Posts tagged ‘Wirth’

Niklaus Wirth and the Importance of Being Simple

[This is a verbatim copy of a post in the Communications of the ACM blog, 9 January 2024.]

I am still in shock from the unexpected death of Niklaus Wirth eight days ago. If you allow a personal note (not the last one in this article): January 11, two days from now, was inscribed in my mind as the date of the next time he was coming to my home for dinner. Now it is the date set for his funeral.


Niklaus Wirth at the ACM Turing centenary celebration
San Francisco, 16 June 2012
(all photographs in this article are by B. Meyer)

A more composed person would wait before jotting down thoughts about Wirth’s contributions but I feel I should do it right now, even at the risk of being biased by fresh emotions.

Maybe I should first say why I have found myself, involuntarily, writing obituaries of computer scientists: Kristen Nygaard and Ole-Johan Dahl, Andrey Ershov, Jean Ichbiah, Watts Humphrey, John McCarthy, and most recently Barry Boehm (the last three in this very blog). You can find the list with comments and links to the eulogy texts on the corresponding section of my publication page. The reason is simple: I have had the privilege of frequenting giants of the discipline, tempered by the sadness of seeing some of them go away. (Fortunately many others are still around and kicking!) Such a circumstance is almost unbelievable: imagine someone who, as a student and young professional, discovered the works of Galileo, Descartes, Newton, Ampère, Faraday, Einstein, Planck and so on, devouring their writings and admiring their insights — and later on in his career got to meet all his heroes and conduct long conversations with them, for example in week-long workshops, or driving from a village deep in Bavaria (Marktoberdorf) to Munich airport. Not possible for a physicist, of course, but exactly the computer science equivalent of what happened to me. It was possible for someone of my generation to get to know some of the giants in the field, the founding fathers and mothers. In my case they included some of the heroes of programming languages and programming methodology (Wirth, Hoare, Dijkstra, Liskov, Parnas, McCarthy, Dahl, Nygaard, Knuth, Floyd, Gries, …) whom I idolized as a student without every dreaming that I would one day meet them. It is natural then to should share some of my appreciation for them.

My obituaries are neither formal, nor complete, nor objective; they are colored by my own experience and views. Perhaps you object to an author inserting himself into an obituary; if so, I sympathize, but then you should probably skip this article and its companions and go instead to Wikipedia and official biographies. (In the same vein, spurred at some point by Paul Halmos’s photographic record of mathematicians, I started my own picture gallery. I haven’t updated it recently, and the formatting shows the limits of my JavaScript skills, but it does provide some fresh, spontaneous and authentic snapshots of famous people and a few less famous but no less interesting ones. You can find it here. The pictures of Wirth accompanying this article are taken from it.)


Niklaus Wirth, Barbara Liskov, Donald Knuth
(ETH Zurich, 2005, on the occasion of conferring honorary doctorates to Liskov and Knuth)

A peculiarity of my knowledge of Wirth is that unlike his actual collaborators, who are better qualified to talk about his years of full activity, I never met him during that time. I was keenly aware of his work, avidly getting hold of anything he published, but from a distance. I only got to know him personally after his retirement from ETH Zurich (not surprisingly, since I joined ETH because of that retirement). In the more than twenty years that followed I learned immeasurably from conversations with him. He helped me in many ways to settle into the world of ETH, without ever imposing or interfering.

I also had the privilege of organizing in 2014, together with his longtime colleague Walter Gander, a symposium in honor of his 80th birthday, which featured a roster of prestigious speakers including some of the most famous of his former students (Martin Oderski, Clemens Szyperski, Michael Franz…) as well as Vint Cerf. Like all participants in this memorable event (see here for the program, slides, videos, pictures…) I learned more about his intellectual rigor and dedication, his passion for doing things right, and his fascinating personality.

Some of his distinctive qualities are embodied in a book published on the occasion of an earlier event, School of Niklaus Wirth: The Art of Simplicity (put together by his close collaborator Jürg Gutknecht together with Laszlo Boszormenyi and Gustav Pomberger; see the Amazon page). The book, with its stunning white cover, is itself a model of beautiful design achieved through simplicity. It contains numerous reports and testimonials from his former students and colleagues about the various epochs of Wirth’s work.


