Archive for the ‘Computer science’ Category.

Multirequirements (new paper)

 

As part of a Festschrift volume for Martin Glinz of the university of Zurich I wrote a paper [1] describing a general approach to requirements that I have been practicing and developing for a while, and presented in a couple of talks. The basic idea is to rely on object-oriented techniques, including contracts for the semantics, and to weave several levels of discourse: natural-language, formal and graphical.

Reference

[1] Bertrand Meyer: Multirequirements, to appear in Martin Glinz Festschrift, eds. Anne Koziolek and Norbert Scheyff, 2013, available here.

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ESEC/FSE 2013: 18-26 August, Saint Petersburg, Russia

The European Software Engineering Conference takes place every two years in connection with the ACM Foundations of Software Engineering symposium (which in even years is in the US). The next ESEC/FSE  will be held for the first time in Russia, where it will be the first major international software engineering conference ever. It comes at a time when the Russian software industry is ever more present through products and services offered worldwide. See the conference site here. The main conference will be held 21-23 August 2013, with associated events before and after so that the full dates are August 18 to 26. (I am the general chair.)

Other than ICSE, ESEC/FSE is second to none in the quality of the program. We already have four outstanding keynote speakers:  Georges Gonthier from Microsoft Research, Paola Inverardi from L’Aquila in Italy, David Notkin from U. of Washington (in whose honor a symposium will be held as an associated event of ESEC/FSE, chaired by Michael Ernst), and Moshe Vardi of Rice and of course Communications of the ACM.

Saint Petersburg is one of the most beautiful cities in the world, strewn with gilded palaces, canals, world-class museums (not just the Hermitage), and everywhere mementos of the great poets, novelists, musicians and scientists who built up its fame.

Hosted by ITMO National Research University, the conference will be held in the magnificent building of the Razumovsky Palace on the banks of the Moika river; see here.

The Call for Papers has a deadline of March 1st, so there is still plenty of time to polish your best paper and send it to ESEC/FSE. There is also still time to propose worskhops and other associated events. ESEC/FSE will be a memorable moment for the community and we hope to see many of the readers there.

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Negative variables and the essence of object-oriented programming (new paper)

In modeling object-oriented programs, for purposes of verification (proofs) or merely for a better understanding, we are faced with the unique “general relativity” property of OO programming: all the operations you write (excluding non-OO mechanisms such as static functions) are expressed relative to a “current object” which changes repeatedly during execution. More precisely at the start of a call x.r (…) and for the duration of that call the current object changes to whatever x denotes — but to determine that object we must again interpret x in the context of the previous current object. This raises a challenge for reasoning about programs; for example in a routine the notation f.some_reference, if f is a formal argument, refers to objects in the context of the calling object, and we cannot apply standard rules of substitution as in the non-OO style of handling calls.

In earlier work [1, 2] initially motivated by the development of the Alias Calculus, I introduced a notion of negative variable to deal with this issue. During the execution of a call x.r (…) the negation of x , written x’, represents a back pointer to the calling object; negative variables are characterized by axiomatic properties such as x.x’= Current and x’.(old x)= Current. Alexander Kogtenkov has implemented these ideas and refined them.

Negative variable as back pointer

In a recent paper under submission [3], we review the concepts and applications of negative variables.

References

[1] Bertrand Meyer: Steps Towards a Theory and Calculus of Aliasing, in International Journal of Software and Informatics, 2011, available here.

[2] Bertrand Meyer: Towards a Calculus of Object Programs, in Patterns, Programming and Everything, Judith Bishop Festschrift, eds. Karin Breitman and Nigel Horspool, Springer-Verlag, 2012, pages 91-128, available here.

[3] Bertrand Meyer and Alexander Kogtenkov: Negative Variables and the Essence of Object-Oriented Programming, submitted for publication, 2012. [Updated 13 January 2014: I have removed the link to the draft mentioned in this post since it is now superseded by the new version, soon to be published, and available here.]

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Loop invariants: the musical

 

Actually it is not a musical but an extensive survey. I have long been fascinated by the notion of loop invariant, which describes the essence of a loop. Considering a loop without its invariant is like conducting an orchestra without a score.

In this submitted survey paper written with Sergey Velder and Carlo Furia [1], we study loop invariants in depth and describe many algorithms from diverse areas of computer science through their invariants. For simplicity and clarity, the specification technique uses the Domain Theory technique described in an earlier article on this blog [2] (see also [3]). The invariants were verified mechanically using Boogie, a sign of how much more realistic verification technology has become in recent years.

The survey was a major effort (we worked on it for a year and a half); it is not perfect but we hope it will prove useful in the understanding, teaching and verification of important algorithms.

Here is the article’s abstract:

At the heart of every loop, and hence of all significant algorithms, lies a loop invariant: a property ensured by the initialization and maintained by every iteration so that, when combined with the exit condition, it yields the loop’s final effect. Identifying the invariant of every loop is not only a required step for software verification, but also a key requirement for understanding the loop and the program to which it belongs. The systematic study of loop invariants of important algorithms can, as a consequence, yield insights into the nature of software.

We performed this study over a wide range of fundamental algorithms from diverse areas of computer science. We analyze the patterns according to which invariants are derived from postconditions, propose a classification of invariants according to these patterns, and present its application to the algorithms reviewed. The discussion also shows the need for high-level specification and invariants based on “domain theory”. The included invariants and the corresponding algorithms have been mechanically verified using an automatic program prover. Along with the classification and applications, the conclusions include suggestions for automatic invariant inference and general techniques for model-based specification.

 

References

[1] Carlo Furia, Bertrand Meyer and Sergey Velder: Loop invariants: analysis, classification, and examples, submitted for publication, December 2012, draft available here.

[2] Domain Theory: the Forgotten Step in Program Verification, article from this blog, 11 April 2012, available here.

[3] Domain Theory: Precedents, article from this blog, 11 April 2012, available here

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Why so many features?

 

It is a frequent complaint that production software contains too many features: “I use only  maybe 5% of Microsoft Word!“, with the implication that the other 95% are useless, and apparently without the consideration that maybe someone else needs them; how do you know that what is good enough for you is good enough for everyone?

The agile literature frequently makes this complaint against “software bloat“, and has turned it into a principle: build minimal software.

Is software really bloated? Rather than trying to answer this question it is useful to analyze where features come from. In my experience there are three sources: internal ideas; suggestions from the field; needs of key customers.

1. Internal ideas

A software system is always devised by a person or group, who have their own views of what it should offer. Many of the more interesting features come from these inventors and developers, not from the market. A competent group does not wait for users or prospects to propose features, but comes up with its own suggestions all the time.

This is usually the source of the most innovative ideas. Major breakthroughs do not arise from collecting customer wishes but from imagining a new product that starts from a new basis and proposing it to the market without waiting for the market to request it.

2. Suggestions from the field

Customers’ and prospects’ wishes do have a crucial role, especially for improvements to an existing product. A good marketing department will serve as the relay between the field’s wishes and the development team. Many such suggestions are of the “Check that box!” kind: customers and particularly prospects look at the competition and want to make sure that your product does everything that the others do. These suggestions push towards me-too features; they are necessary to keep up with the times, but must be balanced with suggestions from the other two sources, since if they were the only inspiration they would lead to a product that has the same functionality as everyone else’s, only delivered a few months later, not the best recipe for success.

3. Key customers

Every company has its key customers, those who give you so much business that you have to listen to them very carefully. If it’s Boeing calling, you pay more attention than to an unknown individual who has just acquired a copy. I suspect that many of the supposedly strange features, of products the ones that trigger “why would anyone ever need this?” reactions, simply come from a large customer who, at some point in the product’s history, asked for a really, truly, absolutely indispensable facility. And who are we — this includes Microsoft and Adobe and just about everyone else — to say that it is not required or not important?

It is easy to complain about software bloat, and examples of needlessly complex system abound. But your bloat may be my lifeline, and what I dismiss as superfluous may for you be essential. To paraphrase a comment by Ichbiah, the designer of Ada, small systems solve small problems. Outside of academic prototypes it is inevitable that  a successful software system will grow in complexity if it is to address the variety of users’ needs and circumstances. What matters is not size but consistency: maintaining a well-defined architecture that can sustain that growth without imperiling the system’s fundamental solidity and elegance.

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Computer scientist gallery, updated

After several months of inaction I have updated my “Gallery of Computer Scientists” [1]. It benefits from many recent meetings where the density per square meter of Turing award winners and other brilliant computer scientists was hard to beat, most notably the two extraordinary Turing centenary celebrations  — the ACM event in San Francisco, and Andrei Voronkov’s Manchester conference — and our own LASER summer school of last September which brought together the Gotha of programming language designers. And I still have not included everyone.

I do not know of any photographic collection anywhere that compares to this archive in either quantity or quality of the scientists pictured. My only regret is that I did not start earlier (I missed several giants of the field, to soon departed, such as Dijkstra, Dahl and Nygaard, even though I had many occasions to photograph them). The truth is that I had got impatient with photography and started again only when digital cameras became widely available.

The quality of the pictures themselves varies. It is definitely higher in recent ones: I may have become a better photographer, but it does not hurt that I have more sophisticated cameras than the rudimentary point-and-shoot I was using at the beginning. I should also improve the layout of the page, although I hope you will appreciate the ability to move the cursor around to get large pictures without having to click and go to different pages.

I started this collection because it occurred to me that for a number of reasons I am, more than almost anyone I know, in the position of meeting outstanding people from many different sub-communities of software engineering and the rest of computer science: from program verification, semantics, languages, algorithms to architecture, management, empirical software engineering and many others. I realized that it would be unconscionable not to take advantage of these opportunities and do for computer scientists what Paul Halmos did for mathematicians [2].

Some of the people pictured are more famous than others, but all do interesting work. There is no profound logic to the choice of subjects; it obviously depends on the chances I get, but also on the time I can spend afterwards to sort through the shots (this is not a full-time job). So if you know I took a picture of you and you do not see it on the page, do not take offense: it may be a matter of time, or I may need another opportunity and a better shot.

All the pictures are by me. They are of different styles; I try to capture a personality and a mood. Many shots show a computer scientist in flagrante delicto: doing computer science, as when giving a talk, or engaging in a design discussion around a laptop. Some were taken in more informal settings, such as a long winter walk in the woods. A few reveal some humorous or fancy aspect of the subject’s personality. None has any context or explanation; I will not tell you, for example, why Tony Hoare had, on that day, two hats and two umbrellas. I think it is more fun to let you imagine.

Pictures are only pictures and what matters is the work that all these great people do. Still, I hope you will enjoy seeing what they look like.

References

[1] Bertrand Meyer’s Gallery of Computer Scientists, available here.
[2] Paul Halmos’s photo collection, see here.

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A fundamental duality of software engineering

A couple of weeks ago I proposed a small quiz. (I also stated that the answer would come “on Wednesday” — please understand any such promise as “whenever I find the time”. Sorry.) Here is the answer.

The quiz was:

I have a function:

  • For 0 it yields 0.
  • For 1 it yields 1.
  • For 2 it yields 4.
  • For 3 it yields 9.
  • For 4 it yields 16.

What is the value for 5?

