CODEWITHSUNDEEP
rdata.table
sztal
sztal

Reputation: 298

How is dot (.) alias for list constructor implemented in data.table package?

I wonder how is is the dot alias (.) for the list constructor function implemented in the data.table package. Just to be clear I am thinking about this functionality:

library(data.table)
D = data.table(iris)
x1 = D[, .(Sepal.Length, Sepal.Width)]    # dot alias
x2 = D[, list(Sepal.Length, Sepal.Width)] # standard name
identical(x1, x2)    # TRUE

I tried to find it in the source code on github, but it is too dense for me to understand it in any reasonable amount of time.

EDIT. I know this can be easily done by defining an alias like: . <- list or . <- function(...) list(...). However, this is not exactly what I am looking for. I want to define such an alias, so it only works in the context of a given function/method.

Example.

L <- .(1)   # This throws error
L <- func(.(1)) # This works

Actually I can get what I want using the rlang tools for tidy evaluation. Below is a simple example.

library(rlang)
func <- function(...) {
    . <- list
    eval_tidy(enexpr(x))
}
x1 <- func(.(1))
x2 <- list(1)
identical(x1, x2)   # TRUE

So I wonder how this kind of functionality is implemented in the data.table specifically, since it was developed a way earlier than rlang?

Upvotes: 3

Views: 271

Answers (1)

Roland
Roland

Reputation: 132959

data.table replaces the dot in expressions before they are evaluated. It uses computing on the language.

The relevant function is replace_dot_alias (note that reusing this function means that you need to conform to data.table's license):

replace_dot_alias = function(e) {
  # we don't just simply alias .=list because i) list is a primitive (faster to iterate) and ii) we test for use
  # of "list" in several places so it saves having to remember to write "." || "list" in those places
  if (is.call(e) && !is.function(e[[1L]])) {
    # . alias also used within bquote, #1912
    if (e[[1L]] == 'bquote') return(e)
    if (e[[1L]] == ".") e[[1L]] = quote(list)
    for (i in seq_along(e)[-1L]) if (!is.null(e[[i]])) e[[i]] = replace_dot_alias(e[[i]])
  }
  e
}

An example of usage:

expr <- quote(.(a = b, c = .(sum(d))))
replace_dot_alias(expr)
#list(a = b, c = list(sum(d)))

The modified expression is then executed.

Upvotes: 4

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