问题描述
我已经遇到了流行的 data.table 包，有一件事特别感激我。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
这没有在基础R中定义。事实上，如果你没有加载 data.table 包，它会产生一个错误，如果你试过使用它（例如 a：= 2 ）并显示以下消息：
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:
：= 工作？为什么R让你定义：= 为中缀运算符，而每个其他中缀函数必须由 %% 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"
它并不意味着用于定义中缀函数的％function.name％的替代语法。是 data.table 利用R的一些解析怪癖吗？这是一个黑客？它会在未来修补吗？
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
这是基础R解析器识别的东西，似乎解析为左分配（至少在条款或顺序操作等）。有关详细信息，请参见。
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
至于我可以告诉它是无记录的关心）。但是它是一个函数/运算符，你可以改变
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
正如你所看到的，：部分本身并没有什么特别的。它恰好是复合令牌的开始。
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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