Shortly thereafter I added a hint: the value for 5 is 25, and changed the question to: “What is the value for 6?”. For good measure we can also ask about the value for 1000. Now compare your answer to  what follows.

A good answer for the value at 6 is: 34 . The function in this case is -10 + 5 x + |2 x – 3| + |2 x -7|. It matches the values for the given inputs.

Linear, small values

 

 

 

 

 

 

 

 

 

The value for 1000 is 8980:

Linear function, full range

 

 

 

 

 

 

 

 

 

Another good answer at position 6 is 35.6. It comes up if we assume the function is over reals rather than integers; then a possible formula, which correlates very well (R-square of 0.9997) with the values at the given inputs, is:

869.42645566111 (1 – 0.4325853145802 e-.0467615868913719  (x – 17.7342512233011))2.3116827277657443

Exponential function, initial range

 

 

 

 

 

 

 

 

 

 

with a quite different asymptotic behavior, giving the value 869.4 at position 1000:

Exponential, full range

 

 

 

 

 

 

 

 

 

 

Some readers might have thought of another possibility, the square function x2, which again matches all the given values:

Square function, initial range

 

 

 

 

 

 

 

 

 

 

So which of these answers is right? Each is as good as the others, and as bad. There is in particular no reason to believe that the values given in the quiz’s statement suggest the square function. Any function that fits the given values, exactly (if we stick to integers) or approximately (with reals as simulated on a computer) is an equally worthy candidate. Six inputs, or six thousand, do not resolve the question. At best they are hints.

This difference between a hint and a solution is at the core of software engineering. It is, for example, the difference between a test and a specification. A test tells us that the program works for some values; as Dijkstra famously pointed out, and anyone who has developed a serious program has experienced, it does not tell us that it will work for others. The more successful tests, the more hints; but they are still only hints. I have always wondered whether Dijkstra was explicitly thinking of the Popperian notion of falsifiability: no number of experiments will prove a physical theory (although a careful experiment may boost the confidence in the theory, especially if competing theories fail to explain it, as the famous Eddington expedition did for relativity in 1919 [1]); but a single experiment can disprove a theory. Similarly, being told that our function’s value at 6 is 34 disqualifies the square function and the last one (the exponential), but does not guarantee that the first function (the linear combination) is the solution.

The specification-testing duality is the extension to computer science of the basic duality of logic. It starts with the elementary boolean operators: to prove a or b it suffices to establish a or to establish b; and to disprove a and b it suffices to show that a does not hold or to show that b does not hold. The other way round, to disprove a or b we have to show that a does not hold and to show that b does not hold; to prove that a and b holds, we have to show that a holds and to show that b holds.

Predicate calculus generalizes or to , “there exists”, and and to , “for all”. To prove ∃ x | p (x) (there is an x of which p holds) it suffices to find one value a such that p (a); let’s be pretentious and say we have “skolemized” x. To disprove∀ x | p (x) (p holds of all x) it suffices to find one value for which p does not hold.

In software engineering the corresponding duality is between proofs and tests, or (equivalently) specifications and use cases. A specification is like a “for all”: it tells us what must happen for all envisioned inputs. A test is like a “there exists”: it tells us what happens for a particular input and hence, as in predicate calculus, it is interesting as a disproof mechanism:

  • A successful test brings little information (like learning the value for 5 when trying to figure out what a function is, or finding one true value in trying to prove a or a false value in trying to prove a ).
  • An unsuccessful test brings us decisive information (like a false value for a ): the program is definitely not correct. It skolemizes incorrectness.

A proof, for its part, brings the discussion to an end when it is successful. In practice, testing may still be useful in this case, but only testing that addresses issues not covered by the proof:

  • Correctness of the compiler and platform, if not themselves proved correct.
  • Correctness the proof tools themselves, since most practical proofs require software support.
  • Aspects not covered by the specification such as, typically, performance and usability.

But for the properties it does cover the proof is final.

It is as foolish, then, to use tests in lieu of specifications as it would be to ignore the limitations of a proof. Agile approaches have caused much confusion here; as often happens in the agile literature [2], the powerful insight is mixed up with harmful advice. The insight, which has significantly improved the practice of software development, is that the regression test suite is a key asset of a project and that tests should be run throughout. The bad advice is to ditch upfront requirements and specifications in favor of tests. The property that tests lack and specifications possess is generality. A test is an instance; a thousand tests can never be more than a thousand instances. As I pointed out in a short note in EiffelWorld (the precursor to this blog) a few years ago [3], the relationship is not symmetric: one can generate tests from a specification, but not the other way around.

The same relationship holds between use cases and requirements. It is stunning to see how many people think that use cases (scenarios) are a form of requirements. As requirements they are as useless as one or ten values are to defining a function. Use cases are a way to complement the requirements by describing the system’s behavior in selected important cases. A kind of reality check, to ensure that whatever abstract aims have been defined for the system it still covers the cases known to be of immediate interest. But to rely on use cases as requirements means that you will get a system that will satisfy the use cases — and possibly little else.

When I use systems designed in recent years, in particular Web-based systems, I often find myself in a stranglehold: I am stuck with the cases that the specifiers thought of. Maybe it’s me, but my needs tend, somehow, to fall outside of these cases. Actually it is not just me. Not long ago, I was sitting close to a small-business owner who was trying to find her way through an insurance site. Clearly the site had a planned execution path for employees, and another for administrators. Problem: she was both an employee and the administrator. I do not know how the session ended, but it was a clear case of misdesign: a system built in terms of standard scenarios. Good specification performs an extra step of abstraction (for example using object-oriented techniques and contracts, but this is for another article). Skipping this step means forsaking the principal responsibility of the requirements phase: to generalize from an analysis of the behavior in known cases to a definition of the desired behaviors in all relevant cases.

Once more, as everywhere else in computer science [4], abstraction is the key to solid results that stand the test of time. Definitely better than judging a book by its cover, inferring a function by its first few values, verifying a program by its tests, or specifying a system by its use cases.

References

[1] See e.g. a blog article: Einstein and Eddington, here.

[2] Bertrand Meyer: Agile! The Good, the Hype and the Ugly, 2013, to appear.

[3] Bertrand Meyer: Test or spec? Test and spec? Test from spec!, EiffelWorld column, 2004 available here.

[4] Jeff Kramer: Is Abstraction the Key to Computer Science?, in Communications of the ACM, vol. 50, no. 4, April 2007, pages 36-42,  available from CiteSeer here

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Precedent

Alexander Kogtenkov pointed out to me that precursor work to my papers on the Alias Calculus [1] [2] had been published by John Whaley and Martin Rinard [3]. There are some significant differences; in particular my rules are simpler, and their work is not explicitly presented as a calculus. But many of the basic ideas are the same. The reason I did not cite that paper is simply that I was not aware of it; I am happy to correct the omission.

References

[1] Bertrand Meyer: Towards a Theory and Calculus of Aliasing, in Journal of Object Technology, vol. 9, no. 2, March-April 2010, pages 37-74, available here (superseded by [2])
[2] Bertrand Meyer: Steps Towards a Theory and Calculus of Aliasing, in International Journal of Software and Informatics, 2011, available here (revised and improved version of [1].)
[3] John Whaley and Martin Rinard: Compositional Pointer and Escape Analysis for Java Programs, in POPL 1999, available here.

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Quiz (1): What is this function?

For various reasons there have been no articles in recent weeks; now we are restarting on a regular basis!

A the first topic for this new season, here is a little quiz. I have a function:

  • For 0 it yields 0.
  • For 1 it yields 1.
  • For 2 it yields 4.
  • For 3 it yields 9.
  • For 4 it yields 16.

The question: what is the value for 5?

Answer next Wednesday (at least it will be Wednesday in some time zone).

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The manhood test

 

I came across an obscure and surprisingly interesting article by Cliff Jones [1], about the history of rely-guarantee but with the following extract:

It was perhaps not fully appreciated at the time of [Hoare’s 1969 axiomatic semantics paper] that the roles of pre and post conditions differ in that a pre condition gives permission to a developer to ignore certain possibilities; the onus is on a user to prove that a component will not be initiated in a state that does not satisfy its pre condition. In contrast a post condition is an obligation on the code that is created according to the specification. This Deontic view carries over [to rely-guarantee reasoning].

I use words more proletarian than “deontic”, but this view is exactly what stands behind the concepts of Design by Contract and has been clearly emphasized in all Eiffel literature ever since the first edition of OOSC. It remains, however, misunderstood outside of the Eiffel community; many people confuse Design by Contract with its opposite, defensive programming. The criterion is simple: if you have a precondition to a routine, are you willing entirely to forsake the corresponding checks (conditionals, exceptions…) in the routine body? If not, you may be using the word “contract” as a marketing device, but that’s all. The courage to remove the checks is the true test of adulthood.

The application of Microsoft’s “Code Contracts” mechanism to the .NET libraries fails that test: a precondition may say “buffer not full” or “insertions allowed”, but the code still checks the condition and triggers an exception. The excuse I have heard is that one cannot trust those unwashed developers. But the methodological discipline is lost. Now let me repeat this using clearer terminology: it’s not deontic.

Reference

[1] Cliff Jones: The role of auxiliary variables in the formal development of concurrent programs, in Reflections on the work of C. A. R. Hoare, eds. Jones, Roscoe and Wood, Springer Lecture Notes in Computer Science,  2009, technical report version available here.

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EIS: Putting into Practice the Single Model Principle

Since release 6.2 (November 2008) EiffelStudio has included the EIS system, Eiffel Information System. It has been regularly revised, and significantly improved for the recent 7.1 release.

For us EIS is a key contribution with far-reaching software engineering implications, but many users seem unaware of it, perhaps because we have not been explicit enough about why we think it is important. We would love to have more people try it and give us their feedback. (Please make sure to use the 7.1 version.) Information on EIS can be found in the documentation [1] and also in a blog entry by Tao Feng [2].

EIS connects an Eiffel system with external documents in arbitrary formats; examples of formats currently supported are Microsoft Word and PDF, but you can easily add protocols. Such a connection links an element of the Eiffel text, such as a feature, with an element of the external document, such as a paragraph. Then clicking the Eiffel element in EiffelStudio will open the document at the corresponding place in the external tool (Word, Acrobat etc.); this is the EIS “outgoing” mechanism. Conversely the external element has a back link: clicking in the external tool will open EiffelStudio at the right place; this is the EIS “incoming” mechanism.

For the outgoing mechanism, the link will appear as part of a note clause (with attributes filled by default, you need only edit the URL and any option that you wish to change):

EIS incoming note

The fundamental idea behind EIS is to support the seamless form of software development promoted and permitted by Eiffel, where all phases of a project’s lifecycle are closely linked and the code provides the ultimate reference. Since other documents are often involved, in particular a requirements document (SRS, Software Requirements Specification), it is essential to record their precise associations with elements of the software text. For example a paragraph in the SRS could state that “Whenever the tank temperature reaches 50 degrees, the valve shall be closed”. In the software text, there will be some feature, for example monitor_temperature in the class TANK, reflecting this requirement. The two elements should be linked, in particular to ensure that dependencies appear clearly and that any change in either the requirements or the code triggers the corresponding update to the other side. This is what EIS provides.

