如何在data.table包中实现列表构造函数的点（。）别名？ [英] How is dot (.) alias for list constructor implemented in data.table package?

问题描述

我想知道 list 构造函数的点别名（。）如何在<$中实现c $ c> data.table 包。为了清楚起见，我正在考虑此功能：

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

我试图在源代码中找到它github，但是它太密集了，我无法在任何合理的时间内理解它。

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.

编辑。
我知道可以通过定义一个别名来轻松实现，例如：。 <-列表或。 <-函数（...）列表（...）。但是，这并不是我要找的东西。我想定义这样的别名，因此它仅在给定的函数/方法的上下文中起作用。

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.

示例。

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

实际上，我可以使用 rlang 工具进行整洁的评估，得到想要的东西。下面是一个简单的示例。

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

所以我想知道如何在 data.table 特别是，因为它的开发时间早于 rlang ？

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

推荐答案

data.table在计算表达式之前会替换表达式中的点。

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

相关功能为 replace_dot_alias （请注意，重复使用此功能意味着您需要遵守data.table的许可证）：

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
}

用法示例：

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

然后执行修改后的表达式。

The modified expression is then executed.

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