simplify format_col.default, allowing e.g. vctrs_list_of columns to print well

Author: MichaelChirico

NEWS.md

@@ -117,6 +117,8 @@ rowwiseDT(
 
 15. The auto-printing suppression in `knitr` documents is now done by implementing a method for `knit_print` instead of looking up the call stack, [#6589](https://github.com/Rdatatable/data.table/pull/6589). Thanks to @jangorecki for the report [#6509](https://github.com/Rdatatable/data.table/issues/6509) and @aitap for the fix.
 
+16. A data.table with a column of class `vctrs_list_of` (from package {vctrs}) prints as expected, [#5948](https://github.com/Rdatatable/data.table/issues/5948). Before, they could be printed messily, e.g. printing every entry in a nested data.frame. Thanks @jesse-smith for the report, @DavisVaughan and @r2evans for contributing, and @MichaelChirico for the PR.
+
 ## NOTES
 
 1. There is a new vignette on joins! See `vignette("datatable-joins")`. Thanks to Angel Feliz for authoring it! Feedback welcome. This vignette has been highly requested since 2017: [#2181](https://github.com/Rdatatable/data.table/issues/2181).

R/print.data.table.R

@@ -193,8 +193,6 @@ has_format_method = function(x) {
 format_col.default = function(x, ...) {
   if (!is.null(dim(x)))
     "<multi-column>"
-  else if (has_format_method(x) && length(formatted<-format(x, ...))==length(x))
-    formatted  #PR5224 motivated by package sf where column class is c("sfc_MULTIPOLYGON","sfc") and sf:::format.sfc exists
   else if (is.list(x))
     vapply_1c(x, format_list_item, ...)
   else
@@ -236,6 +234,8 @@ format_list_item.data.frame = function(x, ...) {
   paste0("<", class1(x), paste_dims(x), ">")
 }
 
+# format_list_item.vctrs_list_of = function(, .)
+
 # FR #1091 for pretty printing of character
 # TODO: maybe instead of doing "this is...", we could do "this ... test"?
 # Current implementation may have issues when dealing with strings that have combinations of full-width and half-width characters,

inst/tests/other.Rraw

@@ -1,4 +1,4 @@
-pkgs = c("ggplot2", "hexbin", "plyr", "dplyr", "caret", "zoo", "xts", "gdata", "nlme", "bit64", "knitr", "parallel", "sf", "nanotime", "R.utils", "yaml")
+pkgs = c("bit64", "caret", "dplyr", "gdata", "ggplot2", "hexbin", "knitr", "nanotime", "nlme", "parallel", "plyr", "R.utils", "sf", "vctrs", "xts", "yaml", "zoo")
 # First expression of this file must be as above: .gitlab-ci.yml uses parse(,n=1L) to read one expression from this file and installs pkgs.
 # So that these dependencies of other.Rraw are maintained in a single place.
 # TEST_DATA_TABLE_WITH_OTHER_PACKAGES is off by default so this other.Rraw doesn't run on CRAN. It is run by GLCI, locally in dev, and by
@@ -773,3 +773,9 @@ if (loaded[["nanotime"]]) {
 res <- tables(env=.e)
 test(32, res[, .(NAME,NROW,NCOL,MB)], data.table(NAME="DT",NROW=20000000L,NCOL=15L,MB=2288.0))
 rm(.e, res)
+
+if (loaded[["vctrs"]]) {
+  # vctrs::list_of() columns are treated the same as other list() columns
+  DT = data.table(a = 1, b = list_of(mtcars))
+  test(33, DT, output="<vctrs_list_of>.*<data\\.frame\\[32x11\\]>")
+}