dplyr form for data.table

library(ggplot2)
library(data.table)

data("diamonds", package = "ggplot2")
setDT(diamonds)

arrange_dt <- function(DT, ...){
  setorderv(DT, as.character(as.list(substitute(list(...)))[-1L]))
}
arrange_dt(diamonds, carat)

select_dt <- function(DT, ...){
  cols <- deparse(substitute(list(...)))
  cols <- sub("list\\(", ".(", cols)
  DT[j = eval(parse(text = cols))]
}
select_dt(diamonds, carat, cut, color, price)

distinct_dt <- function(DT, ..., keep_all = FALSE){
  uniqueCols <- as.character(as.list(substitute(list(...)))[-1L])
  if (keep_all){
    unique(DT, by = uniqueCols)
  } else {
    select_dt(unique(DT, by = uniqueCols), ...)
  }
}
distinct_dt(diamonds, cut, color)
distinct_dt(diamonds, cut, color, keep_all = TRUE)

filter_dt <- function(DT, filters){
  z <- substitute(filters)
  DT[eval(z)]
}
filter_dt(diamonds, depth > 60)

group_by_dt <- function(DT, ...){
  setattr(DT, "groups", as.character(as.list(substitute(list(...)))[-1L]))
}
group_by_dt(diamonds, cut, color)

group <- function(DT){
  attr(DT, "groups")
}
group(diamonds)

ungroup <- function(DT){
  setattr(DT, "groups", NULL)
}
ungroup(diamonds) %>>% group

mutate_dt <- function(DT, ...){
  objects <- substitute(list(...))
  namesVar <- names(as.list(objects)[-1L])
  grps <- group(DT)
  if (is.null(grps)) {
    DT[j = `:=`(eval(namesVar), eval(objects))]
  } else {
    DT[j = `:=`(eval(namesVar), eval(objects)), by = grps]
  }
}
mutate_dt(ungroup(diamonds), V5 = depth - table, V6 = depth + table)
mutate_dt(group_by_dt(diamonds, cut, color), V3 = mean(carat), V4 = max(price))

summarise_dt <- function(DT, ...){
  objects <- deparse(substitute(list(...)))
  objects <- sub("list\\(", ".(", objects)
  grps <- group(DT)
  if (is.null(grps)) {
    DT[j = eval(parse(text = objects))]
  } else {
    DT[j = eval(parse(text = objects)), by = grps]
  }
}
summarise_dt(ungroup(diamonds), V3 = mean(carat), V4 = max(price))
summarise_dt(group_by_dt(diamonds, cut, color), V3 = mean(carat), V4 = max(price))