QED

QED is a text editor written circa 1967 by L. Peter Deutsch and Butler W. Lampson at Berkley for the SDS-930 computer. It was the inspiration for the unix text editor ed).

This is a c implementation I made for fun, and isn’t completely finished yet. I did not have access to any source code for the original in making this, and it is based primarily on the 1968 DARPA manual by Angluin and Deutsch, with gaps filled in by (in roughly descending order) the 1967 Communications of the ACM article “An Online Editor“ by Deutsch and Lampson, the behavior of ed, and my own common sense. The goal is to reproduce the user-visible behavior of QED rather than its internal workings, even when there are sources that discuss that topic.

To get into the spirit of things, I wrote it entirely using ed and, as it matured, qed itself (with less sometimes used to look at the code).

Compiling and Running

It requires no external libraries apart from ones that come with c. You can compile it with:

cc qed.c -o qed

and install it with:

sudo mv qed /usr/local/bin/

It does assume you have a Unicode-compatible terminal to render the up-arrow and left-arrow glyphs shown in the manual. I’ve tried to reproduce the experience of using QED as closely as I can in a modern unix environment, but a few changes from the original have been necessary:

• From the manual it appears that the machine this originally ran on was upper-case only. This version has no limitation about editing upper- and lower-case text.
• The manual also suggests that the environment the original QED ran on had a @CONTINUE feature that seems to have allowed the user to “un-quit” the last program that was running, I assume as long as nothing else had been run in the interim that might have overwritten the first program’s memory. The original relied on this in place of any kind of “there are unsaved changes, are you sure you want to quit?” warning; instead it just let you know there were unsaved changes with WRITE OUT and then quit anyway, since you could just @CONTINUE qed if you did actually want to save. Since unix doesn’t have anything like @CONTINUE, my plan is currently that if the user quits without saving, qed will create a save file, maybe ~/.qed_save, with some representation of the program state, and that launching qed again with -c will restore from the save file. I haven’t implemented any of that yet, though.

Apart from these, I have not implemented any features not found in the manual or the article.

To do

Here’s a rough list of things that I haven’t finished implementing yet:

• The TABS command that sets the tab stops. Currently it just inserts ^I tabs when the user presses the tab key. The original appears to have only supported indentation via spaces; if you wanted an actual tab you’d have to do ^V+tab
• The ^K editing key that puts in “comments”, i.e. keeps echoing the user’s keystrokes but doesn’t put them into the buffer until the user presses ^K again
• The ^L special buffer key that copies subsequently-entered text into buffer 1 (I almost thought, given that the command is ^L, that the buffer it used was the letter “ell” not the digit “one” - the two characters are indistinguishable in the typewriter font the manual is written in - but then remembered that QED did not print lower-case letters so in the examples shown in the manual it would have printed #L and not #l if that was the buffer being addressed)
• The ^Y key to re-edit the current line
• In edit mode, ^Q is supposed to restore the original line as it was; it doesn’t do this yet, it just deletes the line you’ve typed the same as in insert mode
• ^Q is supposed to be able to delete multiple lines if you keep pressing it; currently it will only clear the current line
• It needs to print WRITE OUT! if you quit without saving
• I need to implement my -c replacement for @CONTINUE as mentioned above
• Buffer calls are not supposed to echo the buffer contents to the terminal, but they currently do

  Other QED Sources and Projects

  I started this project in 2015, forgot about it for a few years, then picked it up again in early 2019. In the interim some original sources for different versions of QED were published online, collected in the QED archive. These include assembly for the version I’ve been attempting to reproduce here. Once I’ve gotten this project to a point where it is (I think) feature complete, I might take a look (if I can find a reference on SDS-930 assembly), but as it stands this project is not based on that code. There is also a modern port of QED by Leah Neukirchen, but from the name and documentation it is of a later QED editor than the one I’m targetting here.