We envision further tools to track dependencies and in particular to warn users if an element of a connection (e.g. requirement or code) changes, alerting them to the need to check the linked elements on the other side. One of the key goals here is traceability: effective project management, particular during the evolution of a system, requires that all dependencies between the project’s artifact are properly recorded so that it is possible to find out the consequences of any change, proposed or carried out.

The general approach reflects the essential nature of Eiffel development, with its Single Product Principle linking all elements of a software system and minimizing, rather than exaggerating, the inevitable differences of levels of abstraction between requirements, design, code, test plans, test logs, schedules and all the other products of a software project. The core problem of software engineering is change: if we use different tools and notations at each step, and keep the documents separate, we constantly run the risk of divergence between intent and reality. Eiffel by itself offers a good part of the solution by providing a single method (with all its principles, from Design by Contract to open-closed etc.), a single notation (the Eiffel language itself) and a single integrated set of tools (the EiffelStudio IDE) supporting the entire lifecycle; the language, in particular is meant for requirements and design as much as for implementation. The graphical forms (BON and UML, as produced by the Diagram Tool of EiffelStudio in a roundtrip style, i.e. changes to the diagram immediately generate code and changes to the code are reflected in the diagram) directly support these ideas. Of course documents in other formalisms, for example SRS, remain necessary for human consumption; but they should be closely linked to the core project asset, the Eiffel code; hence the need for EIS and its connection mechanisms.

This approach, as I have often noted when presenting it in public, is hard to convey to people steeped in the mindset of the past (UML as separate from code, model-driven development) which magnify the differences between software levels, hence introducing the risk of divergence and making change painful. The Eiffel approach is innovative enough to cause incomprehension or even rejection. (“What, you are not model-driven, but everyone says model-driven is good!” – well, models are bad if they are inaccurate. In the Eiffel approach the model and the program are the same thing, or more precisely the model is the abstract view of the program, obtained through abstraction mechanisms such as deferred classes with contracts and the “contract view” tool of EiffelStudio.)

To be effective, these ideas require proper tool support, for which EIS is a start. But we would like to know if we are on the right track and hence need feedback. We would be grateful if you could try out EIS and tell us what you think, both about the current state of the mechanism and its long-term prospects in the general framework of high-quality, sustainable software development.

References

[1] EIS documentation, here.

[2] Tao Feng, Start using Eiffel Information System, Eiffelroom blog entry of 17 April 2008, available here.

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Domain Theory: precedents

Both Gary Leavens and Jim Horning commented (partly here, partly on Facebook) about my Domain Theory article [1] to mention that Larch had mechanisms for domain modeling and specification reuse. As Horning writes:

The Larch Shared Language was really all about creating reusable domain theories, including theorems about the domains.  See, for example [2] and [3].

I am honored that they found the time to write about the article and happy to acknowledge Larch, one of the most extensive efforts, over several decades, to provide serious notations and tools for specification. Leavens’s and Horning’s messages gave me the opportunity to re-read some Larch papers and discover a couple I did not know.

My article did not try to provide exhaustive references; if it had, Larch would have been among them. I would probably have cited my own paper on M [4], earlier than [3], which introduces a notation for composing specifications; see section 1.4 (“Features of the M method and the associated notation have thus been devised to allow for modular descriptions of systems. A system description may include an interface paragraph that describes the connection of the current specification with others, existing or yet to be written”) and the  presentation of these mechanisms in section 5.

Larch traits, described in [3], pursue a similar aim, but the earlier article cited by Horning [2] is a general, informal discussion of formal specification; it does not mention traits, and in fact does not cite Larch, stating instead “We have experimented with the use of two very different tools, PIE and Affirm, in constructing modest sized algebraic specifications”. Its general observations about the specification task remain useful today, and it does mention reuse in passing.

If we were to look for precedents, the basic source would have to be the Clear specification language of Goguen and Burstall, for which the citations [5, 6, 7] all appear in my M paper [4] and go back further: 1977-1981. Clear made a convincing case for modularizing specifications, and defined supporting language constructs.

Since these early publications, many people have come to realize that reuse and composition can be as useful on the specification side as they are for programming. Typical specification and verification techniques, however, do not take advantage of this idea and tend to make us restart every time from the lowest level. Domain Theory, as outlined in [1], is intended to bring abstraction, which has proved so beneficial in other parts of software engineering, to the world of specification.

References

[1] Domain Theory: The Forgotten step in program verification, an article in this blog, see here.

[2] John V. Guttag, James J. Horning, Jeannette M. Wing: Some Notes on Putting Formal Specifications to Productive Use, in Science of Computer Programming, vol. 2, no. 1, 1982, pages 53-68. (BM note: I found a copy here.)

[3] John V. Guttag, James J. Horning: A Larch Shared Language Handbook, in Science of Computer Programming, vol. 6, no. 2, 1986, pages 135-157. (BM note: I found a copy here, which also has a link to the Larch report.)

[4] Bertrand Meyer: M: A System Description Method, Technical Report TR CS 85-15, University of California, Santa Barbara, 1985, available here.

[5] Rod M. Burstall and Joe A. Goguen: Putting Theories Together to Make Specifications, in Proceedings of 5th International Joint Conference on Artificial Intelligence, Cambridge (Mass.), 1977, pages 1045- 1058.

[6] Rod M. Burstall and Joe A. Goguen: “The Semantics of Clear, a Specification Language,” in Proceedings of Advanced Course on Abstract Software Specifications, Copenhagen, Lecture Notes on Computer Science 86, Copenhagen, Springer-Verlag, 1980, pages 292-332, available here.

[7] Rod M. Burstall and Joe A. Goguen: An Informal Introduction to Specifications using Clear, in The Correctness Problem in Computer Science, eds R. S. Boyer and JJ. S. Moore, Springer-Verlag, 1981, pages 185-213.

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Aliasing and framing: Saint Petersburg seminar next week

In  last Thursday’s session of the seminar, Kokichi Futatsugi’s talk took longer than planned (and it would have been a pity to stop him), so I postponed my own talk on Automatic inference of frame conditions through the alias calculus to next week (Thursday local date). As usual it will be broadcast live.

Seminar page: here, including the link to follow the webcast.

Time and date: 5 April 2012, 18 Saint Petersburg time; you can see the local time at your location here.

Abstract:

Frame specifications, the description of what does not change in a routine call, are one of the most annoying components of verification, in particular for object-oriented software. Ideally frame conditions should be inferred automatically. I will present how the alias calculus, described in recent papers, can address this need.

There may be a second talk, on hybrid systems, by Sergey Velder.

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Seminar sessions in Saint Petersburg: CafeOBJ and the frame issue

The Saint Petersburg software engineering seminar has two sessions today (29 March 2012, 18 local time, see here for the date and time in your area), broadcast live:

  • By Kokichi Futatsugi from KAIST (Japan): Combining Inference and Search in Verification with CafeOBJ.
  • By me: Automatic inference of frame conditions through the alias calculus.

See details including the link for the live webcast on the seminar page. The page also includes links to video recordings of recent sessions.

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A carefully designed Result

 

In the Eiffel user discussion group [1], Ian Joyner recently asked:

A lot of people are now using Result as a variable name for the return value in many languages. I believe this first came from Eiffel, but can’t find proof. Or was it adopted from an earlier language?

Proof I cannot offer, but certainly my recollection is that the mechanism was an original design and not based on any previous language. (Many of Eiffel’s mechanisms were inspired by other languages, which I have always acknowledged as precisely as I could, but this is not one of them. If there is any earlier language with this convention — in which case a reader will certainly tell me — I was and so far am not aware of it.)

The competing conventions are a return instruction, as in C and languages based on it (C++, Java, C#), and Fortran’s practice, also used in Pascal, of using the function name as a variable within the function body. Neither is satisfactory. The return instruction suffers from two deficiencies:

  • It is an extreme form of goto, jumping out of a function from anywhere in its control structure. The rest of the language sticks to one-entry, one-exit structures, as I think all languages should.
  • In most non-trivial cases the return value is not just a simple formula but has to be computed through some algorithm, requiring the declaration of a local variable just to denote that result. In every case the programmer must invent a name for that variable and, in a typed language, include a declaration. This is tedious and suggests that the language should take care of the declaration for the programmer.

The Fortran-Pascal convention does not combine well with recursion (which Fortran for a long time did not support). In the body of the function, an occurrence of the function’s name can denote the result, or it can denote a recursive call; conventions can be defined to remove the ambiguity, but they are messy, especially for a function without arguments: in function f, does the instruction

f := f + 1

add one to the value of the function’s result as computed so far, as it would if f were an ordinary variable, or to the result of calling f recursively?

Another problem with the Fortran-Pascal approach is that in the absence of a language-defined rule for variable initialization a function can return an undefined result, if some path has failed to initialize the corresponding variable.

The Eiffel design addresses these problems. It combines several ideas:

  • No nesting of routines. This condition is essential because without it the name Result would be ambiguous. In all Algol- and Pascal-like languages it was considered really cool to be able to declare routines within routines, without limitation on the depth of recursion. I realized that in an object-oriented language such a mechanism was useless and in fact harmful: a class should be a collection of features — services offered to the rest of the world — and it would be confusing to define features within features. Simula 67 offered such a facility; I wrote an analysis of inter-module relations in Simula, including inheritance and all the mechanisms retained from Algol such as nesting (I am trying to find that document, and if I do I will post it in this blog); my conclusion was the result was too complicated and that the main culprit was nesting. Requiring classes to be flat structures was, in my opinion, one of the most effective design decisions for Eiffel.
  • Language-defined initialization. Even a passing experience with C and C++ shows that uninitialized variables are one of the major sources of bugs. Eiffel introduced a systematic rule for all variables, including Result, and it is good to see that some subsequent languages such as Java have retained that convention. For a function result, it is common to ignore the default case, relying on the standard initialization, as in if “interesting case” then Result:= “interesting value” end without an else clause (I like this convention, but some people prefer to make all cases explicit).
  • One-entry, one-exit blocks; no goto in overt or covert form (break, continue etc.).
  • Design by Contract mechanisms: postconditions usually need to refer to the result computed by a function.

The convention is then simple: in any function, you can use a language-defined local variable Result for you, of the type that you declared for the function result; you can use it as a normal variable, and the result returned by any particular call will be the final value of the variable on exit from the function body.

The convention has been widely imitated, starting with Delphi and most recently in Microsoft’s “code contracts”, a kind of poor-man’s Design by Contract emulation, achieved through libraries; it requires a Result notation to denote the function result in a postcondition, although this notation is unrelated to the mechanisms in the target languages such as C#. As the example of Eiffel’s design illustrates, a programming language is a delicate construction where all elements should fit together; the Result convention relies on many other essential concepts of the language, and in turn makes them possible.

Reference

[1] Eiffel Software discussion group, here.

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New LASER proceedings

Springer has just published in the tutorial sub-series of Lecture Notes in Computer Science a new proceedings volume for the LASER summer school [1]. The five chapters are notes from the 2008, 2009 and 2010 schools (a previous volume [2] covered earlier schools). The themes range over search-based software engineering (Mark Harman and colleagues), replication of software engineering experiments (Natalia Juristo and Omar Gómez), integration of testing and formal analysis (Mauro Pezzè and colleagues), and, in two papers by our ETH group, Is branch coverage a good measure of testing effectiveness (with Yi Wei and Manuel Oriol — answer: not really!) and a formal reference for SCOOP (with Benjamin Morandi and Sebastian Nanz).

