in linked articles.

 /*note - answer1.score < answer2.score, but answer1 is accepted so should
 *still be first in the list of answers.

This is bad. You could work this one out with a limited knowledge of the domain, or from the unit tests. This comment adds nothing.

 * The UIWebView loads its contents asynchronously. If it's still doing
 * that when the test comes to evaluate its content, the content will seem
 * empty and the test will fail. Any solution to this comes down to "hold
 * the test back for a bit", which I've done explicitly here.

This is good. I’ve explained that the code has a surprising shape, but for a reason I understand, and I’ve provided a reference that goes into more detail.

    //Knuth Section 6.2.2 algorithm D.

This is good, if a bit too brief. I’ve cited the reference description (to me, anyway: obviously Knuth got it from somewhere else) of the algorithm. If you want to know why it does what it does, you can go and read the discussion there. If there’s a bug you can compare my implementation with Knuth’s. Of course Knuth wrote more than one book, so I probably should have specified “The Art of Computer Programming” in this comment.

 * The command bus accepts commands from the application and schedules work
 * to fulfil those commands.

This is not what I mean by a comment. It’s API documentation, it happens to be implemented as a comment, but it fills a very particular and better-understood role.

What do other people’s comments look like?

Here are some similarly-annotated comments, from a project I happen to have open (GNUstep-base).

 * If we need space allocated to store a return value,
 * make room for it at the end of the callframe so we
 * only need to do a single malloc.

Explains why the programmer wrote it this way, which is a good thing.

  /* The addition of a constant '8' is a fudge applied simply because
   * some return values write beynd the end of the memory if the buffer
   * is sized exactly ... don't know why.

This comment is good in that explains what is otherwise a very weird-looking bit of code. It would be better if the author had found the ultimate cause and documented that, though.

/* This class stores objects inline in data beyond the end of the instance.
 * However, when GC is enabled the object data is typed, and all data after
 * the end of the class is ignored by the garbage collector (which would
 * mean that objects in the array could be collected).
 * We therefore do not provide the class when GC is being used.

This is a good comment, too. There’s a reason the implementation can’t be used in particular scenarios, here’s why a different one is selected instead.

 *  Make sure the array is 'sane' so that it can be deallocated
 *  safely by an autorelease pool if the '[anObject retain]' causes
 *  an exception.

This is a bad comment, in my opinion. Let’s leave aside for the moment the important issue of our industry’s relationship with mental illness. What exactly does it mean for an array to be ‘sane’? I can’t tell from this comment. I could look at the code, and find out what is done near this comment. However, I could not decide what there contributes to this particular version of ‘sanity’: particularly, what if anything could I remove before it was no longer ‘sane’? Why is it that this particular version of ‘sanity’ is required?

What do other people say about comments?

For many people, the go-to (pun intended) guide on coding practice is, or was, Code Complete, 2nd Edition. As with this blog and APPropriate Behaviour, McConnell promotes the view that comments are part of documentation and that documentation is part of programming as a social activity. From the introduction to Chapter 32, Self-Documenting Code:

Like layout, good documentation is a sign of the professional pride a programmer puts into a program.

He talks, as do some of the authors in 97 Things Every Programmer Should Know, about documenting the design decisions, both at overview and detailed level. That is a specific way to address the “why” question, because while the code shows you what it does it doesn’t express the infinitude of things that it does not do. Why does it not do any of them? Good question, someone should answer it.

Section 32.3 is, in a loose way, a Socratic debate on the value of comments. In a sidebar to this is a quote attributed to “B. A. Sheil”, from an entry in the bibliography, The Psychological Study of Programming. This is the source that most directly connects the view on comments I’ve been expressing above and in earlier articles to the wider discourse. The abstract demonstrates that we’re in for an interesting read:

Most innovations in programming languages and methodology are motivated by a belief that they will improve the performance of the programmers who use them. Although such claims are usually advanced informally, there is a growing body of research which attempts to verify them by controlled observation of programmers’ behavior. Surprisingly, these studies have found few clear effects of changes in either programming notation or practice. Less surprisingly, the computing community has paid relatively little attention to these results. This paper reviews the psychological research on programming and argues that its ineffectiveness is the result of both unsophisticated experimental technique and a shallow view of the nature of programming skill.

Here is not only the quote selected by McConnell but the rest of its paragraph, which supplies some necessary context. The emphasis is Sheil’s.

Although the evidence for the utility of comments is equivocal, it is unclear what other pattern of results could have been expected. Clearly, at some level comments have to be useful. To believe otherwise would be to believe that the comprehensibility of a program is independent of how much information the reader might already have about it. However, it is equally clear that a comment is only useful if it tells the reader something she either does not already know or cannot infer immediately from the code. Exactly which propositions about a program should be included in the commentary is therefore a matter of matching the comments to the needs of the expected readers. This makes widely applicable results as to the desirable amount and type of commenting so highly unlikely that behavioral experimentation is of questionable value.

So it turns out that at about the time I was being conceived, so was the opinion on comments (and documentation and code readability in general) to which I ascribe: that you should write for your audience, and your audience probably needs to know more than just what the software is up to.

