Closes #3011 -- adds a method to print expression columns

reword

NAMESPACE

@@ -176,5 +176,6 @@ S3method(unique, ITime)
 export(format_col)
 S3method(format_col, default)
 S3method(format_col, POSIXct)
+S3method(format_col, expression)
 export(format_list_item)
 S3method(format_list_item, default)

NEWS.md

@@ -46,7 +46,7 @@
 
 6. New variable `.Last.updated` (similar to R's `.Last.value`) contains the number of rows affected by the most recent `:=` or `set()`, [#1885](https://github.com/Rdatatable/data.table/issues/1885).
 
-7. `data.table` printing now supports customizable methods for both columns and list column row items, part of [#1523](https://github.com/Rdatatable/data.table/issues/1523). `format_col` is S3-generic for customizing how to print whole columns; `format_list_item` is S3-generic for customizing how to print each row of a list column. Thanks variously to @mllg, who initially filed [#3338](https://github.com/Rdatatable/data.table/pulls/3338) with the seed of the idea, @franknarf1 who earlier suggested the idea of providing custom formatters, @fparages who submitted a patch to improve the printing of timezones for [#2842](https://github.com/Rdatatable/data.table/issues/2842), and @MichaelChirico for the ultimate implementation. See `?print.data.table` for examples.
+7. `data.table` printing now supports customizable methods for both columns and list column row items, part of [#1523](https://github.com/Rdatatable/data.table/issues/1523). `format_col` is S3-generic for customizing how to print whole columns; `format_list_item` is S3-generic for customizing how to print each row of a list column. Thanks variously to @mllg, who initially filed [#3338](https://github.com/Rdatatable/data.table/pulls/3338) with the seed of the idea, @franknarf1 who earlier suggested the idea of providing custom formatters, @fparages who submitted a patch to improve the printing of timezones for [#2842](https://github.com/Rdatatable/data.table/issues/2842), @RichardRedding for pointing out an error relating to printing wide `expression` columns in [#3011](https://github.com/Rdatatable/data.table/issues/3011), and @MichaelChirico for the ultimate implementation. See `?print.data.table` for examples.
 
 #### BUG FIXES
 

R/print.data.table.R

@@ -148,6 +148,9 @@ format_col.default = function(x, ...) {
 # #2842 -- different columns can have different tzone, so force usage in output
 format_col.POSIXct = function(x, ...) format(x, usetz = TRUE, ...)
 
+# #3011 -- expression columns can wrap to newlines which breaks printing
+format_col.expression = function(x, ...) format(char.trunc(as.character(x)), ...)
+
 format_list_item.default = function(x, ...) {
   if (is.null(x)) return ("") # NULL item in a list column
   if (is.atomic(x) || inherits(x, "formula")) # FR #2591 - format.data.table issue with columns of class "formula"

inst/tests/tests.Rraw

@@ -14141,19 +14141,22 @@ DT = data.table(
   t1 = as.POSIXct('2018-05-01 12:34:56', tz = 'UTC'),
   t2 = as.POSIXct('2018-05-01 12:34:56', tz = 'Asia/Singapore')
 )
-test(1992.1, DT, output = 'UTC.*\\+08')
+test(2032.1, DT, output = 'UTC.*\\+08')
+
+# #3011 -- default expression printing can break format_col.default
+test(2032.2, print(data.table(e = expression(1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13))), output = '1 + 2 + 3')
 
 # format_col generic is used
 format_col.complex = function(x, ...) sprintf('(%.1f, %.1fi)', Re(x), Im(x))
 registerS3method("format_col", "complex", format_col.complex)
 x = data.table(z = c(1 + 3i, 2 - 1i, pi + 2.718i))
-test(1992.2, x, output = '(1.0, 3.0i)')
+test(2032.3, x, output = '(1.0, 3.0i)')
 
 # format_list_item() generic is used
 format_list_item.myclass <- function(x, ...) paste0("<", class(x)[1L], ":", x$id, ">")
 registerS3method("format_list_item", "myclass", format_list_item.myclass)
 DT = data.table(row = 1:2, objs = list(structure(list(id = "foo"), class = "myclass"),  structure(list(id = "bar"), class = "myclass")))
-test(1992.3, print(DT), output = "myclass:foo.*myclass:bar")
+test(2032.4, print(DT), output = "myclass:foo.*myclass:bar")
 
 ###################################
 #  Add new tests above this line  #

man/print.data.table.Rd

@@ -26,6 +26,7 @@
   format_col(x, \dots)
   \method{format_col}{default}(x, \dots)
   \method{format_col}{POSIXct}(x, \dots)
+  \method{format_col}{expression}{x, \dots)
 
   format_list_item(x, \dots)
   \method{format_list_item}{default}(x, \dots)