The idea of these LASER tutorial books — which are now a tradition, with the volume from the 2011 school currently in preparation — is to collect material from the presentations at the summer school, prepared by the lecturers themselves, sometimes in collaboration with some of the participants. Reading them is not quite as fun as attending the school, but it gives an idea.

The 2012 school is in full preparation, on the theme of “Advanced Languages for Software Engineering” and with once again an exceptional roster of speakers, or should I say an exceptional roster of exceptional speakers: Guido van Rossum (Python), Ivar Jacobson (from UML to Semat), Simon Peyton-Jones (Haskell), Roberto Ierusalimschy (Lua), Martin Odersky (Scala), Andrei Alexandrescu (C++ and D),Erik Meijer (C# and LINQ), plus me on the design and evolution of Eiffel.

The preparation of LASER 2012 is under way, with registration now open [3]; the school will take place from Sept. 2 to Sept. 8 and, like its predecessors, in the wonderful setting on the island of Elba, off the coast of Tuscany, with a very dense technical program but time for enjoying the beach, the amenities of a 4-star hotel and the many treasures of the island. On the other hand not everyone likes Italy, the sun, the Mediterranean etc.; that’s fine too, you can wait for the 2013 proceedings.

References

[1] Bertrand Meyer and Martin Nordio (eds): Empirical Software Engineering and Verification, International Summer Schools LASER 2008-2010, Elba Island, Italy, Revised Tutorial Lectures, Springer Verlag, Lecture Notes in Computer Science 7007, Springer-Verlag, 2012, see here.

[2] Peter Müller (ed.): Advanced Lectures on Software Engineering, LASER Summer School 2007-2008, Springer Verlag, Lecture Notes in Computer Science 7007, Springer-Verlag, 2012, see here.

[3] LASER summer school information and registration form, http://se.ethz.ch/laser.

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ERC Advanced Investigator Grant: Concurrency Made Easy

In April we will be starting the  “Concurrency Made Easy” research project, the result of a just announced Advanced Investigator Grant from the European Research Council. Such ERC grants are awarded to a specific person, rather than a consortium of research organizations as in the usual EU funding scheme. The usual amount, which applies in my case, is 2.5 million euros (currently almost 3 .3 million dollars) over five years, on a specific theme. According to the ERC’s own description [1],

ERC Advanced Grants allow exceptional established research leaders of any nationality and any age to pursue ground-breaking, high-risk projects that open new directions in their respective research fields or other domains.

This is the most sought-after research funding instrument of the EU, with a success rate of about 12% [2], out of a group already preselected by the host institutions. What makes ERC Advanced Investigator Grants so coveted is the flexibility of the scheme (no constraints on the topic, light administrative baggage) and the trust that an award implies in a particular researcher and his ability to carry out advanced research.

The name of the CME project clearly signals its ambition: to turn concurrent programming into a normal, unheroic part of programming. Today adding concurrency to a program, usually in the form of multithreading, is very hard, complexity and risk of all kinds. Everyone is telling us that we must rethink programming, retrain programmers and revamp curricula to put the specific reasoning modes of concurrent programming at the center. I don’t think this can work; thinking concurrently is just too hard to become the default mode. Instead, we should adapt programming languages, theories and tools so that programmers can continue to apply the reasoning schemes that have proved so successful in classical programming, especially object-oriented programming with the benefit of Design by Contract.

The starting point is the SCOOP model, to which I started an introduction in an earlier article of this blog [3], with a sequel yet to come. SCOOP is a minimal extension to the O-O framework to support concurrency, yielding very simple (the S in the acronym) solutions to concurrent programming problems. As part of the CME project we plan to develop it in many different directions and establish a sound and effective formal basis.

I have put the project description — the scientific part of the actual proposal text accepted by the ERC — online [4].

In the next few weeks I will be publishing here specific announcements for the positions we are seeking to fill very quickly; they include postdocs, PhD students, and one research engineer. We are looking for candidates with excellent knowledge and practice of concurrency, Eiffel, formal techniques etc. The formal application procedure will be Web-based and is not in place yet but you can contact me if you fit the profile and are interested.

We can defeat the curse: concurrent programming (an obligatory condition of any path towards a successful future for information technology) does not have to be black magic. It can be made simple and efficient. Such is the challenge of the CME project.

References

[1] European Research Council: Advanced Grants, available here.

[2] European Research Council: Press release on 2011 Advanced Investigator Grants, 24 January 2012, available here.

[3] Concurrent Programming is Easy, article from this blog, available here.

[4] CME Advanced Investigator Grant project description, available here.

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Never design a language

It is a common occurrence in software development. Someone says: “We should design a language”. The usual context is that some part of the development requires a rich functionality set, and it appears appropriate to provide a flexible solution through a specialized language. As an example, in the development of an airline’s frequent flyer program on which I once worked the suggestion came to design a “Flyer Award Language” , with instructions appropriate for that application domain: record a trip, redeem an award, provide a statement of available miles and so on. A common term for such notations is DSL, for Domain-Specific Language.

Designing a language in such a context is almost always a bad idea (and I am not sure why I wrote “almost”). Languages are endless objects of discussion, usually on the least important aspects, which are also the most visible and those on which everyone has a strong opinion: concrete syntactic properties. People might pretend otherwise (“let’s not get bogged down on syntax, this is just one possible form”) but syntax is what the discussions will get bogged down to — keywords or symbols, this order or that order of operands, one instruction with several variants vs. several instructions… — at the expense of discussing the fundamental issues of functionality.

Worse yet, even if a language will be part of the solution it is usually just one facet to the solution. As was already explained in detail in [1], any useful functionality set will naturally be useful through several interfaces: a textual notation with concrete syntax may be one of them, but other possible ones include an API (Abstract Program Interface) for use from other software elements, a Graphical User Interface, a web user interface, yet another for web services (typically WSDL or some other XML or JSON format).

In such cases, starting with a concrete textual language is pretty silly, since it cannot yield the others directly (it would have to be parsed and further analyzed, which does not make sense). Of all the kinds of interface listed, the most fundamental one is the API: it describes the raw functionality, excluding any choice of syntax but including, thanks to contracts, elements of semantics. For example, a class AWARD in our frequent flyer application might include the feature


             redeem_for_upgrade (c: CUSTOMER; f : FLIGHT)
                                     — Upgrade c to next class of service on f.
                       require
                                    c /= holder
implies holder.allowed_substitute (c)
                                    f.permitted_for_upgrade
(Current)
                                    c.booked
( f )
                       
ensure
                                    c.class_of_service
( f ) =  old c.class_of_service ( f ) + 1

There is of course no implementation as this declaration only specifies an interface, but it says what needs to be said: to redeem the award for an upgrade, the intended customer must be either the holder of the award or an allowed substitute; the flight must be available for an upgrade with the current award (including the availability of enough miles); the intended customer must already be booked on the flight; and the upgrade will be for the next class of service.

These details are the kind of things that need to be discussed and agreed before the API is finalized. Then one can start discussing about a textual form (a DSL), a graphical interface, a web services interface. They all consist of relatively simple layers to be superimposed on a solidly defined and precisely specified basis. Once you have that basis, you can have all the fun you like arguing over everyone’s favorite forms of concrete syntax; it cannot hurt the project any more. Having these discussions early, at the expense of the more fundamental issues, is a great danger.

One of the key rules for successful software construction — as for many other ventures of course, especially in science and technology — is to distinguish the essential from the auxiliary, and consequently to devote proper attention to the essential issues while avoiding disputations of auxiliary issues. To define functionality, API is essential; language is auxiliary.

So when should you design a language? Never. Well, hardly ever.

Reference

[1] Bertrand Meyer: Introduction to the Theory of Programming Languages, Prentice Hall, 1990.

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Guest article: funding great research

In a blog article posted in its original version on this blog [1] and in a revised version on the Communications of the ACM blog [2], I emphasized the relevance of incremental research. Recently Mikkel Thorup sent me some interesting comments, which I am publishing here as the first Guest Column of this blog.

References

[1] Bertrand Meyer: One Cheer for Incremental Research, in the present blog, 10 August 2009, available here

[2] Bertrand Meyer: Long Live Incremental Research, in Communications of the ACM Blog, 13 June 2011, available here.

Guest article by Mikkel Thorup: Funding Great Research

Research foundations want great research projects. However, a while back Bertrand Meyer wrote an interesting blog post: Long Live Incremental Research [2]. With examples he showed that many of the greatest results of research could not possibly be the projected results of great sounding project descriptions. His conclusion is that we should drop the high-flying ambitions from project descriptions, and instead support more incremental research proposals, hoping that great stuff will happen on the way. Indeed incremental research is perfect for research projects with predictable deliverables. However, I suggest the opposite conclusion; namely that we for some of the funding drop the project description.

The basic idea is that foundations should encourage researchers to look for results far better than those that can reasonably be projected. In particular, researchers should be free to follow their inspiration when they see new exiting opportunities. This is not done by tying researchers to incremental projects. Instead we can sometimes switch to result based funding, that is, funding based on results already achieved (with emphasis on the more recent past). Such result based funding is more like rewards for great results, and it offers researchers the perfect incentive to do their very best so as to secure future funding.

Consider a researcher with a history of brilliant ideas taking research in surprising new directions. If we try casting this as a project, the referees will rightly complain: “It is not clear how the applicant will come up with a brilliant idea, nor is it clear what the surprise will be”. With such lack of focus and feasibility, a low project score is expected.  If the project description has a predefined weight of, say, 40%, then the overall score will be too low for funding, regardless of the researcher’s established track record of succeeding in unlikely situations.  However, research needs great new ideas. Therefore we need some result based funding so that we can support researchers with a proven talent for generating great new ideas even if we do not quite understand how it will happen.

The above problem is often very real in my field of theoretical computer science. Like in other fields, theoretical research is only interesting if it contains surprises (otherwise it is more like development). A project plan would make sense if the starting point was a surprising idea or approach that it would take years to develop, but in theory, the most exciting ideas are often strikingly simple. When first you have such an idea, you are typically close to done, ready to start writing a paper. Thus, if you have a great idea when you apply for a grant, you will typically be done long before you get the grant. The essence of the research is thus the unpredictable search for powerful ideas and insights. The most appropriate project description is therefore just a description of the importance of the area to be researched and the type of results aimed for. The track record shows which researchers have the talent to succeed.

Dropping the how-part of the project description will greatly increase methodological diversity, allowing researchers to use the strategy that has proved most suitable for their area and their own talent and skills.  As a simple example, Bertrand suggested funding incremental research, hoping that great surprising things would turn up on the way. My strategy is the opposite. I try to spend as much time as possible on overly ambitious targets. Most of the time I fail, but I rarely come home empty-handed, for by studying the unknown I nearly always discover something new, sometimes even more interesting than the original target. From the perspective of ambition, I see it as an advantage that I minimize time spend on easy targets, but foundations seem to prefer that you take a planned path with some guaranteed targets on the way. The point here is not to argue whether one strategy is superior to the other, but rather to embrace the diversity of strategies that may work depending on the area and the individual researcher.

