Overriding a method of data.table - failing to perfectly forward arguments - r

Question

I'm able to successfully modify the behaviour of [.data.frame, but fail to do so for [.data.table.
For data.frame:
# Exact same signature as "[.data.frame" :
"[.my.data.frame" <- function (x, i, j,
drop = if (missing(i)) TRUE
else length(cols) == 1) {
if(!missing(j) && j==8 ) {
cat("Oy vey\n")
}
NextMethod()
}
df <- data.frame(a=1,b=2)
class(df) <- c("my.data.frame", class(df))
# Works as expected:
df[1,2] # 2
df[1,8] # Oy Vey NULL
df[1,] # 1 2
However, for (the considerably more complicated) data.table:
# Exact same signature as "[.data.table" :
"[.my.data.table" <- function (x, i, j, by, keyby, with = TRUE, nomatch = getOption("datatable.nomatch"),
mult = "all", roll = FALSE,
rollends = if (roll == "nearest") c(TRUE, TRUE)
else if (roll >= 0) c(FALSE, TRUE) else c(TRUE, FALSE),
which = FALSE, .SDcols, verbose = getOption("datatable.verbose"),
allow.cartesian = getOption("datatable.allow.cartesian"),
drop = NULL, on = NULL) {
if(!missing(j) && j==8 ) {
cat("Oy vey\n")
}
NextMethod()
}
dt <- data.table(a=1,b=2)
class(dt) <- c("my.data.table", class(dt))
dt[1,2] # ERROR: i is not found in calling scope and it is not a column of type logical. When the first argument inside DT[...] is a single symbol, data.table looks for it in calling scope.
I know better than to pass arguments to NextMethod. It looks like I must call [.data.table explicitly, capture and pass the arguments as unevaluated promises - but all my attempts with quote, substitute or match.call have so far failed. Any insight would be appreciated.

Answer 1

I've found a partial solution, posting here in hope someone might improve on it.
"[.my.data.table" <- function (x, ...) {
# Modifications and tests galore - which can be tricky with this signature
class(x) <- class(x)[-1]
ret <- x[...]
class(x) <- c("my.data.table", class(x))
ret
}
I still consider this partial, because actually doing something in the function probably involves at least something like arglist <- list(...), and this fails when [ is called like this -
dt[1,]
Other directions are still very welcome.