That Sheil reference also contains a cautionary tale about the “value” of comments:

Weissman found that appropriate comments caused hand simulation to proceed significantly faster, but with significantly more errors.

That’s a reference to Laurence Weissman’s 1974 PhD Thesis.

posted by Graham at 15:16  

Monday, July 15, 2013

Did that restructuring work actually help?

Before getting into the meat of this post, I’d like to get into the meta of this post. This essay, and I imagine many in this blog [Ed: by which I meant the blog this has been imported from], will be treading a fine line. The intended aim is to question accepted industry practice, and find results consistent or inconsistent with the practice as a beneficial task to perform. I’m more likely to select papers that appear to refute the practice, as that’s more interesting and makes us introspect the way we work more than does affirmation. The danger is that this skates too close to iconoclasm, as expressed in the Goto Copenhagen talk title Is it just me or is everything shit?. My intention isn’t to say that whatever we’re doing is wrong, just to provide some healthy inspection and analysis of our industry.

Legacy Software Restructuring: Analyzing a Concrete Case

The thread in this paper is that metrics that have long been used to measure the quality of source code—metrics related to coupling and cohesion—may not actually be relevant to the problems developers have to solve. Firstly, the jargon:

  • coupling refers to the connections between the part of the software (module, class, function, whatever) under consideration and the rest of the software system. Received wisdom is that lower coupling (i.e. fewer connections that are less-tightly intertwined) is better.
  • cohesion refers to the relatedness of the tasks performed by the (module, class, function, whatever) under consideration. The more different responsibilities a component provides or uses, the lower its cohesion. Received wisdom is that higher cohesion (i.e. fewer responsibilities per module) is better.

We’re told that striving for low coupling and high cohesion will make the parts of our software reusable and replaceable, and will reduce the number of code sites we need to change when we want to fix bugs in the future. The focus of this paper is on whether the metrics we use as proxies for these properties actually represent enhancements to the code; in other words, whether we have a systematic way to decide whether a change is an improvement or not.


The way in which the authors test their metrics is necessarily problematic. There is no objective standard against which they can be prepared—if there were, we’d have an objective standard and we could all go home. They hypothesise that any restructuring effort by a development team must represent an improvement in the codebase: if you didn’t think a change was better, why would you make that change?

Necessity is one such reason. Consider the following thought process:

I need to add this feature to my product, this change was unforeseen at design time, so the architecture doesn’t really support it. I’m not very happy about the this, but shoehorning it in here is the simplest way to support what I need.

To understand other problems with this methodology, a one-paragraph introduction to the postmodern philosophy of software engineering is required. Software, it says, supports not some absolute set of requirements that were derived from studying the universe, but the ad-hoc set of interactions between the various people who engage with the software system. Indeed, the software system itself modifies these interactions, creating a feedback loop that in fact modifies the requirements of the software that was created. Some of the results of this philosophy[*] are expressed in “Manny” Lehman’s Laws of Software Engineering, which are also cited by the paper I’m talking about here. The authors offer one of Lehman’s Laws as:

[*] I don’t consider Lehman’s laws to be objectively true of software artefacts, but to be hypotheses that arise from a particular philosophy of software. I also think that philosophy has value.

Considering Lehman’s law of software evolution, such systems would already have suffered a decrease in their quality due to the maintenance. This would increase the probability that the restructuring has a better modular quality.

This statement is inconsistent. On the one hand, this change improves the quality of some software. On the other hand, the result of a collection of such changes is to decrease the quality. Now there’s nothing to say that a particular change won’t be an improvement; but there’s also nothing to say that the observed change has this property.[*] The postmodern philosophy adds an additional wrinkle: even if this change is better, it’s only better _as perceived by the people currently working with the system_. Others may have different ideas. We saw, in discussing the teaching of programming, that even experienced programmers can have difficulty reading somebody else’s code. I wouldn’t find it a big stretch to posit that different people have different ideas of what constitutes “good” modular decomposition, and that therefore a different set of programmers would think this change to be worse.

[*]Actually I think the sentence in the paper might just be broken; remember that I found this on the preprint server so it might not have been reviewed yet. One of Lehman’s laws says that, for “E-type” software (by which he means systems that evolve with their environment—in other words, systems where a postmodern appraisal is applicable), the software will gradually be perceived as _reducing_ in quality if no maintenance work goes into it. That’s because the system is evolving while the software isn’t; the requirements change without the software catching up.

Results and Discussion

The authors found that, for three particular revisions of Eclipse, the common metrics for coupling and cohesion did not monotonically “improve” with successive restructuring efforts. In some cases, both coupling and cohesion decreased in the same effort. In addition, they found that the number and extent of cyclic dependencies between Java packages increased with every successive version of the platform.

It’s not really possible to choose a conclusion to draw from these results:

  • maybe the dogma of increasing cohesion and decreasing coupling is misleading.
  • maybe the metrics used to measure those properties were poorly chosen (though they are commonly-chosen).
  • maybe the Eclipse developers use some other measurement of quality that the authors didn’t ask about.
  • maybe some of the Eclipse engineers do take these properties into account, and some others don’t, and we [can’t – added on import] even draw general conclusions about Eclipse.