Perhaps more seriously, if a target is hard to achieve, it may be because it requires a crazy approach that would not look reasonable to anyone else, but which may work for a researcher thanks to his special talents and intuition. Indeed I have often been positively surprised seeing how others succeeded using an approach I had myself dismissed.  As a project, such crazy approaches would fail on perceived feasibility, but the point in result based funding is that researchers are free to use whatever approach they find most efficient. Funding is given to those who prove successful. This gives the perfect incentive to do great work, securing future funding.

Result-based funding would also reduce resources needed to evaluate applications. It is very hard for a general panel to evaluate the methodology and success probability of a project.  Moreover, it requires an intimate knowledge of a field to evaluate how big a difference a result would make relative to what is already known. However, handling published results, we know what happened and we can rely on peer-review for the difference it made to the field. All the panel has to do is to evaluate how the successes meet with the objectives of the foundation.

Let us, as an example, take something like the ERC Advanced Investigator Grant which welcomes high risk high gain research. It would seem that aiming for surprising breakthroughs in an important area would fall well within this scope. Having researchers with proven skills explore the area and follow their inspiration may be the optimal strategy. Uncertainty about what they would find should not be worse than high risk. In fact, based on past performance, it may be safe to assume that they will discover something interesting if not ground-breaking. However, when projects are scored on focused feasibility, such projects will fail even if their expected return is very high. It has to be possible to get a high overall score for promising research even if standard project parameters like focus and feasibility would be counterproductive.  At the end of the day, what we want are results, not project descriptions, so what should determine the overall score is which proposal is expected to yield the greatest results.

Long live great research!

Mikkel Thorup

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How to design software

 

 

I think I recently understood how software should be designed — or at least, since I have informally practiced the method for some time, how to explain it. Maybe not absolutely all types of software, but the most important kind: APIs (abstract program interfaces). The key task in software design is to define proper interfaces; if it is done right, everything else will fall into place, and if it is done wrong, there will be no end of problems everywhere else.

To put a Swiss theme to the description I may call this approach the Gotthard method. You are building a tunnel, starting from both sides at the same time, the northern, rainy, German-speaking part, and the southern, sunny, Italian-speaking part. (Actual languages and climates may vary.)

Tunnel through a mountain

For the method to work it is really important that the two crews should meet somewhere in the middle:

Gotthard crews meeting

In software design we are typically confronted with two views:

  • The client view: application software needs certain abstractions and functionalities that will make it easy to produce clear, simple, extendible, reusable client programs.
  • The supplier view: the necessary mechanisms are usually available in a raw form directly reflecting the underlying platform, a combination of hardware and software facilities.

Good design is a negotiation and iteration process that tries to reconcile the two views, working top-down from the client side and bottom-up from the supplier side, just as you would work when digging a tunnel between Unterwald and the Tessin.

As an example, consider a Web-oriented API. On the supplier side, we have a stateless protocol with essentially one mechanism: processing a request and sending a response. On the client side, we want to enable the building of applications, such as an e-commerce site, which need to pretend that they are working with stateful sessions, just as with a classical client-server GUI setup. The task of building software is to provide what the client application needs, in terms that make sense to the client and with all the abstractions that it needs — in our example, SESSION, STATE, USER and so on.

Since these higher-level abstractions are not directly provided by the supplier side, they need to be implemented or, to use a more appropriate term, faked. After all, everything in computer science is about faking: pretending that we have machines that we really don’t, simply by building them conceptually, in the form of APIs, in terms of machines that we have already built (bottom-up approach) or hope to build (top-down approach). “Building” a machine here means— except for the bottom-most machines, down at the level of the hardware, which very few programmers ever use directly anyway —faking them again, in terms of simpler ones. Fakes all the way down.

The process of software design then consists of developing intermediate levels of abstraction until we reach a compromise: a set of abstractions that satisfy the needs of application programmers and are efficiently implementable (or better yet, already implemented as part of this negotiation process) on the basis of what was available in the first place.

A poorly functioning software process will be more like yoyo design: trying something too abstract, then something too low-level and so on, converging too late if at all. Effective design is like boring a tunnel using modern engineering techniques, which rely on a clear understanding of where the crews start on both sides and make sure they end up meeting in the right place.

 

Photo reference: Herrenknecht AG, www.herrenknecht.com.

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Ado About The Resource That Was (Not)

 

After a few weeks of use, Microsoft Outlook tends in my experience to go into a kind of thrashing mode where the user interface no longer quite functions as it should, although to the tool’s credit it does not lose information. Recently I have been getting pop-up warnings such as

 

A required resource was

 

A required resource was what? The message reminded me of an episode in a long-ago game of Scrabble, in which I proposed ADOABOUT as a word. “Ado about what? ”, the other players asked, and were not placated by my answer.

The message must have been trying to say  that a required resource was missing, or not found, but at the time of getting the final detail Outlook must have run out of UI resources and hence could not summon the needed text string. Not surprising, since running out of resources is precisely what caused the message to appear, in a valiant attempt to tell the user what is going on. (Valiant but not that useful: if you are not a programmer on the Outlook development team but just a customer trying to read email, it is not absolutely obvious how the message, even with the missing part, helps you.) The irony in the example is that the title bar suggests the problem arose in connection with trying to display the “Social Connector” area, a recent Outlook feature which I have never used. (Social connector? Wasn’t the deal about getting into computer science in the first place that for the rest of your life you’d be spared the nuisance of social connections? One can no longer trust anything nowadays.)

We can sympathize with whoever wrote the code. The Case Of The Resource That Was (Not) is an example of a general programming problem which we may call Space Between Your Back And Wall  or SBYBAW:  when you have your back against the wall, there is not much maneuvering space left.

A fairly difficult case of the SBYBAW problem arises in garbage collection, for example for object-oriented languages. A typical mark-and-sweep garbage collector must traverse the entire object structure to remove all the objects that have not been marked as reachable from the stack. The natural way to write a graph traversal algorithm is recursive: visit the roots; then recursively traverse their successors, flagging visited objects in some way to avoid cycling. Yes, but the implementation of a recursive routine relies on a stack of unpredictable size (the longest path length). If we got into  garbage collection, most likely it’s that we ran out of memory, precisely the kind of situation in which we cannot afford room for unpredictable stack growth.

In one of the early Eiffel garbage collectors, someone not aware of better techniques had actually written the traversal recursively; had the mistake not been caught early enough, it would no doubt have inflicted unbearable pain on humankind. Fortunately there is a solution: the Deutsch-Schorr-Waite algorithm [1], which avoids recursion on the program side by perverting the data structure to  replace some of the object links by recursion-control links; when the traversal’s execution proceeds along an edge, it reverses that edge to permit eventual return to the source. Strictly speaking, Deutsch-Schorr-Waite still requires a stack of booleans — to distinguish original edges from perverted ones — but we can avoid a separate stack (even just  a stack of booleans, which can be compactly represented in a few integers) by storing these booleans in the mark field of the objects themselves. The resulting traversal algorithm is a beauty — although it is fairly tricky, presents a challenge for verification tools, and raises new difficulties in a multi-threaded environment.

Deutsch-Schorr-Waite is a good example of “Small Memory Software” as studied in a useful book of the same title [2]. The need for Small Memory Software does not just arise for embedded programs running on small devices, but also in mainstream programming whenever we face the SBYBAW issue.

The SBYBAW lesson for the programmer is tough but simple. The resources we have at our disposal on a computing system may be huge, but they are always finite, and our programs’ appetite for resources will eventually exhaust them. At that stage, we have to deal with the SBYBAW rule, which sounds like a tautology but is an encouragement to look for clever algorithms:  techniques for freeing resources when no resources remain must not request new resources.

References

[1] Deutsch-Schorr-Waite is described in Knuth and also in [2]. Someone should start a Wikipedia entry.

[2] James Noble and Charles Weir: Small Memory Software: Patterns for Systems with Limited Memory, Addison-Wesley, 2001.

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The Modes and Uses of Scientific Publication

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Recycled(This article was initially published in the CACM blog.)
Publication is about helping the advancement of humankind. Of course.

Let us take this basis for granted and look at the other, possibly less glamorous aspects.

Publication has four modes: Publicity; Exam; Business; and Ritual.

1. Publication as Publicity

The first goal of publication is to tell the world that you have discovered something: “See how smart I am!” (and how much smarter than all the others out there!). In a world devoid of material constraints for science, or where the material constraints are handled separately, as in 19th-century German universities where professors were expected to fund their own labs, this would be the only mode and use of publication. Science today is a more complex edifice.

A good sign that Publication as Publicity is only one of the modes is that with today’s technology we could easily skip all the others. If all we cared about were to make our ideas and results known, we would simply put out our papers on ArXiv or just our own Web page. But almost no one stops there; researchers submit to conferences and journals, demonstrating how crucial the other three modes are to the modern culture of science.

2. Publication as Exam

Academic careers depend on a publication record. Actually this is not supposed to be the case; search and tenure committees are officially interested in “impact,” but any candidate is scared of showing a short publication list where competitors have tens or (commonly) hundreds of items.

We do not just publish; we want to be chosen for publication. Authors are proud of the low acceptance rates of conferences at which their papers have been accepted; in the past few years it has in fact become common practice, in publication lists attached to CVs, to list this percentage next to each accepted article. Acceptance rates are carefully tracked; see for example [2] for software engineering.

As Jeff Naughton has pointed out [1], this mode of working amounts to giving researchers the status of students forced to take exams again and again. Maybe that part is inevitable; the need to justify ourselves anew every morning may be an integral part of being a scientist, especially one funded by other people’s money. Two other consequences of this phenomenon are, I believe, more damaging.

The first risk directly affects the primary purpose of publication (remember the advancement of humankind?): a time-limited review process with low acceptance rates implies that some good papers get rejected and some flawed ones accepted. Everyone in software engineering knows (and recent PC chairs have admitted) that getting a paper accepted at the International Conference on Software Engineering is in part a lottery; with an acceptance rate hovering around 13%, this is inevitable. The mistakes occur both ways: papers accepted or even getting awards, then shown a few months later to be inaccurate; and innovative papers getting rejected because some sentence rubbed the referees the wrong way, or some paper was not cited. With a 4-month review cycle, and the next deadline coming several months later, the publication of a truly important result can be delayed significantly.

The second visible damage is publication inflation. Today’s research environment channels productive research teams towards an LPU (Least Publishable Unit) publication practice, causing an explosion of small contributions and the continuous decrease of the ratio of readers to writers. When submitting a paper I have always had, as my personal goal, to be read; but looking at the overall situation of computer science publication today suggests that this is not the dominant view: the overwhelming goal of publication is publication.

3. Publication as Business

Publishing requires an infrastructure, and money plays a role. Conferences in particular are a business. They have a budget to balance, not always an easy task, although a truly successful conference can be a big money-maker for its sponsor, commercial or non-profit. The financial side of conference publication has its consequences on authors: if you do not pay your fees, not only will you be unable to participate, but your paper will not be published.

One can deplore these practices, in particular their effect on authors from less well-endowed institutions, but they result from today’s computer science publication culture with its focus on the conference, what Lance Fortnow has called “A Journal in a Hotel”.

