unnest in data.table

library(data.table)
library(pipeR)
DT <- data.table(a = list(list(1:5), list(2:4), list(1:5)), b = 1:3,
                 c = list(list(0:4), list(6:8), list(7:11)),  d = 2:4)
#         a b      c d
# 1: <list> 1 <list> 2
# 2: <list> 2 <list> 3
# 3: <list> 3 <list> 4
autoFind <- function(DT){
  setdiff(names(DT), names(DT)[sapply(DT, function(x) any(class(x) %in% "list"))])
}
extendTbl <- function(DT, groupbyVar = autoFind(DT)){
  expr <- setdiff(names(DT), groupbyVar) %>>%
    (paste0(., "=unlist(",  ., ")")) %>>%
    paste0(collapse = ",") %>>% (paste0(".(", ., ")"))
  return(DT[ , eval(parse(text = expr)), by = groupbyVar])
}
extendTbl(DT)
#     b d a  c
#  1: 1 2 1  0
#  2: 1 2 2  1
#  3: 1 2 3  2
#  4: 1 2 4  3
#  5: 1 2 5  4
#  6: 2 3 2  6
#  7: 2 3 3  7
#  8: 2 3 4  8
#  9: 3 4 1  7
# 10: 3 4 2  8
# 11: 3 4 3  9
# 12: 3 4 4 10
# 13: 3 4 5 11
extendTbl(DT, c("b", "d"))
#     b d a  c
#  1: 1 2 1  0
#  2: 1 2 2  1
#  3: 1 2 3  2
#  4: 1 2 4  3
#  5: 1 2 5  4
#  6: 2 3 2  6
#  7: 2 3 3  7
#  8: 2 3 4  8
#  9: 3 4 1  7
# 10: 3 4 2  8
# 11: 3 4 3  9
# 12: 3 4 4 10
# 13: 3 4 5 11