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GUI terminal widget

4,085 bytes added, 29 May
Insert content of https://lists.gnu.org/archive/html/octave-maintainers/2019-05/msg00115.html.
Way back in January 2012, I posted the following message about changing
the terminal widget in the GUI to handle input differently.


http://lists.gnu.org/archive/html/octave-maintainers/2012-01/msg00416.html

I included a simple example written with gtkmm to illustrate the idea of
having the terminal widget in the GUI be in control of input and output
and feed lines of text to the interpreter. To properly handle input
that spans multiple lines, a push parser is used instead of the current
pull parser. It worked as an example, but was not much help for Octave
since we are using Qt. So now I've reworked it using Qt and you can
find my sources here:

http://hg.octave.org/jwe-qt-gui-with-push-parser

See the NOTES file for build instructions. That file also contains the
following list of open questions that will need to be resolved if we are
going to attempt a switch.

* With this arrangement, would the interpreter have to run in a
separate thread? As the example shows, it's not absolutely necessary.
It could offer some advantages, but only if it is possible for the GUI
to do useful things while the interpreter is busy.

* If the interpreter is not running in a separate thread but but the
graphics engine is, then what happens to graphics callbacks when the
parser is in the middle of parsing an expression? Or is this not an
issue because separate parsers can be used even if there is only one
evaluator? Could it ultimately be possible to have the evaluator
running in multiple threads?

* If the interpreter does run in a separate thread, we still must
wait for it to calculate and return a result so we can synchronize input
and output. Otherwise, we may print the next prompt before the output
from the previous expression is evaluated and printed. You'll see this
behavior if you build the example program with
CALC_USE_INTERPRETER_THREAD defined.

* The example parser currently also performs evaluations and computes
results immediately so it doesn't properly handle expression lists that
are split across multiple lines. Octave wouldn't have this problem
because we already build a parse tree then execute it once it is complete.

* Do we need text position markers to keep track of the prompt
position (beginning of current line) when inserting or clearing text?
This doesn't seem necessary in the current example, but it doesn't have
functions that can clear the screen or otherwise redraw prior output
that would cause the position of the cursor in the window to change.

* The example program doesn't attempt handle multi-line prompts or
prompts with invisible characters (color specifications, for example).
Fixing that will make the redisplay function significantly more complex.
See, for example, how complicated the default rl_redisplay function is
in the readline library. Unless we actually write a terminal emulator
(like the current terminal widgets) then it is not possible to use
readline's rl_redisplay function directly.

* We'll need to implement a pager ourselves, since "less" won't work
in this simplified terminal widget.

* The system function may need to be modified so that external
programs that expect to be running in a terminal will continue to work
properly. On Unixy systems, this job can be done with ptys. I guess
Windows systems can use a hidden console? But if these things are
required, are we more or less back to were we were before since we used
a pty and hidden console to implement the terminal widgets? I believe
the Emacs start-process function must do similar things, so we might be
able to reuse that code.

* If readline runs in the terminal widget, who owns the command-line
history? Either way, if the GUI is in control of keyboard input, it
will need access to the history list and Octave will also need access
for the history functions.

Now that I have an almost working (if quite simplistic) example in Qt, I
will attempt to modify Octave to use this approach. From that, I expect
many more questions to come up.