Niklaus Wirth (right)
with F.L. Bauer, one of the founders of German computer science
Zurich,22 June 2005

Various epochs and many different topics. Like a Renaissance man, or one of those 18-th century “philosophers” who knew no discipline boundaries, Wirth straddled many subjects. It was in particular still possible (and perhaps necessary) in his generation to pay attention to both hardware and software. Wirth is most remembered for his software work but he was also a hardware builder. The influence of his PhD supervisor, computer design pioneer and UC Berkeley professor Harry Huskey, certainly played a role.

Stirred by the discovery of a new world through two sabbaticals at Xerox PARC (Palo Alto Research Center, the mother lode of invention for many of today’s computer techniques) but unable to bring the innovative Xerox machines to Europe, Wirth developed his own modern workstations, Ceres and Lilith. (Apart from the Xerox stays, Wirth spent significant time in the US and Canada: University of Laval for his master degree, UC Berkeley for his PhD, then Stanford, but only as an assistant professor, which turned out to be Switzerland’s and ETH’s gain, as he returned in 1968,)



Lilith workstation and its mouse
(Public display in the CAB computer science building at ETH Zurich)

One of the Xerox contributions was the generalized use of the mouse (the invention of Doug Englebart at the nearby SRI, then the Stanford Research Institute). Wirth immediately seized on the idea and helped found the Logitech company, which soon became, and remains today, a world leader in mouse technology.
Wirth returned to hardware-software codesign late in his career, in his last years at ETH and beyond, to work on self-driving model helicopters (one might say to big drones) with a Strong-ARM-based hardware core. He was fascinated by the goal of maintaining stability, a challenge involving physics, mechanical engineering, electronic engineering in addition to software engineering.
These developments showed that Wirth was as talented as an electronics engineer and designer as he was in software. He retained his interest in hardware throughout his career; one of his maxims was indeed that the field remains driven by hardware advances, which make software progress possible. For all my pride as a software guy, I must admit that he was largely right: object-oriented programming, for example, became realistic once we had faster machines and more memory.

Software is of course what brought him the most fame. I struggle not to forget any key element of his list of major contributions. (I will come back to this article when emotions abate, and will add a proper bibliography of the corresponding Wirth publications.) He showed that it was possible to bring order to the world of machine-level programming through his introduction of the PL/360 structured assembly language for the IBM 360 architecture. He explained top-down design (“stepwise refinement“), as no one had done before, in a beautiful article that forever made the eight-queens problem famous. While David Gries had in his milestone book Compiler Construction for Digital Computers established compiler design as a systematic discipline, Wirth showed that compilers could be built simply and elegantly through recursive descent. That approach had a strong influence on language design, as will be discussed below in relation to Pascal.

The emphasis simplicity and elegance carried over to his book on compiler construction. Another book with the stunning title Algorithms + Data Structures = Programs presented a clear and readable compendium of programming and algorithmic wisdom, collecting the essentials of what was known at the time.

And then, of course, the programming languages. Wirth’s name will forever remained tied to Pascal, a worldwide success thanks in particular to its early implementations (UCSD Pascal, as well as Borland Pascal by his former student Philippe Kahn) on microcomputers, a market that was exploding at just that time. Pascal’s dazzling spread was also helped by another of Wirth’s trademark concise and clear texts, the Pascal User Manual and Report, written with Kathleen Jensen. Another key component of Pascal’s success was the implementation technique, using a specially designed intermediate language, P-Code, the ancestor of today’s virtual machines. Back then the diversity of hardware architectures was a major obstacle to the spread of any programming language; Wirth’s ETH compiler produced P-Code, enabling anyone to port Pascal to a new computer type by writing a translator from P-Code to the appropriate machine code, a relatively simple task.

