结合管道和magrittr点（。）占位符 [英] Combining pipes and the magrittr dot (.) placeholder

问题描述

我对R相当陌生，我想了解％>％运算符和 的用法。（点）占位符。作为一个简单的示例，以下代码有效

I am fairly new to R and I am trying to understand the %>% operator and the usage of the " ." (dot) placeholder. As a simple example the following code works

library(magrittr)
library(ensurer)
ensure_data.frame <- ensures_that(is.data.frame(.))
data.frame(x = 5) %>% ensure_data.frame

但是以下代码失败

ensure_data.frame <- ensures_that(. %>% is.data.frame)
data.frame(x = 5) %>% ensure_data.frame

现在我将占位符传递到is.data.frame方法中。

where I am now piping the placeholder into the is.data.frame method.

我猜这是我对点占位符的局限性/解释的理解滞后，但是有人可以澄清吗？

I am guessing that it is my understanding of the limitations/interpretation of the dot placeholder that is lagging, but can anyone clarify this?

推荐答案

问题是magrittr对匿名函数具有简写形式：

The "problem" is that magrittr has a short-hand notation for anonymous functions:

. %>% is.data.frame

大致与

function(.) is.data.frame(.)

换句话说，当点是（最左侧）左侧时，管道具有特殊的行为。

In other words, when the dot is the (left-most) left-hand side, the pipe has special behaviour.

您可以通过几种方式逃避行为，例如

You can escape the behaviour in a few ways, e.g.

(.) %>% is.data.frame

或LHS与不相同的任何其他方式。
看起来似乎是不受欢迎的行为，但是通常在这样的示例中，实际上并不需要传递第一个表达式，因此 is.data.frame（。）与<$一样具有表现力。 c $ c>。 ％>％is.data.frame 和
例子，例如

or any other way where the LHS is not identical to . In this particular example, this may seem as undesirable behaviuour, but commonly in examples like this there's really no need to pipe the first expression, so is.data.frame(.) is as expressive as . %>% is.data.frame, and examples like

data %>% 
some_action %>% 
lapply(. %>% some_other_action %>% final_action)

$ b可以说
$ b

比

can be argued to be clearner than

data %>% 
some_action %>%
lapply(function(.) final_action(some_other_action(.)))

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