GUI terminal widget: Difference between revisions

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Way back in January 2012, I posted the following message about changing
== Background ==
the terminal widget in the GUI to handle input differently.


Way back in January 2012, jwe [https://lists.gnu.org/archive/html/octave-maintainers/2012-01/msg00416.html 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
Jwe 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 jwe has reworked it using Qt and you can find the sources here:


I included a simple example written with gtkmm to illustrate the idea of
https://hg.octave.org/jwe-qt-gui-with-push-parser
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 [https://hg.octave.org/jwe-qt-gui-with-push-parser/file/tip/NOTES 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.


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
== Open questions ==
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  
# 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.
graphics engine is, then what happens to graphics callbacks when the  
# 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?
parser is in the middle of parsing an expression?  Or is this not an  
# 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.
issue because separate parsers can be used even if there is only one  
# 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.
evaluator?  Could it ultimately be possible to have the evaluator  
# 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.
running in multiple threads?
# 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.
  * If the interpreter does run in a separate thread, we still must  
# 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.
wait for it to calculate and return a result so we can synchronize input  
# 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.
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.