# [Rd] getGraphicsEvent() questions, minor feature/tweak request, and patch(es).

Hey all, new R user and first timer here.

I've been using getGraphicsEvent() on an X11-Xlib device in a kind of
interactive loop, and would like to be able to stop it from printing
newlines in the console when I don't actually want to print a prompt.
Even an empty "" value still causes a newline right now.

To not break any code that depends on this behavior, I figured getting
C_getGraphicsEvent to accept an NA value would be the way to go. It
strikes me as highly unlikely that there's any code out there built on
expecting getGraphicsEvent() to error out when getting a 'prompt = NA'
parameter, so this seems like a safe change to make.

After rooting about in the R source for a bit I've found a way to
implement this change myself in src/main/gevents.c, and I've included a
patch as attachment. What I don't know is if there's a better or at
least more conventional way to be checking for the NA value.

Coercing with asChar() and only comparing against R_NaString causes NULL
values, functions, expression objects and potentially others to become
valid prompt values as well, so I'm using TYPEOF() to restrict accepted
values further. I've found manually entered NA values are interpreted as
being of LGLSXP, so that's the only type I'm accepting NA values for. Is
there a better way? There's an isna() in library/utils/src/io.c, but
that module doesn't seem to be available to the linker in the current
build script for gevents.c's module, which would make it a more invasive
patch to use. I'm also not certain it'd work the way I'd want it to,
from looking at its internals.

As far as I can tell, the prompt value is only used in two places;
do_getGraphicsEvent() in src/main/gevents.c, and GA_eventHelper() in
library/grDevices/src/devWindows.c. The latter has a sanity check for
string-ness already, so making NA a possibility should be safe there, as
well.

That apparent lack of use elsewhere leads me to a question about the
help file for getGraphicsEvent(). In it, the claim is made that 'prompt'
is used to display a prompt text on the device. This might be true in MS
Windows, but for X11 the prompt value seems to be solely used for
Rprintf() to the console. Is this discrepency a bug or missing feature
in the X11 implementation, an incorrect wording in the help, or an
interpretation error on my side? Whatever it is, I've also included a
minor patch for the help file to at least indicate support for 'prompt =
NA'.

Apologies if I'm doing this all wrong, or if I'm missing obvious reasons
why these patches are unacceptable. As I said, first timer here and new
to R in general. ;-)

- Richard Bodewits

Index: src/main/gevents.c
===================================================================
--- src/main/gevents.c  (revision 71269)
+++ src/main/gevents.c  (working copy)
@@ -135,7 +135,7 @@
checkArity(op, args);

prompt = CAR(args);
-    if (!isString(prompt) || !length(prompt)) error(_("invalid prompt"));
+    if ((!isString(prompt) || !length(prompt)) && (TYPEOF(prompt) != LGLSXP ||
asChar(prompt) != R_NaString)) error(_("invalid prompt"));

/* NB:  cleanup of event handlers must be done by driver in onExit handler
*/

@@ -159,8 +159,10 @@
if (!count)
error(_("no graphics event handlers set"));

-       Rprintf("%s\n", CHAR(asChar(prompt)));
-       R_FlushConsole();
+    if (TYPEOF(prompt) != LGLSXP || asChar(prompt) != R_NaString) {
+        Rprintf("%s\n", CHAR(asChar(prompt)));
+        R_FlushConsole();
+    }

/* Poll them */
while (result == R_NilValue) {

Index: src/library/grDevices/man/getGraphicsEvent.Rd
===================================================================
--- src/library/grDevices/man/getGraphicsEvent.Rd       (revision 71269)
+++ src/library/grDevices/man/getGraphicsEvent.Rd       (working copy)
@@ -24,12 +24,12 @@

}
\arguments{
-  \item{prompt}{prompt to be displayed to the user in the graphics window}
+  \item{prompt}{prompt to be displayed to the user in the graphics window, or
NA for no prompt}
\item{onMouseDown}{a function to respond to mouse clicks}
\item{onMouseMove}{a function to respond to mouse movement}
\item{onMouseUp}{a function to respond to mouse button releases}
\item{onKeybd}{a function to respond to key presses}
-  \item{consolePrompt}{prompt to be displayed to the user in the console}
+  \item{consolePrompt}{prompt to be displayed to the user in the console, or
NA for no prompt}
\item{which}{which graphics device does the call apply to?}
\item{...}{items including handlers to be placed in the event environment}
\item{env}{an environment to be used as the event environment}

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