So this paper doesn’t demonstrate that cohesion and coupling metrics are wrong. But it does raise the important question: might they be not right? If you’re relying on some code metrics derived from received wisdom or dogma, it’s time to question whether they really apply to what you do.

posted by Graham at 10:59  

Monday, July 15, 2013

Teaching Programming to People. It’s easy, right?

I was doing a literature search for a different subject (which will appear soon), and found a couple of articles related to teaching programming. I don’t know if you remember when you learnt programming, but you probably found it hard. I’ve had some experience of teaching programming: specifically, teaching C to undergraduates. Said undergraduates, as it happens, weren’t on a computing course (they studied Physics), and only turned up to the few classes they had a year because attendance was mandatory. The lectures, which weren’t compulsory, had fewer students showing up.

Teaching Python to Undergraduates

When I took the course, we were taught a Pascal variant on NeXTSTEP. I have some evidence that Algol had been the first programming language taught on the course; probably on an HLH Orion minicomputer. While Pascal was developed in part as a vehicle to teach structured programming concepts, the academics in the computing course at my department were already starting to see it as a toy language with no practical utility. Such justification was used to look for a different language to teach. As you can infer from the previous paragraph, we settled on C: but not before a test where interested students (myself included) who had, for the most part, already taken the Pascal course. The experiences with Python were written up in a Masters’ thesis by Michael Williams, the student who had converted the (Pascal-based, of course) teaching materials to Python.

Like Wirth, when Guido van Rossum designed Python he had teaching in mind; though knowing the criticisms of Pascal he also made it extensible so that it could be used as a “real” language. This extensibility was put to use in the Python experiment at Oxford, giving students the numpy module which they used mainly for its matrix datatype (an important facility in Physics).

What the report shows is that it’s possible to teach someone enough Python to get onto problems with numerical computation in a day; although clearly this is also true of Pascal and C. One interesting observation is the benefit of enforced layout (Python’s meaningful indentation) to both the students, who reported that they did not find it difficult to indent a program correctly; and to the teachers, who found that because students were coerced into laying out their code consistently, it was easier to read and understand the intention of code.

An interesting open question is whether that means enforced indentation leads to more efficient code reviews in general, not just in an expert/neophyte relationship. Many developers using languages that don’t enforce layout choose to add the enforcement themselves. Whether this is an issue at all when modern IDEs can lay out code automatically (assuming developers with enough experience of the IDE to use that feature) also needs answering.

The conclusion of this study was that Python is appropriate as a teaching language for Oxford’s Physics course, though clearly it was not adopted and C was favoured. Why was this? As this decision was made after the report was produced, it doesn’t say, and my own recollection is hazy. I recall the “not for real world use” lobby was involved, that it was also possible to teach C in the time involved, and that while many people wanted to teach Java this was overruled due to a desire to avoid OO. The spurned Java crowd preferred C for its Java-like syntax.

Wait, C?

The next part of this story wasn’t published, but I’ll cover it anyway just for completeness. The year after this Python study, the teaching course did an A/B test where half of the first year course was taught Python, and half C. Whatever conclusions were drawn from this test, C won out so either there was no significant difference in that type of course or the “real worldness” of C was thought to be greater than that of Python. I remember both being given as justifications, but don’t know whether either or both were retrofitted.

Whatever the cause, teaching C was sufficiently not bad that the course is still based on the language.

Going back to that Java decision. How good is Java as a teaching language?

Analyses of Student Programming Errors In Java Programming Courses

I’m going to use the results of this paper to argue that Java is not good as a teaching language.

Programming errors can be categorized as syntax, semantic and logic. A syntax error is an error due to incorrect grammar. Syntax errors are often detected by a program called a compiler, if the language is a compiled language such as Java. A semantic error is an error due to misuse of a programming concept, despite correct syntactic structure. Semantic errors are caught when the program code is compiled. A logic error occurs when the program does not solve the problem that the programmer meant for it to solve.

[Notice that the study is only investigating errors: it’s not completely obvious but “bugs” aren’t included. The author’s only reporting on things that are either compiler or runtime errors in Java-land, like typos and indices out of bounds.]

Categorization of errors of the present study into syntax, semantic, runtime and logic revealed that syntax errors made up 94.1%, semantic errors 4.7% and logic
errors 1.2%.

In the ideal world, a programming course teaches students the principles of programming and how to combine these to solve some computational problem. In learning these things, you expect people to make semantic and logic errors: they don’t yet know how these things work. Syntax errors, on the other hand, are the compiler’s way of saying “meh, you know what you meant but I couldn’t be bothered to work it out”, or “I require you to jump through some hoops and you didn’t”.

You don’t want syntax errors when you’re teaching programming. You want people to struggle with the problems, not the environment in which those problems are presented. Imagine failing a student because they pushed the door to the exam room when it was supposed to be pulled: that’s a syntax error. One of the roles of a demonstrator in a computing course is to be the magic compiler pixie, fixing syntax errors so the students can get back on track.

OK, so not Java. What else is out there?