Here I have a confession to make: other than the clear and simple keyword-based syntax, I never liked Pascal much. I even have a snide comment in my PhD thesis about Pascal being as small, tidy and exciting as a Swiss chalet. In some respects, cheekiness aside, I was wrong, in the sense that the limitations and exclusions of the language design were precisely what made compact implementations possible and widely successful. But the deeper reason for my lack of enthusiasm was that I had fallen in love with earlier designs from Wirth himself, who for several years, pre-Pascal, had been regularly churning out new language proposals, some academic, some (like PL/360) practical. One of the academic designs I liked was Euler, but I was particularly keen about Algol W, an extension and simplification of Algol 60 (designed by Wirth with the collaboration of Tony Hoare, and implemented in PL/360). I got to know it as a student at Stanford, which used it to teach programming. Algol W was a model of clarity and elegance. It is through Algol W that I started to understand what programming really is about; it had the right combination of freedom and limits. To me, Pascal, with all its strictures, was a step backward. As an Algol W devotee, I felt let down.
Algol W played, or more precisely almost played, a historical role. Once the world realized that Algol 60, a breakthrough in language design, was too ethereal to achieve practical success, experts started to work on a replacement. Wirth proposed Algol W, which the relevant committee at IFIP (International Federation for Information Processing) rejected in favor of a competing proposal by a group headed by the Dutch computer scientist (and somewhat unrequited Ph.D. supervisor of Edsger Dijkstra) Aad van Wijngaarden.

Wirth recognized Algol 68 for what it was, a catastrophe. (An example of how misguided the design was: Algol 68 promoted the concept of orthogonality, roughly stating that any two language mechanisms could be combined. Very elegant in principle, and perhaps appealing to some mathematicians, but suicidal: to make everything work with everything, you have to complicate the compiler to unbelievable extremes, whereas many of these combinations are of no use whatsoever to any programmer!) Wirth was vocal in his criticism and the community split for good. Algol W was a casualty of the conflict, as Wirth seems to have decided in reaction to the enormity of Algol 68 that simplicity and small size were the cardinal virtues of a language design, leading to Pascal, and then to its modular successors Modula and Oberon.

Continuing with my own perspective, I admired these designs, but when I saw Simula 67 and object-oriented programming I felt that I had come across a whole new level of expressive power, with the notion of class unifying types and modules, and stopped caring much for purely modular languages, including Ada as it was then. A particularly ill-considered feature of all these languages always irked me: the requirement that every module should be declared in two parts, interface and implementation. An example, in my view, of a good intention poorly realized and leading to nasty consequences. One of these consequences is that the information in the interface part inevitably gets repeated in the implementation part. Repetition, as David Parnas has taught us, is (particularly in the form of copy-paste) the programmer’s scary enemy. Any change needs to be checked and repeated in both the original and the duplicate. Any bug needs to be fixed in both. The better solution, instead of the interface-implementation separation, is to write everything in one place (the class of object-oriented programming) and then rely on tools to extract, from the text, the interface view but also many other interesting views abstracted from the text.

In addition, modular languages offer one implementation for each interface. How limiting! With object-oriented programming, you use inheritance to provide a general version of an abstraction and then as many variants as you like, adding them as you see fit (Open-Closed Principle) and not repeating the common information. These ideas took me towards a direction of language design completely different from Wirth’s.

One of his principles in language design was that it should be easy to write a compiler — an approach that paid off magnificently for Pascal. I mentioned above the beauty of recursive-descent parsing (an approach which means roughly that you parse a text by seeing how it starts, deducing the structure that you expect to follow, then applying the same technique recursively to the successive components of the expected structure). Recursive descent will only work well if the language is LL (1) or very close to it. (LL (1) means, again roughly, that the first element of a textual component unambiguously determines the syntactic type of that component. For example the instruction part of a language is LL (1) if an instruction is a conditional whenever it starts with the keyword if, a loop whenever it starts with the keyword while, and an assignment variable := expression whenever it starts with a variable name. Only with a near-LL (1) structure is recursive descent recursive-decent.) Pascal was designed that way.

A less felicitous application of this principle was Wirth’s insistence on one-pass compilation, which resulted in Pascal requiring any use of indirect recursion to include an early announcement of the element — procedure or data type — being used recursively. That is the kind of thing I disliked in Pascal: transferring (in my opinion) some of the responsibilities of the compiler designer onto the programmer. Some of those constraints remained long after advances in hardware and software made the insistence on one-pass compilation seem obsolete.

