In fact, I'd be very interested to know what Wirth thinks about Smalltalk; I haven't read a statement from him about it anywhere, or heard anything from him about it in the many interviews; interestingly, Kay asked him a question at the Turing Price lecture and even introduced himself by name, and you didn't get the impression that Wirth knew him.
I also find it remarkable that while Wirth refers to the windows concept and Smalltalk in the HOPL paper, which was written as recently as 2005, Cedar does not use overlapping windows at all, but rather - like the Oberon system - a tiling window manager; the Oberon user interface looks very similar to Cedar and has little resemblance to the Smalltalk-80 user interface.
There's a hint of what he thinks about Smalltalk in his book "Programming with Oberon" (co-authored with Martin Reiser). See sections 12.6.1 and, in particular, 12.6.2 (pages 236-239): https://people.inf.ethz.ch/wirth/ProgInOberonWR.pdf
Thanks for the reference; unfortunately we don't know who of the two authors has written the paragraph, and it is also not very specific. Meanwhile I found the following paragraph in Wirth's tutorial titled "Programming in Oberon" (of which a version can be downloaded from https://people.inf.ethz.ch/wirth/Oberon/PIO.pdf, and the first version of which apparently is from 2004); on page 57 we can read:
"The designers of Smalltalk clearly wished to present not only a new language, but also a new
paradigm, a new approach to programming. To be effective and convincing in this endeavor, they
provided also a new terminology to underscore a different quality of programming. In this effort,
record-structured variables became objects, their associated procedures became methods, a data
type became a class, and calling a procedure is now termed sending a message to an object. This is
denoted as object.method(message)"
It is interesting to note that "object.method(message)" has nothing to do with Smalltalk syntax at all (which supports my assumption that he had little to no personal contact with it). In his 1988 papers Smalltalk is just referenced as one of many OO languages, next to Simula 67, ObjectPascal and C++.
§§12.6.1–2 are on pp. 253/338 to 256/338. Initially I went to the wrong pages. Definitely worth a read.
Interestingly his point (3) on p.219 (236/338) ("Within the text of a specific action procedure, a regional type guard is required to allow access to the state variables defined in the extension of the base type") was precisely Abadí and Cardelli's motivation for defining the ς-calculus. This limitation means that Oberon's type system is insufficient to statically guarantee the safety of object methods ("action procedures") and this is an obviously desirable thing. He mentions another similar limitation on the Oberon type system on p. 237 (254/338): "The fact that the compiler cannot check whether a message is ‘understood by the object’ may be an advantage or a disadvantage, depending on the type of the application."
> In fact, I'd be very interested to know what Wirth thinks about Smalltalk; I haven't read a statement from him about it anywhere, or heard anything from him about it in the many interviews
I'd be interested to know as well. It might be worth writing him a physical letter asking about it. There is a mailing address for his department at ETH here (https://people.inf.ethz.ch/wirth/index.html) and I'm willing to bet they know how to forward him correspondence.
His home address and phone number are even in the phonebook, but I don't consider myself or the subject important enough to bother him with it. But maybe someone else here has insider information. That said, there's plenty of circumstantial evidence that he either didn't know Smalltalk in the nineties or didn't think it was worth mentioning. After all, Xerox themselves did not use Smalltalk in their products either, and even at PARC the other teams tended to work with BCPL or Mesa rather than Smalltalk. I read an interview with Larry Tesler where he describes that for a demonstration of a Smalltalk application they had to fast forward the movie so that the menus would pop up fast enough. Not surprisingly, Tesler then brought Object Pascal to life at Apple in collaboration with Wirth, instead of using Smalltalk as he had in his previous job (even though Apple had a license - that was used nota bene later for Squeak).
I also find it remarkable that while Wirth refers to the windows concept and Smalltalk in the HOPL paper, which was written as recently as 2005, Cedar does not use overlapping windows at all, but rather - like the Oberon system - a tiling window manager; the Oberon user interface looks very similar to Cedar and has little resemblance to the Smalltalk-80 user interface.