The Oxford Physics investigation didn’t publish any results on the difficulty of teaching C. Thankfully, the Other Place is more forthcoming. Tim Love, author of Tim Love’s Cricket for the Dragon 32 and teacher of C++ to Engineering undergraduates blogged about difficulties encountered defining functions in C++. There’s no frequency information here, just representative problems. While most of the problems are semantic rather than syntactic, with some logic problems too, we can’t really compare these with the Java analysis above anyway.

The sorts of problems described in this blog post are largely the kind of problems you want, or at least don’t mind, people experiencing while you’re teaching them programming: as long as they get past them, and understand the difference between what they were trying and what eventually worked.

In that context, comments like this are worrying:

I left one such student to read the documentation for a few minutes, but when I returned to him he was none the wiser. The Arrays section of the doc might be sub-optimal, but it can’t be that bad – it’s much the same as last year’s.

So the teacher knows that the course might have problems, but not what they are or how to correct them despite seeing the failure modes in first person. This is not isolated: the Oxford course linked above has not changed substantially since the version I wrote (which added all the Finder & Xcode stuff).

So, we know something about the problems encountered by students of programming. Do we know anything about how they try to solve those problems?

An analysis of patterns of debugging among novice computer science students

We’ve already seen that students didn’t think to use the interactive interpreter feature of Python when the course handbook stopped telling them about it. In this paper, Ahmadzadeh et al modified the Java compiler to collect analytics about errors encountered by students on their course (the methodology looks very similar to the other Java paper, above). An interesting statistic noticed in §3.1, a statistical analysis of compiler errors:

It can be seen from this table that the error that is most common amongst all the subjects is failing to define a variable before it is used. This was almost always the highest frequency error when teaching a range of different concepts.

It’s possible that you could avoid 30-50% of the problems discovered in this study by using a language that doesn’t require explicit declaration of variables. Would the errors then be replaced by something else? Maybe.

In section 4, the authors note that there’s a distinction between being able to debug effectively and being able to program well: most people who are good at debugging are also good programmers, but a minority of good programmers are good at debugging. Of course this is measuring a class of neophytes so it’s possible that this gap eventually closes, but more work would need to be done to demonstrate or disprove that.

I notice that the students in this test are (at least, initially) fixing problems introduced into a program by someone else. Is that skill related to fixing problems in your own code? Might you be more frustrated if you think you’ve finished an assignment only to find there’s a problem you don’t understand in it? Does debugging someone else’s program support the educational goal? This paper suggests that the skills are in fact different, which is why “good” programmers can be “bad” debuggers: they understand programming, but not the problem solved by someone else’s code. They also suggest that “bad” programmers who do well at fixing bugs do it because they understand the aim of the program, and can reason about what the software should be doing. Perhaps being good at fixing bugs means more understanding of specifications than of code—traditionally the outlook of the tester (who is called on to find bugs but rarely to fix them).

Conclusions and Questions

There’s a surprising amount of data out there on the problems faced by students being taught programming—some of it leads directly to actionable conclusions, or at least testable hypotheses. Despite that, some courses look no different from courses I taught in 2004-2006 nor indeed any different from a course I took in 2000-2001.

Judicious selection of language could help students avoid some of the “syntactic” problems in programming, by choosing languages with less ceremony. Whether such a change would lead to students learning faster, enjoying the topic more, or just bumping up against a different set of syntax errors needs to be tested. But can we extrapolate from this? Are environments that are good for student programmers good for novices in general, including inexperienced professionals? Can this be taken further? Could we conclude that some languages waste time for all programmers, or that becoming expert in programming just means learning to cope with your environment’s idiosyncrasies?

And what should we make of this result that being good at programming and debugging do not go together? Should a programming course aim to develop both skills, or should specialisation be noticed and encouraged early? [Is there indeed a degree in software testing offered at any university?]

But, perhaps most urgently, why are so many different groups approaching this problem independently? Physics and Engineering academics are not experts at teaching computing, and as we’ve seen science code is not necessarily the best code. Could someone aggregate these results from various courses and produce the killer course in undergraduate computing?

posted by Graham at 10:57  

Monday, July 15, 2013

Elegant Object-oriented Software Design via Interactive, Evolutionary Computation

The abstract of this paper from the ArXiv had me concerned:

Design is fundamental to software development but can be demanding to perform. Thus to assist the software designer, evolutionary computing is being increasingly applied using machine-based, quantitative fitness functions to evolve software designs. However, in nature, elegance and symmetry play a crucial role in the reproductive fitness of various organisms. In addition, subjective evaluation has also been exploited in Interactive Evolutionary Computation (IEC). Therefore to investigate the role of elegance and symmetry in software design, four novel elegance measures are proposed based on the evenness of distribution of design elements. In controlled experiments in a dynamic interactive evolutionary computation environment, designers are presented with visualizations of object-oriented software designs, which they rank according to a subjective assessment of elegance. For three out of the four elegance measures proposed, it is found that a significant correlation exists between elegance values and reward elicited. These three elegance measures assess the evenness of distribution of (a) attributes and methods among classes, (b) external couples between classes, and (c) the ratio of attributes to methods. It is concluded that symmetrical elegance is in some way significant in software design, and that this can be exploited in dynamic, multi-objective interactive evolutionary computation to produce elegant software designs.

