SWI-Prolog stream I/O and tab completion in the swipl-window

Question

What I'm Doing

I am currently working on creating a SWI-Prolog module that adds tab-completion capability to the swipl-win window. So far I've actually gotten it to where it reads a single character at a time without stopping/returning anything until a tab character is typed. I have also already written a predicate that returns all possible completions of an incompletely typed term by using substring-matching on a list of current terms (obtained via current_functor/2, current_arithmetic_function/1, current_predicate/2, etc [the predicate used will eventually be based off of context]).

If you want to see my code, it is here...just keep in mind that I'm not exactly a Prolog master yet (friendly tips are more than welcome).

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What I'm Thinking

I realize that when I actually implement my main completion predicate (still unwritten), I'll have to figure out what the last "word" is in the input stream. I'm debating on whether I should create a new stream with everything in the input stream so far (so I don't have to change the position in the input stream/go back to the beginning) or write to a string...if I take the second approach, I'll start over on the string whenever a delimiting character is inputted (characters that start a new "word", like space, comma, parentheses, operators, etc.) so there won't be any searching through the stream every time tab is pressed.

However, there is another thing: When the user is navigating through and modifying a typed but not-yet-submitted query (via arrow keys and backspace and such), a separate stream is necessary to handle mid-stream completion. A string will do just fine if completion is requested at the end of a stream (handling backspace is as easy as lopping off the last character of the string), but since the string would only contain the current "word", tabber.pl would be at a loss in instances like that. Unless, of course, the current-word string would update and find the current word that the cursor is in as the user navigated and typed mid-stream... (could I use at_end_of_stream(Stream) for that?)

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What I'm Asking

How do you think I ought to approach this (string or stream)? The store-to-string method and the make-a-new-stream way both sound like they each have their advantages, so I'm pretty sure the solution will be some sort of combination of both. Any ideas, corrections, or suggestions on accomplishing my goal? (pun intended)

In order to figure that out and really do this correctly, I think I'll also have to know how SWI-Prolog use the input and output streams in the swipl-win window. (It's obviously accepting input, but does it use the output stream to write to the window as you type [into the input stream]?)

Answer 1

Getting this done without changing the C code underlying the swipl-win.exe console will be hard. This also relates to a thread on the mailing list starting here. The completion caller is in src/pl-ntmain.c, do_complete() for Windows and src/os/pl-rl.c, prolog_completion() for the GNU readline based completion used on Unix systems.

The first step to make is lead these two and the upcoming one described in the referenced thread back to Prolog using a callback. That requires a small study of the design of the completion interfaces to arrive at a suitable Prolog callback. I guess that should pass in some representation of the entire line and the caret location and return a list of completions from the caret. With that, anyone can write their own smart completer.