What most characterized Wirth’s approach to design — of languages, of machines, of software, of articles, of books, of curricula — was his love of simplicity and dislike of gratuitous featurism. He most famously expressed this view in his Plea for Lean Software article. Even if hardware progress drives software progress, he could not accept what he viewed as the lazy approach of using hardware power as an excuse for sloppy design. I suspect that was the reasoning behind the one-compilation-pass stance: sure, our computers now enable us to use several passes, but if we can do the compilation in one pass we should since it is simpler and leaner.
As in the case of Pascal, this relentless focus could be limiting at times; it also led him to distrust artificial intelligence, partly because of the grandiose promises its proponents were making at the time. For many years indeed, AI never made it into ETH computer science. I am talking here of the classical, logic-based form of AI; I had not yet had the opportunity to ask Niklaus what he thought of the modern, statistics-based form. Perhaps the engineer in him would have mollified his attitude, attracted by the practicality and well-defined scope of today’s AI methods. I will never know.

As to languages, I was looking forward to more discussions; while I wholeheartedly support his quest for simplicity, size to me is less important than simplicity of the structure and reliance on a small number of fundamental concepts (such as data abstraction for object-oriented programming), taken to their full power, permeating every facet of the language, and bringing consistency to a powerful construction.

Disagreements on specifics of language design are normal. Design — of anything — is largely characterized by decisions of where to be dogmatic and where to be permissive. You cannot be dogmatic all over, or will end with a stranglehold. You cannot be permissive all around, or will end with a mess. I am not dogmatic about things like the number of compiler passes: why care about having one, two, five or ten passes if they are fast anyway? I care about other things, such as the small number of basic concepts. There should be, for example, only one conceptual kind of loop, accommodating variants. I also don’t mind adding various forms of syntax for the same thing (such as, in object-oriented programming, x.a := v as an abbreviation for the conceptually sound x.set_a (v)). Wirth probably would have balked at such diversity.

In the end Pascal largely lost to its design opposite, C, the epitome of permissiveness, where you can (for example) add anything to almost anything. Recent languages went even further, discarding notions such as static types as dispensable and obsolete burdens. (In truth C is more a competitor to P-Code, since provides a good target for compilers: its abstraction level is close to that of the computer and operating system, humans can still with some effort decipher C code, and a C implementation is available by default on most platforms. A kind of universal assembly language. Somehow, somewhere, the strange idea creeped into people’s minds that it could also be used as a notation for human programmers.)

In any case I do not think Niklaus followed closely the evolution of the programming language field in recent years, away from principles of simplicity and consistency; sometimes, it seems, away from any principles at all. The game today is mostly “see this cute little feature in my language, I bet you cannot do as well in yours!” “Oh yes I can, see how cool my next construct is!“, with little attention being paid to the programming language as a coherent engineering construction, and even less to its ability to produce correct, robust, reusable and extendible software.

I know Wirth was horrified by the repulsive syntax choices of today’s dominant languages; he could never accept that a = b should mean something different from b = a, or that a = a + 1 should even be considered meaningful. The folly of straying away from conventions of mathematics carefully refined over several centuries (for example by distorting “=” to mean assignment and resorting to a special symbol for equality, rather than the obviously better reverse) depressed him. I remain convinced that the community will eventually come back to its senses and start treating language design seriously again.

One of the interesting features of meeting Niklaus Wirth the man, after decades of studying from the works of Professor Wirth the scientist, was to discover an unexpected personality. Niklaus was an affable and friendly companion, and most strikingly an extremely down-to-earth person. On the occasion of the 2014 symposium we were privileged to meet some of his children, all successful in various walks of life: well-known musician in the Zurich scene, specialty shop owner… I do not quite know how to characterize in words his way of speaking (excellent) English, but it is definitely impossible to forget its special character, with its slight but unmistakable Swiss-German accent (also perceptible in German). To get an idea, just watch one of the many lecture videos available on the Web. See for example the videos from the 2014 symposium mentioned above, or this full-length interview recorded in 2018 as part of an ACM series on Turing Award winners.