The “design” of a software system is, to me, a part of the social science aspect of software engineering. Does the design make it easy for me to work out how the software functions? Can I see what I need to change to fix some problem? If I don’t fix this problem, can you also see what you’d need to change? Does working with this design cause an emotional response?

With this mindset, it’s hard to understand how an objective metric of software design can be formulated. Without that understanding, it’s impossible to see any value in letting a software system design another software system that human developers are going to work on. In fact, it seems entirely back to front: the design is (to me) the part that needs a combination of experience, insight and serendipity to create. If a computer can then automatically fill some of the details in a way that saves time and reduces error, that would be useful. Doing it the other way around means human programmers become blue-collar subordinates to the (literal) software architect.

So, I didn’t exactly jump into the rest of this paper with an open mind, which I recognised was a problem I needed to deal with so I ploughed on anyway. I started by reading “A Survey on Search-based Software Design”, along with some of the other references, with a view to working out just what it was that these researchers are trying to automate. In the event, this post took a couple of weeks to write at my usual “whenever I get a chance” rate—there was a lot to understand.

What’s Going On?

The idea is that certain principles in object-oriented design can be assigned a quantitative value: a “score”, if you will. So you could score a design on how tightly-coupled the classes are, on how many responsibilities each class has, and on other features. You can also decide that a good design would aim to lower or increase particular scores; for example that looser coupling and fewer responsibilities are “better”. You could decide that some designs are just “stillborn”, and no matter how well they do on some metrics you’re never going to use them: a circular reference, or a class with no responsibilities, might instantly be discarded.

Now imagine deriving some initial design for your software, for example from a collection of use cases. (You may be wondering how, and that’s a good question: if your initial guess at the design is derived automatically from the use cases, then the use cases themselves need to be pretty precise, complete and unambiguous. In other words, they need to be written in a computer programming language.[*]) You score that design according to the criteria you defined, then make some “mutations” (which, in the case of evolutionary software design, means applying design patterns from a catalogue). The mutations that score better, you keep, combining them and mutating them further. Eventually you should have a collection of design candidates that are all much better than the initial guess.

[*]As a thought experiment, take the use-case diagram for a cinema booking system used as one of the inputs for this paper’s methods. Try designing a software system to implement these use cases; every time you have a question that isn’t answered by the diagram, make a note but _don’t assume an answer_. How many questions do you end up with? Are you happy using a design in which these questions are unanswered? My guess is you’ll be OK to leave some of them, designing the software to be flexible to different answers. But some will cause more problems unless they’re addressed.

Is This Useful?

I don’t feel like I got over the bias that I went into this post with: that the point of software design is to communicate something about that software among the various people who will be working on it. Computer-generated design is like computer-generated prose, when viewed from that perspective: uncannily close, but no substitute for the real thing.

What you could get from a technique like this are proposals for improvement on designs: indeed one branch (or clade, perhaps) of research in which genetic algorithms are applied to software design is in refactoring. One can imagine future UML tools (or IDEs) offering suggestions at the architecture level, just as current IDEs can offer suggestions for individual lines or methods.

That’s basically what this paper is driving at: the “interactive” part of Interactive Evolutionary Computation. Human participants created the first versions of the designs, and qualitatively evaluated the later iterations (which were both produced and also evaluated by the software). Ultimately, software designers were called upon to reward the “better” designs and to decide when to stop the iteration: i.e. they chose whether to accept the “suggestions” made by the evolutionary algorithm.

So is this technique a step on the way to having that tool? Looking at table 4, you might think that the software did create better designs than the humans in two thirds of cases. Such is the danger of bold typeface. Look again at the standard deviations. Unfortunately, discussing the results with a tame statistician, we couldn’t agree that the analysis in the paper shows the significant results the authors claim. As an example, it’s not clear that pairing any two metrics actually makes sense, or that just because one measurement comes out consistently lower overall it’s a better indicator of “elegance” than another (which might vary more between designs: something we’re not told here).

The authors are on clearer footing when evaluating the relationship between the rewards given and the metrics: ignoring the software algorithm completely, do people consistently think of some particular property of a software design as indicative of elegance? While they’ve only got 7 participants (who, assuming you know the group, can probably be de-anonymised based on the information presented…just saying), and it’s risky to draw general conclusions from such a small number of people[*], there are early indications here of consistency.

[*]particularly as they’re all in academia, and probably all in the same institution.

posted by Graham at 10:52  

Monday, July 15, 2013

On Scientific Computing

Or: Not everyone works the way you work

Currently doing the rounds on Twitter is a paper from the ArXiV called Best Practices for Scientific Computing—a paper with 13 authors and 6 pages,including a page-long collection of references. Here’s the abstract:

Scientists spend an increasing amount of time building and using software. However, most scientists are never taught how to do this efficiently. As a result, many are unaware of tools and practices that would allow them to write more reliable and maintainable code with less effort. We describe a set of best practices for scientific software development that have solid foundations in research and experience, and that improve scientists’ productivity and the reliability of their software.

