piping with dot inside dplyr::filter

Question

I'm struggling to pipe stuff to another argument inside the function filter from dplyr using %>% margritr.

I would assume that this should work:

library(dplyr)
library(margritr)

d <- data.frame(a=c(1,2,3),b=c(4,5,6))
c(2,2) %>% filter(d, a %in% .)

But I get this:

# Error in UseMethod("filter_") : 
#  no applicable method for 'filter_' applied to an object of class "c('double', 'numeric')"

I would expect it to work in the same way as this:

filter(d, a %in% c(2,2))
#   a b
# 1 2 5

What am I doing wrong?

Answer 1

The pipe is designed to compose the function around its first argument when you pass it. When you want to circumvent this behavior, you can generate an anonymous environment that is more flexible. You do this with curly braces, just like when you're writing a function.

5 %>% 
  {filter(iris, Sepal.Length == .)}

For why this works, writing {somefunctions(x, y)} is equivalent to writing function(...) {somefunctions(x, y)}. So the function above ignores its arguments, but just evaluates the variables in its environment. The . pronoun is defined for it by the pipe, and it searches for other variables (like iris) in the global environment.