On the “down-to-earth” part: computer scientists, especially of the first few generations, tend to split into the mathematician types and the engineer types. He was definitely the engineer kind, as illustrated by his hardware work. One of his maxims for a successful career was that there are a few things that you don’t want to do because they are boring or feel useless, but if you don’t take care of them right away they will come back and take even more of your time, so you should devote 10% of that time to discharge them promptly. (I wish I could limit that part to 10%.)

He had a witty, subtle — sometimes caustic — humor. Here is a Niklaus Wirth story. On the seventh day of creation God looked at the result. (Side note: Wirth was an atheist, which adds spice to the choice of setting for the story.) He (God) was pretty happy about it. He started looking at the list of professions and felt good: all — policeman, minister, nurse, street sweeper, interior designer, opera singer, personal trainer, supermarket cashier, tax collector… — had some advantages and some disadvantages. But then He got to the University Professor row. The Advantages entry was impressive: long holidays, decent salary, you basically get to do what you want, and so on; but the Disadvantages entry was empty! Such a scandalous discrepancy could not be tolerated. For a moment, a cloud obscured His face. He thought and thought and finally His smile came back. At that point, He had created colleagues.

When the computing world finally realizes that design needs simplicity, it will do well to go back to Niklaus Wirth’s articles, books and languages. I can think of only a handful of people who have shaped the global hardware and software industry in a comparable way. Niklaus Wirth is, sadly, sadly gone — and I still have trouble accepting that he will not show up for dinner, on Thursday or ever again — but his legacy is everywhere.

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Niklaus Wirth birthday symposium, 20 February, Zurich

In honor of Niklaus Wirth’s 80-th birthday we are organizing a symposium at ETH on February 20, 2014. This is a full-day event with invited talks by:

  • Vint Cerf
  • Hans Eberlé
  • Michael Franz
  • me
  • Carroll Morgan
  • Martin Odersky
  • Clemens Szyperski
  • Niklaus Wirth himself

From the symposium’s web page:

Niklaus Wirth was a Professor of Computer Science at ETH Zürich, Switzerland, from 1968 to 1999. His principal areas of contribution were programming languages and methodology, software engineering, and design of personal workstations. He designed the programming languages Algol W, Pascal, Modula-2, and Oberon, was involved in the methodologies of structured programming and stepwise refinement, and designed and built the workstations Lilith and Ceres. He published several text books for courses on programming, algorithms and data structures, and logical design of digital circuits. He has received various prizes and honorary doctorates, including the Turing Award, the IEEE Computer Pioneer, and the Award for outstanding contributions to Computer Science Education.

Participation is free (including breaks, lunch and the concluding “Apéro”) but space is strictly limited and we expect to run out of seats quickly. So if you are interested (but only if you are certain to attend) please register right away.

Symposium page and access to registration form: here.

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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.


[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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In praise of Knuth and Liskov

In November of 2005, as part of the festivities of its 150-th anniversary, the ETH Zurich bestowed honorary doctorates on Don Knuth and Barbara Liskov. I gave the speech (the “laudatio”). It was published in Informatik Spektrum, the journal of Gesellschaft für Informatik, the German Computer Society, vo. 29, no. 1, February 2006, pages 74-76; I came across it recently and thought others might be interested in this homage to two great computer scientists.  The beginning was in German; I translated it into English. I also replaced a couple of German expressions by their translations: “ETH commencement” for ETH-Tag (the official name of the annual ceremony) and “main building” for Hauptgebäude.

I took this picture of Wirth, Liskov and Knuth (part of my gallery of computer scientists)  later that same day.



 In an institution, Ladies and Gentlement, which so proudly celebrates its hundred-and-fiftieth anniversary, a relatively young disciplines sometimes has cause for envy. We computer scientists are still the babies, or at least the newest kids on the block. Outside of this building, for example, you will see streets bearing such names as Clausius, yet there is neither a Von Neumann Lane nor a a Wirth Square. Youth, however,  also has its advantages; perhaps the most striking is that we still can, in our own lifetime, meet in person some of the very founders of our discipline. No living physicist has seen Newton; no chemist has heard Lavoisier. For us, it works. Today, Ladies and Gentlemen, we have the honor of introducing two of the undisputed pioneers of informatics.