Let me start with an anecdote. It’s 2004, and I’ve just started working as a systems manager in a university computing lab. My job is partly to maintain the computers in the lab, partly to teach programming and numerical computing to Physics undergraduates, and partly to write software that will assist in said teaching. As part of this work I started using version control, both for my source code and for some of the configuration files in /etc on the servers. A more experienced colleague saw me doing this and told me that I was just generating work for myself, that this wasn’t necessary for the small things I was maintaining.

Move on now to 2010, and I’m working in a big scientific facility in the UK. Using software and a lot of computers, we’ve got something that used to take an entire PhD to finish down to somewhere between one and eight hours. I’m on the software team, and yes we’re using version control to track changes to the software and to understand what version is released. Well, kindof, anyway. The “core” is in version control, but one of its main features is to provide a scripting environment and DSL in which scientists at the “lab benches”, if you will, can write up scripts that automate their experiments. These scripts are not (necessarily) version-controlled. Worse, the source code is deployed to the experimental stations so someone who discovers a bug in the core can fix it locally without the change being tracked in version control.

So, a group does an experiment at this facility, and produces some interesting results. You try to replicate this later, and you get different results. Could be software-related, right? All you need to do is to use the same software that the original group used…unfortunately, you can’t. It’s gone.

That’s an example of how scientists failing to use the tools from software development could be compromising their science. There’s a lot of snake oil in the software field, both from people wanting you to use their tools/methodologies because you’ll pay them for it, and from people who have decided that “their” way of working is correct and that any other way is incorrect. You need to be able to cut through all of that nonsense to find out how particular tools or techniques impact the actual work you’re trying to achieve. Current philosophy of science places a high value on reproducibility and auditing. Version control supports that, so it would be beneficial for programmers working in science to use version control.

But version control is only one of the 10 recommendation sections in this paper (another is about using the computer to record history, something that I’ll assume is covered well by the above discussion). That leaves eight other sections, which each contain numbered pronouncements about how scientists should write software.

Were you surprised?

I expect, if you write software in the commercial sector, you wouldn’t find any of their suggestions contentious: examples include naming things meaningfully, using a consistent convention for names and layout, don’t repeat yourself, and so on. I included this paper here to start discussion of an important point.

What goes on in commercial software engineering is not the be-all and end-all of software development. Scientific software has been around for as long as there have been computers to run software on, and indeed not only is some really old software still in use but the people who wrote it are still around and maintaining it. In the aforementioned university lab, one of my tasks was to help a professor who’d been using his home-grown FORTRAN FITS manipulation routines for at least two decades. Every system he’d used it on—most recently PowerPC, MIPS and Alpha workstations—had been big-endian and he didn’t know why it gave the wrong results when used on our new Intel Mac. His postdocs and PhD students were using the same code—in the same FORTRAN language, which he’d either taught them or given them a book on. And then of course when they moved to a different institution they’d take that code and that understanding of code with them.

I imagine that many professional programmers are not surprised by the validity of (m)any of the statements made in this paper, but by the necessity of stating them. No, not everyone uses version control, or thinks that agile is the best thing ever, or uses consistent naming conventions throughout a source file. Indeed in my experience of scientific programming, use of a symbolic debugger wasn’t If you consider all of these problems to be “solved” then you’re really only looking at a limited part of the world of software development. It’s not just scientific computing that doesn’t match that worldview; what about all the people out there for whom programming is a bunch of Excel formulae and maybe the odd VBA macro pasted from a website?

In both commercial and scientific software development, understanding and behaviour is spread by sharing knowledge from masters to apprentices. I think that the reason there’s such a big difference in practice could be due to the longer generations in scientific software. That 20+-year-old FITS code still works, why change it? And those 20+-year-old practices that created the FITS code, well they still work too, don’t they?

Which of these things actually matters?

Based on my own experience I’d assert that all of them are important things for scientific programmers to know about. I’ve argued, hopefully convincingly, that version control has an important part to play in the scientific process: numerical analysis is a key part of many experiments and like the rest of the method it should be available for inspection and repetition. Science is also a collaborative activity, so it makes sense that some of the recommendations would be about collaboration: document the purpose of the code instead of its mechanics, write programs for people.

Could I justify those assertions with figures? Probably not. Is that important? Well, actually it probably is. Of the researchers I’ve worked with (bear in mind this has always been in Physics), even many who are heavily invested in computational methods see programming (rightly) as a means to an end, and aren’t likely to try new-to-them techniques in programming just for the sake of programming. Despite any rational economic benefit, they’d rather stick with what they know and focus on getting new results without any surprises.

If you want to say “it’s better to work this way” or “you’ll get results quicker like this”, to a bunch of physicists, you have to show them data to prove it. A paper like the one I’m discussing here will likely be read, if it:

  1. gets published
  2. said publication happens in a relevant journal
  3. said publication is picked up and circulated in enough news sources that researchers who don’t read the publishing journal get wind

On the other hand, it’s likely that the article’s tone will ensure that it only preaches to the converted. Nothing in the paper says “this is actually better”, just “professional programmers do these things”. Exercises like Software Carpentry are likely to only appeal to people who already have an interest in bettering their own programming abilities. As I said, most researchers I know don’t: they want to publish, and programming is a necessary—albeit complex—tool helping them to achieve that.

Why is this suddenly an issue?