Sometimes the consequences border on the absurd. The ASE conference (Automated Software Engineering) accepts some contributions as “short papers”. Fair enough. At ASE 2009, “short paper” did not mean a shorter conference presentation but the permission to put up a poster and stand next to it for a while and answer passersby’s questions. For that privilege — and the real one: a publication in the conference volume — one had to register for the conference. ASE 2009 was in New Zealand, the other end of the world for a majority of authors. I ceded to the injunction: who was I to tell the PhD student whose work was the core of the submission, and who was so happy to have a paper accepted at a well-ranked conference, that he was not going to be published after all? But such practices are dubious. It would be more transparent to set up an explicit pay-for-play system, with page charges: at least the money would go to a scientific society or a university. Instead we ended up funding (in addition to the conference, which from what I heard was an excellent experience) airlines and hotels.

What makes such an example remarkable is that a reasonable justification exists for every one of its components: a highly selective refereeing process to maintain the value of the publication venue; limiting the number of papers selected for full presentation, to avoid a conference with multiple parallel tracks (and the all too frequent phenomenon of conference sessions whose audience consists of the three presenters plus the session chair); making sure that authors of published papers actually attend the event, so that it is a real conference with personal encounters, not just an opportunity to increment one’s publication count. The concrete result, however, is that authors of short papers have the impression of being ransomed without getting the opportunity to present their work in a serious way. Literally seconds as I was going to hit the “publish” button for the present article, an author of an accepted short paper for ASE 2012 (where the process appears similar) sent an email to complain, triggering a new discussion. We clearly need to find better solutions to resolve the conflicting criteria.

4. Publication as Ritual

Many of the seminal papers in science, including some of the most influential in computer science, defy classification and used a distinctive, one-of-a-kind style. Would they stand a chance in one of today’s highly ranked conferences, such as ICSE in software or VLDB in databases? It’s hard to guess. Each community has developed its own standard look-and-feel, so that after a while all papers start looking the same. They are like a classical mass with its Te Deum, Agnus Dei and Kyrie Eleison. (The “Te Deum” part is, in a conference submission, spread throughout the paper, in the form of adoring citations of the program committee members’ own divinely inspired articles, good for their H-indexes if they bless your own offering.)

All empirical software engineering papers, for example, have the obligatory “Threats to Validity” section, which is has developed into a true art form. The trick is the same as in the standard interview question “What can you say about your own deficiencies?”, to which every applicant know the key: describe a personality trait so that you superficially appear self-critical but in reality continue boasting, as in “sometimes I take my work too much to heart” [3]. The “Threats to Validity” section follows the same pattern: you try to think of all possible referee objections, the better to refute them.

Another part of the ritual is the “related work” section, treacherous because you have to make sure not to omit anything that a PC member finds important; also, you must walk a fine line between criticizing existing research too much, which could offend someone, or not enough, which enables the referee to say that you are not bringing anything significantly new. I often wonder who, besides the referees, reads those sections. But here too it is easier to lament than to fault the basic idea or propose better solutions. We do want to avoid wasting our time on papers whose authors are not aware of previous work. The related work section allows referees to perform this check. Its importance in the selection process has, however, grown out of proportion. It is one thing to make sure that a paper is state-of-the-art, but another to reject it (as often happens) because it fails to cite a particular contribution whose results would not directly affect its own. Here we move from the world of the rational to the world of the ritual. An extreme and funny recent example — funny to me, not necessarily to the coauthors — is a rejection from  APSEC 2011, the Australia-Pacific Software Engineering Conference, based on one review (the others were positive) that stated: “How novel is this? Are [there] not any cloud-based IDEs out there that have [a] similar awareness model integrated into their CM? This is something the related work [section] fails to describe precisely. [4] The ritual here becomes bizarre: as far as we know, no existing system discusses a similar model; the reviewer too does not know of any; but he blasts the paper all the same for not citing work that he thinks must have been done by someone, somehow, somewhere. APSEC is a fine conference — it has to be, from the totally unbiased criterion that it accepted another one of our submissions this year! — and this particular paper may or may not have been ready for publication; judge it for yourself [5]. Such examples suggest, however, that the ritual of computer science publication has its limits.

Publicity, Exam, Business, Ritual: to which one of the four modes of publication are you most attuned? Oh, sorry, I forgot: in your case, it is solely for the advancement of humankind.

References and notes

[1] Jeffrey F. Naughton, DBMS Research: First 50 Years, Next 50 Years, slides of keynote at 26th IEEE International Conference on Data Engineering, 2010, available at lazowska.cs.washington.edu/naughtonicde.pdf .

[2] Tao Xie, Software Engineering Conferences, at people.engr.ncsu.edu/txie/seconferences.htm .

[3] I once saw on French TV a hilarious interview of an entrepreneur who had started a software company in Vietnam, where job candidates just did not know “the code”, and moved on, in response to such a question, to tell the interviewer about being rude to their mother and all the other horrible things they had done in their lives.

[4] The words in brackets were not in the review but I added them for clarity.

[5] Martin Nordio, H.-Christian Estler, Carlo A. Furia and Bertrand Meyer: Collaborative Software Development on the Web, available at arxiv.org/abs/1105.0768 .

(This article was first published on the CACM blog in September 2011.)

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John McCarthy

John McCarthyJohn McCarthy, who died last week at the age of 84, was one of the true giants of computer science. Most remarkable about his contributions are their diversity, their depth, and how they span both theory and practice.

To talk about him it is necessary first to dispel an unjustly negative connotation. McCarthy was one of the founders of the discipline of artificial intelligence, its most forceful advocate and the inventor of its very name. In the “AI Winter” episode of the late 1970s and 1980s, that name suffered some disrepute as a result of a scathing report by James Lighthill blaming AI researchers for over-promising. In fact the promoters of AI may not have delivered exactly what they announced (who can accurately predict science?); but what they delivered is astounding. Many breakthroughs in computer science, both in theory (advances in lambda calculus and the theory of computation) and in the practice of programming (garbage collection, functional programming languages), can directly be traced to work in AI. Part of the problem is a phenomenon that I heard John McCarthy himself describe:  “As soon as it works, no one calls it AI any more.” Automatic garbage collection was once advanced artificial intelligence; now it is just an algorithm that makes sure your smartphone does not freeze up. In a different field, we have become used to computers routinely beating chess champions, a feat that critics of AI once deemed unthinkable.

The worst over-promises came not from researchers in the field such as McCarthy, who understood the difficulties, but from people like Herbert Simon, more of a philosopher, who in 1965 wrote that “machines will be capable, within twenty years, of doing any work a man can do.” McCarthy’s own best-known over-promise was to take up David Levy on his 1968 bet that no computer would be able to beat him within ten years. But McCarthy was only mistaken in under-estimating the time span: Deep Blue eventually proved him right.

The word that comes most naturally to mind when thinking about McCarthy is “brilliant.” He belonged to that category of scientists who produce the fundamental insights before anyone else, even if they do not always have the patience to finalize the details. The breathtaking paper that introduced Lisp [1] is labeled “Part 1”; there was never a “Part 2.” (Of course we have a celebrated example in computer science, this one from a famously meticulous author, of a seven-volume treaty which never materialized in full.) It was imprudent to announce a second part, but the first was enough to create a whole new school of programming. The Lisp 1.5 manual [2], published in 1962, was another masterpiece; as early as page 13 it introduces — an unbelievable feat, especially considering that the program takes hardly more than half a page — an interpreter for the language being defined, written in that very language! The more recent reader can only experience here the kind of visceral, poignant and inextinguishable jealously that overwhelms us the first time we realize that we will never be able to attend the première of Don Giovanni at the Estates Theater in Prague on 29 October, 1787 (exactly 224 years ago yesterday — did you remember to celebrate?). What may have been the reaction of someone in “Data Processing,” such as it was in 1962, suddenly coming across such a language manual?

These years, 1959-1963, will remain as McCarthy’s Anni Mirabiles. 1961 and 1962 saw the publication of two visionary papers [3, 4] which started the road to modern program verification (and where with the benefit of hindsight it seems that he came remarkably close to denotational semantics). In [4] he wrote

Instead of debugging a program, one should prove that it meets its specifications, and this proof should be checked by a computer program. For this to be possible, formal systems are required in which it is easy to write proofs. There is a good prospect of doing this, because we can require the computer to do much more work in checking each step than a human is willing to do. Therefore, the steps can be bigger than with present formal systems.

Words both precise and prophetic. The conclusion of [3] reads:

It is reasonable to hope that the relationship between computation and mathematical logic will be as fruitful in the next century as that between analysis and physics in the last. The development of this relationship demands a concern for both applications and for mathematical elegance.

“A concern for both applications and mathematical elegance” is an apt characterization of McCarthy’s own work. When he was not busy designing Lisp, inventing the notion of meta-circular interpreter and developing the mathematical basis of programming, he was building the Lisp garbage collector and proposing the concept of time-sharing. He also played, again in the same period, a significant role in another milestone development, Algol 60 — yet another sign of his intellectual openness and versatility, since Algol is (in spite of the presence of recursion, which McCarthy championed) an imperative language at the antipodes of Lisp.

McCarthy was in the 1960s and 70s the head of the Artificial Intelligence Laboratory at Stanford. For some reason the Stanford AI Lab has not become as legendary as Xerox PARC, but it was also the home to early versions of  revolutionary technologies that have now become commonplace. Email, which hardly anyone outside of the community had heard about, was already the normal way of communicating, whether with a coworker next door or with a researcher at MIT; the Internet was taken for granted; everyone was using graphical displays and full-screen user interfaces; outside, robots were playing volley-ball (not very successfully, it must be said); the vending machines took no coins, but you entered your login name and received a bill at the end of the month, a setup which never failed to astonish visitors; papers were printed with sophisticated fonts on a laser printer (I remember a whole group reading the successive pages of Marvin Minsky’s  frames paper [5] directly on the lab’s XGP, Xerox Graphics Printer, as  they were coming out, one by one, straight from MIT). Arthur Samuel was perfecting his checkers program. Those who were not programming in Lisp were hooked to SAIL, “Stanford Artificial Intelligence Language,” an amazing design which among other insights convinced me once and for all that one cannot seriously deal with data structures without the benefit of an automatic serialization mechanism. The building itself, improbably set up amid the pastures of the Santa Cruz foothills, was razed in the eighties and the lab moved to the main campus, but the spirit of these early years lives on.

McCarthy ran the laboratory in an open and almost debonair way; he was a legend and somewhat intimidating, but never arrogant and in fact remarkably approachable. I took the Lisp course from him; in my second or third week at Stanford, I raised my hand and with the unflappable assurance of the fully ignorant slowly asked a long question: “In all the recursive function definitions that you have shown so far, termination was obvious because there is some ‘n’ that decreases for every recursive call, and we treat the case ‘n = 0’ or ‘n = 1’ in a special, non-recursive way. But things won’t always be so simple. Is there some kind of grammatical criterion on Lisp programs that we could use to ascertain whether a recursive definition will always lead to a terminating computation?” There was a collective gasp from the older graduate students in the audience, amazed that a greenhorn would have the audacity to interrupt the course with such an incompetent query. But instead of dismissing me, McCarthy proceeded, with a smile, to explain the basics of undecidability. He had the same attitude in the many seminars that he taught, often on topics straddling computer science and philosophy, in a Socratic style where every opinion was welcome and no one was above criticism.

