January 2021 - Oberon - mailman3.inf.ethz.ch

Text in V4.
by peter＠easthope.ca 10 Jan '21

10 Jan '21

Recently installed Linux V4 and looked at a few Mod files. One of the more obvious differences from ETH Oberon is the Text. V4 Text has what might be referred to as collapsible elements, marked by characters approximated as "><". "Collapsible" is borrowed from the similar idea in HTML5. https://www.w3schools.com/bootstrap/bootstrap_collapse.asp I don't know the official name in V4. Input welcome. Recognizing the distinct forms of Text, the section, https://en.wikibooks.org/wiki/Oberon#Oberon_Text , should be subordinate to ETH Oberon. V4 Text should be described separately. Again, input welcome. What other capabilities not in ETH Oberon Text, should be recognized in V4 Text? Thanks, ... P. L. -- Tel: +1 604 670 0140 Bcc: peter at easthope. ca

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Re: [Oberon] Java byte-code and Pascal P-Code (was: Text in V4.)
by Chris Burrows 10 Jan '21

10 Jan '21

> -----Original Message----- > From: Oberon [mailto:oberon-bounces@lists.inf.ethz.ch] On Behalf Of > Skulski, Wojciech > Sent: Monday, 11 January 2021 4:23 AM > To: ETH Oberon and related systems > Subject: Re: [Oberon] [EXT] Text in V4. > > I also vaguely remember that Java was inspired by System 3 and there > were references to it in the very early Java publications. But later > they disappeared. Perhaps unintentionally. > Java is a programming language, not an operating system. You may be thinking of claims that the Java 'byte-code' was inspired by Pascal's P-Code machine which was initially developed by Wirth's team at ETH for the portable Pascal-P compiler: https://homepages.cwi.nl/~steven/pascal/book/0intro.html According to Wikipedia this was based on Wirth's earlier work with the Euler compiler in 1965: https://en.wikipedia.org/wiki/Euler_(programming_language) as mentioned here: https://en.wikipedia.org/wiki/P-code_machine Most of its fame occurred later with its use in Pascal systems on microcomputers in the 1980's. e.g. the UCSD P-System and Apple Pascal. Subsequently, modified versions appeared in the form of M-code for Modula-2: https://www.cfbsoftware.com/modula2/ I believe an Oberon version, O-Code, was used in lectures at ETH in the 2000's. Regards, Chris Burrows CFB Software https://www.astrobe.com/RISC5

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First stable release v1.0 of Extended Oberon available
by Andreas Pirklbauer 09 Jan '21

09 Jan '21

I accidentally continued to run some scripts in the background that automatically updated Github with the latest version - I am sorry. Thanks for those who pointed this out. The script has now been removed, and so the release v1.0 is now (actually) “stable” ;-)

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Re: [Oberon] Compiler Construction by Niklaus Wirth
by peter＠easthope.ca 07 Jan '21

07 Jan '21

From: Paul Reed <paulreed(a)paddedcell.com> Date: Thu, 07 Jan 2021 10:29:21 +0000 > I completely agree. As a printed book, it's only 131 pages, and > wonderfully concise. ... > > It's really worth trying to get a physical copy - the act of picking it > up and turning it to see how thin it is, gives one a great deal of > courage. Abe Books provides access to many vendors. https://www.abebooks.com/ A search is easily performed with the results sorted by "Lowest Total Price". Currently the cheapest copy of _Compiler_Construction_ is 43.09 US$, delivery in the USA included. In the present economy a fair price. The next cheapest copy is 559.14 US$ and the third cheapest is a new copy at 1108.19 US$. Yes, a new softcover copy. I have no connection to the book selling business. Simply offering the information for anyone interested to acquire a copy as Paul suggests. Regards, ... P. L. -- Tel: +1 604 670 0140 Bcc: peter at easthope. ca

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LOOP conversion
by Andreas Pirklbauer 07 Jan '21

07 Jan '21

> I know that Andreas has earned a PhD in this field before > he wrote his little compiler. So I am asking seriously. Actually, I don’t hold a PhD. But all you need is to thoroughly read NW’s book Compiler Construction. Better than any PhD for this task!