It isn’t. A very quick search for errors in scientific computing yielded papers published across the last two decades, and I could probably find more. The abstracts for these (I did say it was a very quick search) include some pining for the use of skills from software engineering, or a closer focus by software engineering researchers on scientific computing projects.

What can be done about it?

That’s a very good question. If we knew what to do to improve the quality of any software production effort, there’d be a lot more good software in the world :). If the techniques from commercial software really would help make scientific software better, why wait for the scientists to apply them? Plenty of scientific software is open source, so in the case of things like analysis tools and automatic tests, sufficiently motivated individuals could just apply those things then demonstrate to the project maintainers how much of a difference they’ve made. Sure, there will be problems: I once worked on some software that could only be successfully executed if there was a particle accelerator connected to your workstation. But the first thing I did was to make a virtual particle accelerator – demonstrating how much easier it was to make progress if you could do it away from the experiment.

This brings me onto another option: scientific computing teams can employ commercial developers. I’ve seen it happen, I’ve seen it work and I’ve seen it fail. The ways in which it work include sharing of knowledge from both disciplines, discussing and improving practices. The ways in which it fails come down to frustration on both sides: scientific programmers feel that refactoring is change for change’s sake, perhaps, and software engineers think that not using their favourite practices is the realm of cowboys. That means that for a cross-discipline software team to work, it needs good leadership: the team needs to be designed to appreciate the different skills and viewpoints brought by the different members. And now we’ve gone fully out of the realm of science into management techniques.

posted by Graham at 10:49  

Monday, July 15, 2013

Representativeness in Software Engineering Research

The first paragraph describes the context of this post in relation to the blog on which it originally appeared, not

For this post, I wanted to go a little bit meta. One focus of this blog will be on whether results from academic software engineering are applicable to the work I do as a commercial software developer, so it was hard to pass up this Microsoft Research paper on representativeness of research.

In a nutshell, the problem is this: imagine that you read some study that shows, for example, that schedule slippage on a software project is significantly lessened if developers are given two digestive biscuits and a cup of tea at 4pm on working days. The study examined the breaktime habits of developers on 500 open source projects. This sounds quite convincing. If this thing with the tea and biscuits is true across five hundred projects, it must be applicable to my project, right?

That doesn’t follow. There are many reasons why it might not follow: the study may be biased. The analysis may be wrong. The biscuit thing may be significant but not the root cause of success. The authors may have selected projects that would demonstrate the biscuit outcome. Projects that had initially signed up but got delayed might have dropped out. This paper evaluates one cause of bias: the projects used in the study aren’t representative of the project you’re doing.

It’s a fallacy to assume that just because a study has a large sample size, its results can be generalised to the population. This only applies in the case that the sample represents an even slice of the population. Imagine a world in which all software projects are either written in Java or LISP. Now it doesn’t matter whether I select 10 projects or 10,000 projects: a sample of LISP practices will not necessarily tell us anything about how to conduct a Java project.

Conversely a study that investigates both Java and LISP projects can—in this restricted universe, and with the usual “all other things being equal” caveat—tell us something generally about software projects independent of language. However, choice of language is only one dimension in which software can be measured: the size, activity, number of developers, licence and other factors can all be relevant. Therefore the possible phase space of important factors can be multidimensional.

In this paper the authors develop a framework, based on work in medicine and other fields, for measuring and maximising representativeness of a sample by appropriate selection of projects along the dimensions of the problem space. They apply the framework to recent research.

What they discovered, tabulated in Table II of the paper, is that while a very small, carefully-selected sample can be surprisingly representative (50 out of ~20k projects represented ~15% of their problem space), the ~200 projects they could find analysed in recent research only scored around 9% on their representativeness metric. However in certain dimensions the studies were highly representative, many covering 100% of the phase space in specific dimensions.


A fact that jumped out at me, because of the field I work in, is that there are 245 Objective-C projects in the universe studied by this paper (the projects indexed on Ohloh) and that not one of these is covered by any of the studies they analysed. That could mean that my own back yard is ripe for the picking: that there are interesting results to be determined by analysing Objective-C projects and comparing those results with received wisdom.

In the discussion section, the authors point out that just because a study is not general, does not mean it is not useful. You may not be able to generalise a result gleaned from analysing (say) Java developer tools to all software development, but if what you’re interested in is Java developer tools then that is not a problem.

What this paper gives us, then, is not necessarily a tool that commercial developers can run out and use. It gives us some important quantitative context on evaluating the research that we do read. And, should we want to analyse our own work and investigate hypotheses about factors affecting our projects, it gives us a framework to understand just how representative those analyses would be.

posted by Graham at 10:47  

Monday, July 15, 2013

Garbage-collected Objective-C

When was a garbage collector added to Objective-C? If you follow Apple’s work with the language, you might be inclined to believe that it was in 2008 when AutoZone was added as part of Objective-C 2.0 (the AutoZone collector has since been deprecated by Apple, and I’m not sure whether anyone else ever adopted it).

