These files comes from the [PPL,MWK] and [PPL,SYS] directories on the Stanford AI Lab PDP-10.
MWK is Mark Kahrs, and here is his recollection on why he made a copy and why the files seem tailored for TENEX:
I read the manual from Harvard and thought it looked very cool. So, when I had a chance, I grabbed the files. I am guessing that I grabbed them from PARC since Ed Taft was part of the PPL project at Harvard with Tom Cheatham. And given that MAXC ran a version of TENEX, it all makes sense.
For completeness, a copy of the Unix version is also included.
According to Thomas Standish,
PPL was written on a DEC PDP-10 at Harvard (1969-1971) by Standish and Ed Taft. It was distributed to Brandeis, Cal Tech, Carnegie-Mellon, First Data Corp., Harvard, Irvine, Johns Hopkins, JPL, NSA, On-Line Systems, Princeton, Santa Barbara, Yale, TSC, and via the standard BBN-Tenex releases to Case, CCA, SRI, USC-ISI, Utah, Xerox PARC. PPL has been implemented on the PDP-11/45 for use in Harvard’s undergraduate education program. It was used from 1971 to 1983 as the principal educational language for undergraduate education at Harvard.
According to Ed Taft,
The language was designed by Tim Standish, based on work that he had done prior to becoming a professor at Harvard. He recruited me to write the PDP-10 implementation, which I did under his guidance. I worked on it sporadically during the school year and more intensively as a summer job working for Tim. He and I jointly wrote the PPL User's Manual. I eventually wrote a senior thesis, "Design and Implementation of PPL".
At Harvard, PPL was used as the programming language for an introductory programming class, NatSci 110, which was taught for many years. Initially, students ran their PPL programs on a commercial PDP-10 time-sharing service. Just before I graduated, a project was started to re-implement PPL for the PDP-11. Harvard had a PDP-11 that had many terminals and was more readily accessible to undergraduates. As I recall, the changes to the language were minor, but there were major improvements in the implementation. One of the key players was Alfred Spector, who now works for Google; I don't remember the other participants.
According to languages.info from TECO EMACS,
A language developed at Harvard for research in extensible languages, which turned into a pedagogic tool. It was used to teach programming systems to non-science majors, and was quite successful in this task. Its flavor is APL-ish, if you know APL; if not, then it's an interpretive, highly interactive language, with a great deal of power, allowing you to define new data types and operators, as well as performing in more mundane ways (e.g. like Basic). PPL is now in use at several universities, most of whom run it under UNIX, and is the subject of at least two books on introductory programming.
Note from RMS:
My first system programming experience was helping a little in implementing PPL. As far as I can see, any advantage it has over other languages for pedagogy lie in its interactive line-number-within-function editor, which somewhat resembles the APL editor. This is easier than using a standard printing terminal text editor, and more so for students. However, any display editor (such as EMACS) ought to even things out for other languages.
Aside from that, PPL does have the advantage of not having strong typing, so that the user is not burdened with data type declarations for variables. This makes less to worry about when writing a simple program. However, Lisp has the same advantage.