为什么 := 允许作为中缀运算符? [英] Why is := allowed as an infix operator?

问题描述

我遇到了流行的 data.table 包，其中一件事特别让我感兴趣.它有一个就地赋值运算符

I have come across the popular data.table package and one thing in particular intrigued me. It has an in-place assignment operator

:=

这在基础 R 中没有定义.事实上，如果您没有加载 data.table 包，如果您尝试使用它会引发错误(例如，a := 2) 与消息:

This is not defined in base R. In fact if you didn't load the data.table package, it would have raised an error if you had tried to used it (e.g., a := 2) with the message:

错误:找不到函数 ":="

另外，为什么 := 有效?为什么 R 允许您将 := 定义为中缀运算符，而其他所有中缀函数都必须被 %% 包围，例如

Also, why does := work? Why does R let you define := as infix operator while every other infix function has to be surrounded by %%, e.g.

`:=` <- function(a, b) {
   paste(a,b)
}

"abc" := "def"

很明显，它并不是用于定义中缀函数的 %function.name% 的替代语法.data.table 是否利用了 R 的一些解析怪癖?是黑客吗?以后会打补丁"吗?

Clearly it's not meant to be an alternative syntax to %function.name% for defining infix functions. Is data.table exploiting some parsing quirks of R? Is it a hack? Will it be "patched" in the future?

推荐答案

这是基本 R 解析器识别的东西，并且似乎解析为左分配(至少在操作的顺序等方面).有关详细信息，请参阅 C 源代码.

It is something that the base R parser recognizes and seems to parse as a left assign (at least in terms or order of operations and such). See the C source code for more details.

as.list(parse(text="a:=3")[[1]])
# [[1]]
# `:=`
# 
# [[2]]
# a
# 
# [[3]]
# [1] 3

据我所知，它是无证的(就基本 R 而言).但它是一个可以改变其行为的函数/运算符

As far as I can tell it's undocumented (as far as base R is concerned). But it is a function/operator you can change the behavior of

`:=`<-function(a,b) {a+b}
3 := 7
# [1] 10

如您所见，:"部分本身并没有什么特别之处.它恰好是复合标记的开始.

As you can see there really isn't anything special about the ":" part itself. It just happens to be the start of a compound token.

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