With a slightly wider knowledge of the language’s history, you can push this date back a bit. The GNUstep project—a Free Software reimplementation of Apple’s (formerly NeXT’s) APIs—has been using the Boehm–Demers–Weiser collector for a while. How long? I can’t tell exactly, but a keyword search in the project’s version control logs makes me think that most of the work to support it was done by one person in mid-2002:

r13976 | nico | 2002-06-26 15:34:16 +0100 (Wed, 26 Jun 2002) | 3 lines

Do not add -lobjc_gc -lgc flags when compiling with gc=yes – should now
be added automatically by gnustep-make

r13971 | nico | 2002-06-25 18:28:56 +0100 (Tue, 25 Jun 2002) | 2 lines

Tidyup for gc=yes with old compilers

r13970 | nico | 2002-06-25 18:23:05 +0100 (Tue, 25 Jun 2002) | 2 lines

Tidied code to compile with gc=yes and older compilers

r13969 | nico | 2002-06-25 13:36:11 +0100 (Tue, 25 Jun 2002) | 3 lines

Tidied some indentation; a couple of insignificant changes to have it compile
under gc

r13968 | nico | 2002-06-25 13:15:04 +0100 (Tue, 25 Jun 2002) | 2 lines

Tidied code which wouldn’t compile with gc=yes and gcc < 3.x

r13967 | nico | 2002-06-25 13:13:19 +0100 (Tue, 25 Jun 2002) | 2 lines

Tidied code which was not compiling with the garbage collector

r13966 | nico | 2002-06-25 13:12:17 +0100 (Tue, 25 Jun 2002) | 2 lines

Tidied code which was not compiling with gc=yes

That was, until fairly recently, the earliest example I knew about. Then I discovered a conference talk by Paulo Ferreira:

Reclaiming storage in an object oriented platform supporting extended C++ and Objective-C applications

This is a paper presented at “1991 International Workshop on Object Orientation in Operating Systems”. 1991. That is—obviously—11 years before GNUstep’s GC work and 17 years before Apple released AutoZone.


The context in which this work was being done is a platform called Comandos. I’d never heard of that before—and I thought I knew Objective-C!

Judging from the report linked above, Comandos is a platform for distributed and parallel object-oriented software, based on UNIX but supporting multiple variants. The fact that it was created in 1986 means that both the languages supported—Objective-C and C++—were new at the time. Indeed the project was contemporary with the development of NeXTSTEP, which was publicly released to developers in 1988.

The 1994 summary report doesn’t mention Objective-C: just C++, Eiffel and a bespoke language called Guide. It’s possible that the platform supported ObjC simply because they used gcc which picked up ObjC support during the life of Comandos; however this seems unlikely as there would be significant work in making Objective-C objects work with their platform’s distributed messaging interface and persistence subsystem.

Why ObjC should be one of two languages mentioned (along with C++) in the 1991 paper on garbage collection, but zero of three mentioned (C++, Eiffel, Guide) in 1994 will have to remain a mystery for now. Looking into the references for Ferreira’s paper, I can find one mention of Objective-C as the inspiration for their own, custom C-based message dispatch system, but no indication that they actually used Objective-C.

The Garbage Collector

I’m not really an expert at garbage collectors. In fact, I have no idea what I’m doing. I appreciate them when they’re around, and leak or crash things occasionally when they’re not.

To my uneducated eye, the description of the Ferreira 1991 garbage collector and Apple’s description of their collector (no link I’m afraid, it was session 940 at WWDC 2008) look quite different. AutoZone is conservative (like B-W-D) and only works on Objective-C objects. Ferreira’s collector operates, like B-W-D, on any memory block including new C++ instances and C heap allocations. Apple’s collector is supposed to avoid blocking wherever it can, a constraint not mentioned in the Ferreira paper.

All of Comandos, GNUstep and Cocoa (Apple’s Objective-C framework) have systems for distributed objects that complicate collection: does some remote process have a handle on memory in my address space? The proxy system used by Cocoa and GNUstep make it easy to answer this question. Comandos used a different technique, where objects local to a process were “volatile” and objects shared between processes were “persistent”. Persistent objects were subject to a different lifecycle management process, so the Ferreira GC didn’t interact with them.

As an aside, Apple’s garbage collector also needed to provide a “mixed mode”—support for code that could be loaded into either a garbage-collected or manually managed process.


Memory management is hard. Making programmers do it themselves leads to all sorts of problems. Doing it automatically is also hard, and many different approaches have been tried over the last few decades. Interestingly, Apple has (for the moment) settled on a “none of the above” approach, using a compiler-inserted reference counting system based on the manual ownership tracking previously implemented by the frameworks.

What interests me most about this paper on Objective-C garbage collection is not so much its technical content (which it’s actually rather light on, containing only conversational overviews of the algorithms and no information about results), but the fact that it existed at all and I, as someone who considers himself an experienced Objective-C programmer, did not know anything about it or its project.

That’s why I started this blog [Ed: referring to the blog these posts are imported from] by discussing it. A necessary prerequisite to deciding whether the literature has something useful to tell us is knowing about its existence. I’m really surprised that it took so long for me to find out about something that’s almost directly related to my everyday work. Mind you, maybe I shouldn’t feel too bad: the author of AutoZone told me he hadn’t heard of it, either.

posted by Graham at 10:46  
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