You can find the sources here: https://github.com/dtoffe/adsp-pl0
Some interesting notes:
- The original compiler does not define nor implements "read" or "write" statements, as was the norm in later PL/0 implementations. - The source code in the book corresponds to the implementation in the CDC 6000 that Wirth had at hand back then. That machine used the CDC Display Code:
https://en.wikipedia.org/wiki/CDC_display_code
a 6 bit character code from before the ASCII times. Among its 64 characters, it includes single symbols for "<>", "<=" and ">=". In an attempt to change the code the least to make it run as original as possible, I changed those three character to #, { and }, as the symbols in the lexer are implemented using an array of char (only the assignment is treated as a special case).
- The interpreter printing out the values of every variable assignment, noted by another reader of this post, was a way of getting some information of the running program, since the compiler does not implement read or write statements.
- Following the compiler code, full of single letter variable names, was not the most exciting part.
<- is somewhat better, but, again, for such a common operation, a single character is just more convenient. Sure, we could have editors that turn "=" into := or <- but now we're getting too fancy especially for something pedagogical.
I also don't mind the -> for C pointers; and certainly don't mind the <= >= or even == conventions (although at least today's compilers warn when they see "if (a=b) ...".
Ultimately, humans won't be writing code anymore anyway ( ;-) ?) so maybe the issue is entirely moot.
j = 5;
i = <|>
j = 5;
i = 0;
j = 5;
> I also don't mind the -> for C pointers
Mmm. These two opinions should be contradictory if held on principle as opposed being held out of impression.
it = next(it);
if ((*it)->node->op == EQ) ...
it := next(it);
if it.node.op = EQ ...
Technically, if you don't make assignment an expression, you can even get away with using "=" for both. And "->" exists only because structs originally weren't really typechecked; you could take any pointer and just do "->struct_field" at it, and the compiler would auto-cast.
That is sad, but the revised editions seem to be published online.
I learned first on Pascal & Modula-2 and only picked up C later and while I appreciated its terse minimalism at the time and through the 90s, I actually don't at all now. I find it less readable.
PL/0, PS/2, CP/M, etc.
And then Gary Kildall also seemed to like it with CP/M and PL/M, but those were after IBM had used it and I'd guess Gary was just copying IBM.
Between just those two influences you cover a huge portion of the mainframe and micro computer worlds during the 60s-80s