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LOOP conversion
by Andreas Pirklbauer 07 Jan '21

07 Jan '21

> We are at your mercy. Maybe mercy will be enveloping me (or someone else!) one day ;-) Although the EOS compiler is still just under 3000 source lines of code, I feel that rather too many things have already been added to it already. We don’t want a second Modula-2..

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LOOP conversion
by Andreas Pirklbauer 07 Jan '21

07 Jan '21

> I could understand this direction if Andreas told us that adopting his compiler > is plainly impossible. This would be a technical necessity. I can understand this. I have a sneaking suspicion that the participants in this discussion have (probably?) already spent more time on this question than the time it would take to add LOOP to the compiler ;-)

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Re: [Oberon] LOOP conversion
by Chris Burrows 07 Jan '21

07 Jan '21

> -----Original Message----- > From: Oberon [mailto:oberon-bounces@lists.inf.ethz.ch] On Behalf Of > August Karlstrom > Sent: Thursday, 7 January 2021 6:43 AM > To: oberon(a)lists.inf.ethz.ch > Subject: Re: [Oberon] Re (2): LOOP conversion > > On 2021-01-06 19:41, peter(a)easthope.ca wrote: > > Unrefined oberservations. > > > > (1) Isomorphism exists between LOOP texts and WHILE texts. > > > > (2) Loops can be nested in a tree structure. At each level there > can > > be multiple loops and the number of levels is arbitrary. > > > > (3) In the general case a continuation flag is needed for each > level. > > Eg. VAR continue0, continue1 ... : BOOLEAN; > > > > (4) A LOOP having only one exit in the form "LOOP IF a THEN EXIT > END; > > ... END" can be replaced with "WHILE ~a DO ... END". > > > > (5) A LOOP having only one exit in the form "LOOP ... IF a THEN > EXIT > > END END" can be replaced with "REPEAT ... UNTIL a". > > > > (6) Pattern "LOOP statements0 IF a THEN EXIT END; statements1 END" > > becomes "continue := TRUE; WHILE continue DO; statements0 IF a THEN > > continue := FALSE ELSE statements1 END END". > > We should also note that this is only the first step to make the > program compile. If comprehensibility is the goal the variables named > "continue", "done" etc. should ideally be replaced with something > more descriptive and specific. When skimming through the program a > loop like > > continue := TRUE; > WHILE continue DO > ... > END; > > is not very informative. After the loop we know that `continue' is > false but what does that imply? > Indeed! While it is true that LOOPs can be mechanically (or even automatically) converted by techniques like those listed, the end result is not going to be much of an improvement on the original, and can even be less comprehensible. If you encounter a LOOP with multiple EXITs it should be treated as a warning: 'BEWARE: Quick and dirty code ahead!'. If possible the code should be 'refactored'. This involves going back to square one (the requirements) and redesigning the algorithm. Here is a real-life example that I tripped over in my own Component Pascal (a superset of Oberon-2) code yesterday. Yes - I can be lazy too when the system allows me to be :( In the spirit of 'practise what you preach' I decided to refactor it to eliminate the LOOP. The objective is to write an array of characters (s), to a file, ensuring that a NULL termination character is always written, even if the array doesn't include one: i := 0; LOOP ch := s[i]; Write(r, ch); INC(i); IF ch = 0X THEN EXIT END ; IF i = LEN(s) THEN Write(r, 0X); EXIT END END If this was mechanically converted the result would be something like this: i := 0; continue := TRUE; REPEAT ch := s[i]; Write(r, ch); INC(i); IF ch = 0X THEN continue := FALSE END ; IF continue THEN IF i = LEN(s) THEN Write(r, 0X); continue := FALSE END END UNTIL continue I can see why observers might scratch their heads and not be convinced that this was any improvement. After some consideration, my cleaner solution is: i := 0; REPEAT IF i = LEN(s) THEN ch := 0X ELSE ch := s[i] END; Write(f, ch); INC(i) UNTIL ch = 0X P.S. One of the nice, subtle, improvements of Oberon-07 over Component Pascal / Oberon-2 is noted in an additional sentence in Section 9.1 of the Oberon Language Report: "Strings can be assigned to any array of characters, provided the number of characters in the string is LESS than that of the array. (A NULL CHARACTER IS APPENDED)". Hence, if I was writing this in Oberon-07 it might be possible to simplify it further to: i := 0; REPEAT ch := s[i]; Write(f, ch); INC(i) UNTIL ch = 0X Regards, Chris Burrows CFB Software https://www.astrobe.com

