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

问题描述

我是 R 的新手，我正在尝试理解 %>% 运算符和."(点)占位符的用法.作为一个简单的例子，下面的代码有效

库(magrittr)图书馆(保证人)ensure_data.frame <- ensure_that(is.data.frame(.))data.frame(x = 5) %>% ensure_data.frame

但是下面的代码失败了

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

我现在将占位符传送到 is.data.frame 方法中.

我猜是我对点占位符的限制/解释的理解滞后，但有人可以澄清这一点吗?

解决方案

问题"在于 magrittr 有匿名函数的简写符号:

<预><代码>.%>% is.data.frame

与

大致相同

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

换句话说，当点位于(最左侧)左侧时，管道具有特殊的行为.

您可以通过几种方式逃避该行为，例如

(.) %>% is.data.frame

或 LHS 与 不同的任何其他方式.在这个特定的例子中，这可能看起来是不受欢迎的行为，但通常在这样的例子中，真的不需要管道第一个表达式，所以 is.data.frame(.) 和 一样具有表现力>.%>% is.data.frame，和像

这样的例子

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

可以说比

更清楚

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

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

However the following code fails

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

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

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

解决方案

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

. %>% is.data.frame

is roughly the same as

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

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

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

(.) %>% is.data.frame

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)

can be argued to be clearner than

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

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