How to extract / subset an element from a list with the magrittr %>% pipe?

Question

Since the introduction of the %>% operator in the magrittr package (and it's use in dplyr), I have started to use this in my own work.

One simple operation has me stumped, however. Specifically, this is the extraction (or subsetting) of elements from a list.

An example: In base R I would use $, [ or [[ to extract an element from a list:

iris$Species
iris[["Species"]]

I can achieve the same using the %>% pipe:

iris %>%
  subset(select = "Species") %>%
  head

  Species
1  setosa
2  setosa
3  setosa
4  setosa
5  setosa
6  setosa

Or

iris %>%
  `[[`("Species") %>%
  levels

[1] "setosa"     "versicolor" "virginica" 

However, this feels like a messy, clunky solution.

Is there a more elegant, canonical way to extract an element from a list using the %>% pipe?

Note: I don't want any solution involving dplyr, for the simple reason that I want the solution to work with any R object, including lists and matrices, not just data frames.

Answer 1

In v 1.5 of magrittr on CRAN you can use the %$% operator:

iris %$% 
  Species %>%
  levels

It is essentially a wrapper around with but nicer than

iris %>% 
  with(Species %>% levels)

or

iris %>%
  with(Species) %>%
  levels

It is designed to be convinient when functions don't have their own data argument, e.g. with plot you can do

iris %>% 
  plot(Sepal.Length ~ Sepal.Width, data = .)

but e.g. with ts.plot you can't do that, so now:

iris %$%
  ts.plot(Sepal.Length)

[yeah, I know the example makes no sense, but it illustrates the point]

Note also that [<- and [[<- also have aliases, inset and inset2..