medium- and long-term plans for tests/knitr.R

[aitap]

• I slightly botched print.data.table: check for knitr at sys.calls()-6 #6563 by aiming it at patch-1.16.2 instead of master. Should we cherry-pick 9de2567 into master too?
• Do we want a more reliable secret handshake with knitr? For example,

  • Have knitr set options(knitr.is.auto.printing = TRUE) and detect that in print.data.table

  • signalCondition(<please_dont_autoprint_me>) from print.data.table and have knitr detect that

  • If you want the exact printing behavior of data.table:::print.data.table and want the result to be a table, you can register an S3 method on xfun::record_print (data.table doesn't have to import xfun).

    Originally posted by @yihui in fuse does not work for quote yihui/litedown#38 (comment)

    See Solve the knitr auto-printing problem by registering a method for knit_print #6589

• Do we want to petition R developers to allow returning invisibly from [? The change is just one line, eval = 0 to eval = 200, but it does break R's own tests/reg-tests-2.Rout.save, making matrix(5:-6, 3)[2, invisible(3)] also invisible, and might break a lot of other stuff.

[aitap]

Given the new interest for #3029, perhaps we do want to ask R developers to make .Primitive("[") stop forcing visibility? The complete change that passes LC_ALL=C.UTF-8 make check-devel is below, but this may or may not break a lot of CRAN/Bioconductor packages. (Do we have or know someone who has the infrastructure to run R CMD check on a lot of R packages with two different versions of R?)

Index: src/main/names.c
===================================================================
--- src/main/names.c	(revision 87260)
+++ src/main/names.c	(working copy)
@@ -104,7 +104,7 @@
 {"(",		do_paren,	0,	1,	1,	{PP_PAREN,   PREC_FN,	  0}},
 {".subset",	do_subset_dflt,	1,	1,	-1,	{PP_FUNCALL, PREC_FN,	  0}},
 {".subset2",	do_subset2_dflt,2,	1,	-1,	{PP_FUNCALL, PREC_FN,	  0}},
-{"[",		do_subset,	1,	0,	-1,	{PP_SUBSET,  PREC_SUBSET, 0}},
+{"[",		do_subset,	1,	200,	-1,	{PP_SUBSET,  PREC_SUBSET, 0}},
 {"[[",		do_subset2,	2,	0,	-1,	{PP_SUBSET,  PREC_SUBSET, 0}},
 {"$",		do_subset3,	3,	0,	2,	{PP_DOLLAR,  PREC_DOLLAR, 0}},
 {"@",		do_AT,		0,	0,	2,	{PP_DOLLAR,  PREC_DOLLAR, 0}},
Index: src/main/subset.c
===================================================================
--- src/main/subset.c	(revision 87260)
+++ src/main/subset.c	(working copy)
@@ -697,6 +697,7 @@
 
     /* Method dispatch has failed, we now */
     /* run the generic internal code. */
+    R_Visible = TRUE;
     return do_subset_dflt(call, op, ans, rho);
 }
 
Index: tests/reg-S4.Rout.save
===================================================================
--- tests/reg-S4.Rout.save	(revision 87260)
+++ tests/reg-S4.Rout.save	(working copy)
@@ -738,7 +738,6 @@
 > x <- new("C1"); y <- new("C2")
 > x[1, drop=FALSE]
 drop in C1-[ : FALSE 
-NULL
 > y[1, drop=FALSE]
 drop in C2-[ : FALSE 
 drop in C1-[ : FALSE