He also had a facetious side. At the end of a talk by McCarthy at SRI, Tony Hoare, who was visiting for a few days, asked a question; McCarthy immediately rejoined that he had expected that question, summoned to the stage a guitar-carrying researcher from the AI Lab, and proceeded with the answer in the form of a prepared song.

The progress of science and technology is a collective effort; it takes many people to turn new insights into everyday reality. The insights themselves come from a few individuals, a handful in every generation. McCarthy was one of these undisputed pioneers.

 

References

[1] John McCarthy: Recursive Functions of Symbolic Expressions and Their Computation by Machine, Part I, in Communications of the ACM, vol. 3, no. 4, 1960, pages 184-195.

[2] John McCarthy, Paul W. Abrahams, Daniel J. Edwards, Timothy P. Hart, Michael I. Levin, LISP 1.5 Programmer’s Manual, MIT, 1962. Available at Amazon  External Linkand also as a PDF External Link.

[3] John McCarthy: A Basis for a Mathematical Theory of Computation, first version in Proc. Western Joint Computer Conference, 1961, revised version in Computer Programming and Formal Systems, eds. P. Braffort and D. Hirschberg, North Holland, 1963. Available in various places on the Web, e.g. here External Link.

[4] John McCarthy: Towards a Mathematical Science of Computation, in IFIP Congress 1962, pages 21-28, available in various places on the Web, e.g. here External Link.

[5] Marvin Minsky:  A Framework for Representing Knowledge, MIT-AI Laboratory Memo 306, June 1974, available here External Link.

 

(This article was first published on my ACM blog.  I am resuming regular Monday publication.)

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The story of our field, in a few short words

 

(With all dues to [1], but going up from four to five as it is good to be brief yet not curt.)

At the start there was Alan. He was the best of all: built the right math model (years ahead of the real thing in any shape, color or form); was able to prove that no one among us can know for sure if his or her loops — or their code as a whole — will ever stop; got to crack the Nazis’ codes; and in so doing kind of saved the world. Once the war was over he got to build his own CPUs, among the very first two or three of any sort. But after the Brits had used him, they hated him, let him down, broke him (for the sole crime that he was too gay for the time or at least for their taste), and soon he died.

There was Ed. Once upon a time he was Dutch, but one day he got on a plane and — voilà! — the next day he was a Texan. Yet he never got the twang. The first topic that had put him on  the map was the graph (how to find a path, as short as can be, from a start to a sink); he also wrote an Algol tool (the first I think to deal with all of Algol 60), and built an OS made of many a layer, which he named THE in honor of his alma mater [2]. He soon got known for his harsh views, spoke of the GOTO and its users in terms akin to libel, and wrote words, not at all kind, about BASIC and PL/I. All this he aired in the form of his famed “EWD”s, notes that he would xerox and send by post along the globe (there was no Web, no Net and no Email back then) to pals and foes alike. He could be kind, but often he stung. In work whose value will last more, he said that all we must care about is to prove our stuff right; or (to be more close to his own words) to build it so that it is sure to be right, and keep it so from start to end, the proof and the code going hand in hand. One of the keys, for him, was to use as a basis for ifs and loops the idea of a “guard”, which does imply that the very same code can in one case print a value A and in some other case print a value B, under the watch of an angel or a demon; but he said this does not have to be a cause for worry.

At about that time there was Wirth, whom some call Nick, and Hoare, whom all call Tony. (“Tony” is short for a list of no less than three long first names, which makes for a good quiz at a party of nerds — can you cite them all from rote?) Nick had a nice coda to Algol, which he named “W”; what came after Algol W was also much noted, but the onset of Unix and hence of C cast some shade over its later life. Tony too did much to help the field grow. Early on, he had shown a good way to sort an array real quick. Later he wrote that for every type of unit there must be an axiom or a rule, which gives it an exact sense and lets you know for sure what will hold after every run of your code. His fame also comes from work (based in part on Ed’s idea of the guard, noted above) on the topic of more than one run at once, a field that is very hot today as the law of Moore nears its end and every maker of chips has moved to  a mode where each wafer holds more than one — and often many — cores.

Dave (from the US, but then at work under the clime of the North) must not be left out of this list. In a paper pair, both from the same year and both much cited ever since,  he told the world that what we say about a piece of code must only be a part, often a very small part, of what we could say if we cared about every trait and every quirk. In other words, we must draw a clear line: on one side, what the rest of the code must know of that one piece; on the other, what it may avoid to know of it, and even not care about. Dave also spent much time to argue that our specs must not rely so much on logic, and more on a form of table.  In a later paper, short and sweet, he told us that it may not be so bad that you do not apply full rigor when you chart your road to code, as long as you can “fake” such rigor (his own word) after the fact.

Of UML, MDA and other such TLAs, the less be said, the more happy we all fare.

A big step came from the cold: not just one Norse but two, Ole-J (Dahl) and Kris, came up with the idea of the class; not just that, but all that makes the basis of what today we call “O-O”. For a long time few would heed their view, but then came Alan (Kay), Adele and their gang at PARC, who tied it all to the mouse and icons and menus and all the other cool stuff that makes up a good GUI. It still took a while, and a lot of hit and miss, but in the end O-O came to rule the world.

As to the math basis, it came in part from MIT — think Barb and John — and the idea, known as the ADT (not all TLAs are bad!), that a data type must be known at a high level, not from the nuts and bolts.

There also is a guy with a long first name (he hates it when they call him Bert) but a short last name. I feel a great urge to tell you all that he did, all that he does and all that he will do, but much of it uses long words that would seem hard to fit here; and he is, in any case, far too shy.

It is not all about code and we must not fail to note Barry (Boehm), Watts, Vic and all those to whom we owe that the human side (dear to Tom and Tim) also came to light. Barry has a great model that lets you find out, while it is not yet too late, how much your tasks will cost; its name fails me right now, but I think it is all in upper case.  At some point the agile guys — Kent (Beck) and so on — came in and said we had got it all wrong: we must work in pairs, set our goals to no more than a week away, stand up for a while at the start of each day (a feat known by the cool name of Scrum), and dump specs in favor of tests. Some of this, to be fair, is very much like what comes out of the less noble part of the male of the cow; but in truth not all of it is bad, and we must not yield to the urge to throw away the baby along with the water of the bath.

I could go on (and on, and on); who knows, I might even come back at some point and add to this. On the other hand I take it that by now you got the idea, and even on this last day of the week I have other work to do, so ciao.

Notes

[1] Al’s Famed Model Of the World, In Words Of Four Signs Or Fewer (not quite the exact title, but very close): find it on line here.

[2] If not quite his alma mater in the exact sense of the term, at least the place where he had a post at the time. (If we can trust this entry, his true alma mater would have been Leyde, but he did not stay long.)

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Nastiness in computer science

 

Recycled(This article was originally published in the CACM blog.)
 

Are we malevolent grumps? Nothing personal, but as a community computer scientists sometimes seem to succumb to negativism.

They admit it themselves. A common complaint in the profession (at least in academia) is that instead of taking a cue from our colleagues in more cogently organized fields such as physics, who band together for funds, promotion, and recognition, we are incurably fractious. In committees, for example, we damage everyone’s chances by badmouthing colleagues with approaches other than ours. At least this is a widely perceived view (“Circling the wagons and shooting inward,” as Greg Andrews put it in a recent discussion). Is it accurate?

One statistic that I have heard cited is that in 1-to-5 evaluations of projects submitted to the U.S. National Science Foundation the average grade of computer science projects is one full point lower than the average for other disciplines. This is secondhand information, however, and I would be interested to know if readers with direct knowledge of the situation can confirm or disprove it.

More such examples can be found in the material from a recent keynote by Jeffrey Naughton, full of fascinating insights (see his Powerpoint slides External Link). Naughton, a database expert, mentions that only one paper out of 350 submissions to SIGMOD 2010 received a unanimous “accept” from its referees, and only four had an average accept recommendation. As he writes, “either we all suck or something is broken!

Much of the other evidence I have seen and heard is anecdotal, but persistent enough to make one wonder if there is something special with us. I am reminded of a committee for a generously funded CS award some time ago, where we came close to not giving the prize at all because we only had “good” proposals, and none that a committee member was willing to die for. The committee did come to its senses, and afterwards several members wondered aloud what was the reason for this perfectionism that almost made us waste a great opportunity to reward successful initiatives and promote the discipline.

We come across such cases so often—the research project review that gratuitously but lethally states that you have “less than a 10% chance” of reaching your goals, the killer argument  “I didn’t hear anything that surprised me” after a candidate’s talk—that we consider such nastiness normal without asking any more whether it is ethical or helpful. (The “surprise” comment is particularly vicious. Its real purpose is to make its author look smart and knowledgeable about the ways of the world, since he is so hard to surprise; and few people are ready to contradict it: Who wants to admit that he is naïve enough to have been surprised?)

A particular source of evidence is refereeing, as in the SIGMOD example.  I keep wondering at the sheer nastiness of referees in CS venues.

We should note that the large number of rejected submissions is not by itself the problem. Naughton complains that researchers spend their entire careers being graded, as if passing exams again and again. Well, I too like acceptance better than rejection, but we have to consider the reality: with acceptance rates in the 8%-20% range at good conferences, much refereeing is bound to be negative. Nor can we angelically hope for higher acceptance rates overall; research is a competitive business, and we are evaluated at every step of our careers, whether we like it or not. One could argue that most papers submitted to ICSE and ESEC are pretty reasonable contributions to software engineering, and hence that these conferences should accept four out of five submissions; but the only practical consequence would be that some other venue would soon replace ICSE and ESEC as the publication place that matters in software engineering. In reality, rejection remains a frequent occurrence even for established authors.

Rejecting a paper, however, is not the same thing as insulting the author under the convenient cover of anonymity.

The particular combination of incompetence and arrogance that characterizes much of what Naughton calls “bad refereeing” always stings when you are on the receiving end, although after a while it can be retrospectively funny; one day I will publish some of my own inventory, collected over the years. As a preview, here are two comments on the first paper I wrote on Eiffel, rejected in 1987 by the IEEE Transactions on Software Engineering (it was later published, thanks to a more enlightened editor, Robert Glass, in the Journal of Systems and Software, 8, 1988, pp. 199-246 External Link). The IEEE rejection was on the basis of such review gems as:

  • I think time will show that inheritance (section 1.5.3) is a terrible idea.
  • Systems that do automatic garbage collection and prevent the designer from doing his own memory management are not good systems for industrial-strength software engineering.

One of the reviewers also wrote: “But of course, the bulk of the paper is contained in Part 2, where we are given code fragments showing how well things can be done in Eiffel. I only read 2.1 arrays. After that I could not bring myself to waste the time to read the others.” This is sheer boorishness passing itself off as refereeing. I wonder if editors in other, more established disciplines tolerate such attitudes. I also have the impression that in non-CS journals the editor has more personal leverage. How can the editor of IEEE-TSE have based his decision on such a biased an unprofessional review? Quis custodiet ipsoes custodes?

“More established disciplines”: Indeed, the usual excuse is that we are still a young field, suffering from adolescent aggressiveness. If so, it may be, as Lance Fortnow has argued in a more general context, “time for computer science to grow up.” After some 60 or 70 years we are not so young any more.