Barbara Liskov

The first of our honorees today is Professor Barbara Liskov. To understand her contributions it is essential to realize the unfair competition in which the so-called Moore’s law pits computer software against computing hardware. To match the astounding progress of computing speed and memory over the past five decades, all that we have on the software side is our own intelligence which, it is safe to say, doesn’t double every eighteen months at constant price. The key to scaling up is abstraction; all advances in programming methodology have relied on new abstraction techniques. Perhaps the most significant is data abstraction, which enables us to organize complex systems on the basis of the types of objects they manipulate, defined in completely abstract terms. This is the notion of abstract data type, a staple component today of every software curriculum, including in the very first programming course here ETH. it was introduced barely thirty years ago in a seemingly modest article in SIGPLAN Notices — the kind of publication that hardly registers a ripple in science indexes — by Barbara Liskov and Stephen Zilles. Few papers have had a more profound impact on the theory and practice of software development than this contribution, “Programming with Abstract Data Types”.

The idea of abstract data types, or ADTs, is one of those Egg of Christopher Columbus moments; a seemingly simple intuition that changes the course of things. An ADT is a class of objects described in terms not of their internal properties, but of the operations applicable to them, and the abstract properties of these operations. Not by what they are, but by what they have. A rather capitalistic view of the world, but well suited to the description of complex systems where each part knows as little as possible about the others to protect itself about their future changes.

An abstraction such as ETH-Commencement could be described in a very concrete way: it happens in a certain place, consists of one event after another, gathers so many people. This is what we computer scientists call an implementation-oriented view, and relying on it means that we can’t change any detail without endangering the consistency of other processes, such as the daily planning of room allocation in the Main Building, which use it. In an ADT view, the abstraction “ETH Commencement” is characterized not by what it is but by what it has: a start, an end, an audience, and operations such as “Schedule the ETH Commencement”, “ Reschedule it”, “Start it”, “End it”. They provide to the rest of the world a clean, precisely specified interface which enables every ADT to use every other based on the minimum properties it requires, thus isolating them from irrelevant internal changes, and providing an irreplaceable weapon in the incessant task of software engineering: battling complexity.

Barbara Liskov didn’t stay with the theoretical concepts but implemented the ideas in the CLU language, one of the most influential of the set of programming languages that in the nineteen-seventies changed our perspective of how to develop good software.

She went on to seminal work on operating systems and distributed computing, introducing several widely applied concepts such as guardians, and always backing her theoretical innovations by building practical systems, from the CLU language and compiler to the Argus and Mercury distributed operating systems. Distributed systems, such as those which banks, airlines and other global enterprises run on multiple machines across multiple networks, raise particularly challenging issues. To quote from the introduction of her article on Argus:

A centralized system is either running or crashed, but a distributed system may be partly running and partly crashed. Distributed programs must cope with failures of the underlying hardware. Both the nodes and the network may fail. The goal of Argus is to provide mechanisms that make it easier for programmers to cope with these problems.

Barbara Liskov’s work introduced seminal concepts to deal with these extremely difficult problems.

Now Ford professor of engineering at MIT, she received not long ago the prestigious John von Neumann award of the IEEE; she has been one of the most influential people in software engineering. We are grateful for how Professor Barbara Liskov has helped shape the field are honored to have her at ETH today.