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Re (2): Re (2): LOOP conversion
by peter＠easthope.ca 06 Jan '21

06 Jan '21

From: August Karlstrom <fusionfile(a)gmail.com> Date: Wed, 6 Jan 2021 21:13:15 +0100 > We should also note that this is only the first step to make the program > compile. I agree August. I've been using A2 since it was first released. I appreciate the need for testing and auditing of all code. > If comprehensibility is the goal the variables named > "continue", "done" etc. should ideally be replaced with something more > descriptive and specific. When skimming through the program a loop like > > continue := TRUE; > WHILE continue DO > ... > END; > > is not very informative. After the loop we know that `continue' is false > but what does that imply? Yes. Any suggestion for a name better than continue is welcome. By the way, I left extra comments to facilitate comparison to the LOOP form. Another criticism of my conversion: the new Menu procedure occupies about 1.3 screen height. Very difficult to read and understand. I might find a way to factor segments out as subroutines. The overall objective is to find a way to bring an older system such as ETH Oberon or V4 into 2021. Regards, ... P. L. -- Tel: +1 604 670 0140 Bcc: peter at easthope. ca

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Re (2): LOOP conversion
by peter＠easthope.ca 06 Jan '21

06 Jan '21

From: "Chris Burrows" <chris(a)cfbsoftware.com> Date: Mon, 4 Jan 2021 20:49:56 +1030 > Here's a trickier example ... found in ETH Oberon - TextFrames.Menu: Hello Chris & all, My solution is 7th row from the bottom, 2nd column of the table. https://en.wikibooks.org/wiki/Oberon/A2#Sources,_Tools_and_Configuration_Te… Mulled it over during supper; then tried some typing with little progress. Before taking it up again realized a better approach, for me at least, was to convert all the LOOPs in the module beginning with the easiest. You can see my conversions of procedures LocateString, LocateChar, FlipSelection and Menu. LocateString was easy enough. > I'd be interested to know how long it takes you ... To convert LocateString, LocateChar and FlipSelection about 5 hours total. One blunder necessitating a backtrack. About 2 hr and 45 minutes to convert Menu. Worked in small steps, compiling and testing at each step. Unrefined oberservations. (1) Isomorphism exists between LOOP texts and WHILE texts. (2) Loops can be nested in a tree structure. At each level there can be multiple loops and the number of levels is arbitrary. (3) In the general case a continuation flag is needed for each level. Eg. VAR continue0, continue1 ... : BOOLEAN; (4) A LOOP having only one exit in the form "LOOP IF a THEN EXIT END; ... END" can be replaced with "WHILE ~a DO ... END". (5) A LOOP having only one exit in the form "LOOP ... IF a THEN EXIT END END" can be replaced with "REPEAT ... UNTIL a". (6) Pattern "LOOP statements0 IF a THEN EXIT END; statements1 END" becomes "continue := TRUE; WHILE continue DO; statements0 IF a THEN continue := FALSE ELSE statements1 END END". > ... time taken to prove to yourself that your conversion is > identical in functionality. Note (1) but I don't have a formal proof. Once I recognized the conversion algorithm it was just a matter of doing it properly. Of course dopey errors are easily made. Automation is the answer. Digression In a 2nd year course I learned a little about proof of correct implementation. How much of the ETH Oberon, V4 and etc. source was proven formally? I'd guess not much but might be wrong. Regards, ... P. L. -- Tel: +1 604 670 0140 Bcc: peter at easthope. ca

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