What is your experience? Is the grass greener elsewhere? Are we just like everyone else, or do we truly have a nastiness problem in computer science?

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European Computer Science Summit 2011

The program for ECSS 2011 (Milan, 7-9 November) has just been put online [1]. The European Computer Science Summit, held yearly since 2005, is the annual conference of Informatics Europe and a unique opportunity to discuss issues of interest to the computer science / informatics research and education community; much of the audience is made of deans, department heads, lab directors, researchers and senior faculty. Keynote speakers this year include Stefano Ceri, Mary Fernández, Monika Henzinger, Willem Jonker, Miron Livny, John Mylopoulos, Xavier Serra and John White.

ECSS is not a typical scientific conference; like Snowbird, its counterpart in the US, it is focused on professional and policy issues, and also a place to hear from technology leaders about their research visions. For me it is one of the most interesting events of the year.

References

[1] ECSS home page including advance program, here.

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Scopus’s view of computer science research

I posted on the Informatics Europe blog  a short note about what Scopus sees as the hottest articles in computer science.

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Specification explosion

To verify software, we must specify it; otherwise there is nothing to verify against. People often cite the burden of specification as the major obstacle toward making verification practical. At issue are not only the effort required to express the goals of software elements (their contracts) but also intermediate assertions, or “verification conditions”, including loop invariants, required by the machinery of verification.

At a Microsoft Software Verification summer school [1] in Moscow on July 18 — the reason why there was no article on this blog last week — Stefan Tobies, one of the lecturers, made the following observation about the specification effort needed to produce fully verified software. In his experience, he said, the ratio of specification lines to program lines is three to one.

Such a specification explosion, to coin a phrase, has to be addressed by any practical approach to verification. It would be interesting to get estimates from others with verification experience.

Reducing specification explosion  is crucial to the Eiffel effort to provide “Verification As a Matter Of Course” [2]. The following three techniques should go a long way:

  • Loop invariant inference. Programmers can be expected to write contracts expressing the purpose of routines (preconditions, postconditions) and classes (class invariants), but often balk at writing the intermediate assertions necessary to prove the correctness of loops. An earlier article [3] mentioned some ongoing work on this problem and I hope to come back to the topic.
  • Frame conventions. As another recent article has discussed [4], a simple language convention can dramatically reduce the number of assertions by making frame conditions explicit.
  • Model-based contracts. This technique calls for a separate article; the basic idea is to express the effect of operations through high-level mathematical models relying on a library that describe such mathematical abstractions as sets, relations, functions and graphs.

The risk of specification explosion is serious enough to merit a concerted defense.

 

References

[1] Summer School in Software Engineering and Verification, details here.

[2] Verification As a Matter Of Course, slides of a March 2010 talk, see an earlier article on this blog.

[3] Contracts written by people, contracts written by machines, an earlier article on this blog.

[4] If I’m not pure, at least my functions are, an earlier article on this blog.

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Towards a Calculus of Object Programs

I posted here a draft of a new article, Towards a Calculus of Object Programs.

Here is the abstract:

Verifying properties of object-oriented software requires a method for handling references in a simple and intuitive way, closely related to how O-O programmers reason about their programs. The method presented here, a Calculus of Object Programs, combines four components: compositional logic, a framework for describing program semantics and proving program properties; negative variables to address the specifics of O-O programming, in particular qualified calls; the alias calculus, which determines whether reference expressions can ever have the same value; and the calculus of object structures, a specification technique for the structures that arise during the execution of an object-oriented program.
The article illustrates the Calculus by proving the standard algorithm for reversing a linked list.

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Testing insights

Lionel Briand and his group at the Simula Research Laboratory in Oslo have helped raise the standard for empirical research in testing and other software engineering practices by criticizing work that in their opinion relies on wrong assumptions or insufficiently supported evidence. In one of their latest papers [1] they take aim at “Adaptive Random Testing” (ART); one of the papers they criticize is from our group at ETH, on the ARTOO extension [2] to this testing method. Let’s examine the criticism!

We need a bit of background on random testing, ART, and ARTOO:

  • Random testing tries inputs based on a random process rather than attempting a more sophisticated strategy; it was once derided as silly [3], but has emerged in recent years as a useful technique. Our AutoTest tool [4], now integrated in EiffelStudio, has shown it to be particularly effective when applied to code equipped with contracts, which provide built-in test oracles. As a result of this combination, testing can be truly automatic: the two most tedious tasks of traditional testing, test case preparation and test oracle definition, can be performed without human intervention.
  • ART, developed by Chen and others [5], makes random testing not entirely random by ensuring that the inputs are spread reasonably evenly in the input domain.
  • ARTOO, part of Ilinca Ciupa’s PhD thesis on testing defended in 2008,   generalized ART to object-oriented programs, by defining a notion of distance between objects; the ARTOO strategy  avoids choosing objects that are too close to each other. The distance formula, which you can find in[2], combines three elementary distances: between the types of the objects involved,  the values in their primitive fields (integers etc.), and, recursively, the objects to which they have references.

Arcuri and Briand dispute the effectiveness of ART and criticize arguments that various papers have used to show its effectiveness. About the ARTOO paper they write

The authors concluded that ART was better than random testing since it needed to sample less test cases before finding the first failure. However, ART was also reported as taking on average 1.6 times longer due to the distance calculations!

To someone not having read our paper the comment and the exclamation mark would seem to suggest that the paper somehow downplays this property of random testing, but in fact it stresses it repeatedly. The property appears for example in boldface as part of the caption to Table 2: In most cases ARTOO requires significantly less tests to find a fault, but entails a time overhead, and again in boldface in the caption to Table 3: The overhead that the distance calculations introduce in the testing process causes ARTOO to require on average 1.6 times more time than RAND to find the first fault.

There is no reason, then, to criticize the paper on this point. It reports the results clearly and fairly.

If we move the focus from the paper to the method, however, Arcuri and Briand have a point. As they correctly indicate, the number of tests to first fault is not a particularly useful criterion. In fact I argued against it in another paper on testing [6]

The number of tests is not that useful to managers, who need help deciding when to stop testing and ship, or to customers, who need an estimate of fault densities. More relevant is the testing time needed to uncover the faults. Otherwise we risk favoring strategies that uncover a failure quickly but only after a lengthy process of devising the test; what counts is total time. This is why, just as flies get out faster than bees, a seemingly dumb strategy such as random testing might be better overall.

(To understand the mention of flies and bees you need to read [6].) The same article states, as its final principle:

Principle 7: Assessment criteria A testing strategy’s most important property is the number of faults it uncovers as a function of time.

The ARTOO paper, which appeared early in our testing work, used “time to first failure” because it has long been a standard criterion in the testing literature, but it should have applied our own advice and focused on more important properties of testing strategies.

The “principles” paper [6] also warned against a risk awaiting anyone looking for new test strategies:

Testing research is vulnerable to a risky thought process: You hit upon an idea that seemingly promises improvements and follow your intuition. Testing is tricky; not all clever ideas prove helpful when submitted to objective evaluation.

The danger is that the clever ideas may result in so much strategy setup time that any benefit on the rest of the testing process is lost. This danger threatens testing researchers, including those who are aware of it.

The idea of ARTOO and object distance remains attractive, but more work is needed to make it an effective contributor to automated random testing and demonstrate that effectiveness. We can be grateful to Arcuri and Briand for their criticism, and I hope they continue to apply their iconoclastic zeal to empirical software engineering work, ours included.

I have objections of my own to their method. They write that “all the work in the literature is based either on simulations or case studies with unreasonably high failure rates”. This is incorrect for our work, which does not use simulations, relying instead on actual, delivered software, where AutoTest routinely finds faults in an automatic manner.

In contrast, however, Arcuri and Briand rely on fault seeding (also known as fault introduction or fault injection):

To obtain more information on how shapes appear in actual SUT, we carried out a large empirical analysis on 11 programs. For each program, a series of mutants were generated to introduce faults in these programs in a systematic way. Faults generated through mutation [allow] us to generate a large number of faults, in an unbiased and varied manner. We generated 3727 mutants and selected the 780 of them with lower detection probabilities to carry out our empirical analysis of faulty region shapes.

In the absence of objective evidence attesting to the realism of fault seeding, I do not believe any insights into testing obtained from such a methodology. In fact we adopted, from the start of our testing work, the principle that we would never rely on fault seeding. The problem with seeded faults is that there is no guarantee they reflect the true faults that programmers make, especially the significant ones. Techniques for fault seeding are understandably good at introducing typographical mistakes, such as a misspelling or the replacement of a “+” by a “-”; but these are not interesting kinds of fault, as they are easily caught by the compiler, by inspection, by low-tech static tools, or by simple tests. Interesting faults are those resulting from a logical error in the programmer’s mind, and in my experience (I do not know of good empirical studies on this topic) seeding techniques do not generate them.

For these reasons, all our testing research has worked on real software, and all the faults that AutoTest has found were real faults, resulting from a programmer’s mistake.

We can only apply this principle because we work with software equipped with contracts, where faults will be detected through the automatic oracle of a violated assertion clause. It is essential, however, to the credibility and practicality of any testing strategy; until I see evidence to the contrary, I will continue to disbelieve any testing insights resulting from studies based on artificial fault injection.

References

[1] Andrea Arcuri and Lionel Briand: Adaptive Random Testing: An Illusion of Effectiveness, in ISSTA 2011 (International Symposium on Software Testing and Analysis), available here.

[2] Ilinca Ciupa, Andreas Leitner, Manuel Oriol and Bertrand Meyer: ARTOO: Adaptive Random Testing for Object-Oriented Software, in ICSE 2008: Proceedings of 30th International Conference on Software Engineering, Leipzig, 10-18 May 2008, IEEE Computer Society Press, 2008, also available here.

[3] Glenford J. Myers. The Art of Software Testing. Wiley, New York, 1979. Citation:

Probably the poorest methodology of all is random-input testing: the process of testing a program by selecting, at random, some subset of all possible input values. In terms of the probability of detecting the most errors, a randomly selected collection of test cases has little chance of being an optimal, or close to optimal, subset. What we look for is a set of thought processes that allow one to select a set of test data more intelligently. Exhaustive black-box and white-box testing are, in general, impossible, but a reasonable testing strategy might use elements of both. One can develop a reasonably rigorous test by using certain black-box-oriented test-case-design methodologies and then supplementing these test cases by examining the logic of the program (i.e., using white-box methods).

[4] Bertrand Meyer, Ilinca Ciupa, Andreas Leitner, Arno Fiva, Yi Wei and Emmanuel Stapf: Programs that Test Themselves, IEEE Computer, vol. 42, no. 9, pages 46-55, September 2009, available here. For practical uses of AutoTest within EiffelStudio see here.

[5] T. Y. Chen, H Leung and I K Mak: Adaptive Random Testing, in  Advances in Computer Science, ASIAN 2004, Higher-Level Decision Making,  ed. M.J. Maher,  Lecture Notes in Computer Science 3321, Springer-Verlag, pages 320-329, 2004, available here.

[6] Bertrand Meyer: Seven Principles of Software testing, in IEEE Computer, vol. 41, no. 10, pages 99-101, August 2008, also available here.

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