Unlist nested list columns in data.table

Question

Unlist nested list column in data.table. Assuming all the list elements are the same type. The list elements are named, the name has to be handled also.
It is somehow opposi

Answer 1

Not sure it is more "canonical" but here is a way to modify l so you can use by=a, considering you know the type of your data in list (with some improvements, thanks to @DavidArenburg):

dt[lengths(l) == 0, l := NA_integer_][, .(nm = names(unlist(l)), ul = unlist(l)), by = a]

#   a nm ul
#1: a c1  6
#2: a c2  4
#3: b  x  2
#4: b  y  4
#5: b  z  3
#6: c NA NA