 Donald Knuth

In computer science and beyond, the name of Donald Knuth carries a unique aura. A professor at Stanford since 1968, now emeritus, he is the only person on record whose job title is the title of his own book: Professor of the Art of Computer Programming. This is for his eponymous multi-volume treatise, which established the discipline of algorithm analysis, and has had more effect than any other computer science publication. The Art of Computer Programming is a marvel of breadth, depth, completeness, mathematical rigor and clarity, not to forget humor. In that legendary book you will find exposed in detail the algorithms and data structures that lie at the basis of all software applications today. A Monte Carlo simulation, as a physicists may use, requires a number sequence that is both very long and very random-looking, even though the computer is a deterministic machine; if the simulation is any good, it almost certainly relies on the devious techniques which The Art of Computer Programming presents for making a perfectly deterministic sequence appear to have no order or other recognizable property. If you are running complex programs on your laptop, and they keep creating millions of software objects without clogging up gigabytes of memory, chances are the author of the garbage collector program is using techniques he learned from Knuth, with such delightful names as “the Buddy System”. If your search engine can at the blink of an eye find a needle of useful information in a haystack of tens of billions of Web pages, it’s most likely because they’ve been indexed using finely tuned data structures, such as hash tables, for which Knuth has been the reference for three decades through volume three, Searching and Sorting.

Knuth is famous for his precision and attention to detail, going so far as to offer a financial reward for every error found in his books, although one suspects this doesn’t cost him too much since people are so proud that instead of cashing the check they have it framed for display. The other immediately striking characteristic of Knuth is how profoundly he is driven by esthetics. This applies to performing arts, as anyone who was in the Fraumünster this morning and found out who the organist was can testify, but even more to his scientific work. The very title “the Art of computer programming” betrays this. Algorithms and data structures for Knuth are never dull codes for computers, but objects of intense esthetic pleasure and friendly discussion. This concern with beauty led to a major turn in his career, which delayed the continuation of the book series by many years but resulted in a development that has affected anyone who publishes scientific text. As he received the page proofs of the second edition of one of the volumes in the late seventies he was so repelled by its physical appearance, resulting from newly introduced computer typesetting technology, that he decided to build a revolutionary font design and text processing system, all by himself, from the ground up. This resulted in a number of publications such as a long and fascinating paper in the Bulletin of the American Mathematical Society entitled “The Letter S”, but even more importantly in widely successful and practical software programs which he wrote himself, TeX and Metafont, which have today become standards for scientific publishing. Here too he has shown the way in quality and rigor, being one of the very few people in the world who promise their software to be free of bugs, and backs that promise by giving a small financial reward for any counter-example.

His numerous other contributions are far too diverse to allow even a partial mention here; they have ranged across wide areas of computer science and mathematics.

To tell the truth, we are a little embarrassed that by bringing Professor Knuth here we are delaying by a bit more the long awaited release of volume 4. But we overcome this embarrassment in time to express our pride for having Donald Erwin Knuth at ETH for this anniversary celebration.

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“Touch of Class” published

My textbook Touch of Class: An Introduction to Programming Well Using Objects and Contracts [1] is now available from Springer Verlag [2]. I have been told of many bookstores in Europe that have it by now; for example Amazon Germany [3] offers immediate delivery. Amazon US still lists the book as not yet published [4], but I think this will be corrected very soon.


The book results from six years of teaching introductory programming at ETH Zurich. It is richly illustrated in full color (not only with technical illustrations but with numerous photographs of people and artefacts). It is pretty big, but designed so that a typical one-semester introductory course can cover most of the material.

Many topics are addressed (see table of contents below), including quite a few that are seldom seen at the introductory level. Some examples, listed here in random order: a fairly extensive introduction to software engineering including things like requirements engineering (not usually mentioned in programming courses, with results for everyone to see!) and CMMI, a detailed discussion of how to implement recursion, polymorphism and dynamic binding and their role for software architecture, multiple inheritance, lambda calculus (at an introductory level of course), a detailed analysis of the Observer and Visitor patterns, event-driven programming, the lure and dangers of references and aliasing, topological sort as an example of both algorithm and API design, high-level function closures, software tools, properties of computer hardware relevant for programmers, undecidability etc.

The progression uses an object-oriented approach throughout; the examples are in Eiffel, and four appendices present the details of Java, C#, C++ and C. Concepts of Design by Contract and rigorous development are central to the approach; for example, loops are presented as a technique for computing a result by successive approximation, with a central role for the concept of loop invariant. This is not a “formal methods” book in the sense of inflicting on the students a heavy mathematical apparatus, but it uses preconditions, postconditions and invariants throughout to alert them to the importance of reasoning rigorously about programs. The discussion introduces many principles of sound design, in line with the book’s subtitle, “Learning to Program Well”.

The general approach is “Outside-In” (also known as “Inverted Curriculum” and described at some length in some of my articles, see e.g. [5]): students have, right from the start, the possibility of working with real software, a large (150,000-line) library that has been designed specifically for that purpose. Called Traffic, this library simulates traffic in a city; it is graphical and of good enough visual quality to be attractive to today’s “Wii generation” students, something that traditional beginners’ exercises, like computing the 7-th Fibonacci number, cannot do (although we have these too as well). Using the Traffic software through its API, students can right from the first couple of weeks produce powerful applications, without understanding the internals of the library. But they do not stop there: since the whole thing is available in open source, students learn little by little how the software is made internally. Hence the name “Outside-In”: understand the interface first, then dig into the internals. Two advantages of the approach are particularly worth noting:

  • It emphasizes the value of abstraction, and particular contracts, not by preaching but by showing to students that abstraction helps them master a large body of professional-level software, doing things that would otherwise be unthinkable at an introductory level.
  • It addresses what is probably today the biggest obstacle to teaching introductory programming: the wide diversity of initial student backgrounds. The risk with traditional approaches is either to fly too high and lose the novices, or stay too low and bore those who already have programming experience. With the Outside-In method the novices can follow the exact path charted from them, from external API to internal implementation; those who already know something about programming can move ahead of the lectures and start digging into the code by themselves for information and inspiration.

(We have pretty amazing data on students’ prior programming knowledge, as  we have been surveying students for the past six years, initially at ETH and more recently at the University of York in England thanks to our colleague Manuel Oriol; some day I will post a blog entry about this specific topic.)

The book has been field-tested in its successive drafts since 2003 at ETH, for the Introduction to Programming course (which starts again in a few weeks, for the first time with the benefit of the full text in printed form). Our material, such as a full set of slides, plus exercises, video recordings of the lectures etc. is available to any instructor selecting the text. I must say that Springer did an outstanding job with the quality of the printing and I hope that instructors, students, and even some practitioners already in industry will like both form and content.

Table of contents

Front matter: Community resource, Dedication (to Tony Hoare and Niklaus Wirth), Prefaces, Student_preface, Instructor_preface, Note to instructors: what to cover?, Contents

PART I: Basics
1 The industry of pure ideas
2 Dealing with objects
3 Program structure basics
4 The interface of a class
5 Just Enough Logic
6 Creating objects and executing systems
7 Control structures
8 Routines, functional abstraction and information hiding
9 Variables, assignment and references
PART II: How things work
10 Just enough hardware
11 Describing syntax
12 Programming languages and tools
PART III: Algorithms and data structures
13 Fundamental data structures, genericity, and algorithm complexity
14 Recursion and trees
15 Devising and engineering an algorithm: Topological Sort
PART IV: Object-Oriented Techniques
16 Inheritance
17 Operations as objects: agents and lambda calculus
18 Event-driven design
PART V: Towards software engineering
19 Introduction to software engineering
PART VI: Appendices
A An introduction to Java (from material by Marco Piccioni)
B An introduction to C# (from material by Benjamin Morandi)
C An introduction to C++ (from material by Nadia Polikarpova)
D From C++ to C
E Using the EiffelStudio environment
Picture credits


[1] Bertrand Meyer, Touch of Class: An Introduction to Programming Well Using Objects and Contracts, Springer Verlag, 2009, 876+lxiv pages, Hardcover, ISBN: 978-3-540-92144-8.

[2] Publisher page for [1]: see  here. List price: $79.95. (The page says “Ships in 3 to 4 weeks” but I think this is incorrect as the book is available; I’ll try to get the mention corrected.)

[3] page: see here. List price: EUR 53.45 (with offers starting at EUR 41.67).

[4] page: see here. List price: $63.96.

[5] Michela Pedroni and Bertrand Meyer: The Inverted Curriculum in Practice, in Proceedings of SIGCSE 2006, ACM, Houston (Texas), 1-5 March 2006, pages 481-485; available online.

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