Is it possible to input an expression with the pipe %>% operator in R?-CodePudding

Why doesn't the pipe operator %>% work in the second example in the following code ?

library(magrittr) 

# Works
job::job({install.packages("gtsummary")})  
     
# Doesn't work
{install.packages("gtsummary")} %>% job::job()  
# Error in code[[1]] : object of type 'symbol' is not subsettable

Is this because the piped object is an expression ? I'm not familiar with expressions in R

CodePudding user response：

Based on docs of magrittr library

Technical notes The magrittr pipe operators use non-standard evaluation

I think the problem comes from NSE algorithm in R

as a workaround you can use the native pip operator like

{install.packages("gtsummary")} |>  job::job()

CodePudding user response：

The problem is that job::job uses non-standard evaluation to grab the code you want to run without running it right away. A more basic examples of a function that does this is

nse <- function(x) {
  as.character(substitute(x))
}

nse(hello)
# [1] "hello"
hello %>% nse
# [1] "."

So when you run nse(hello), note that hello is not a variable but the function can use substitute() to get the code for the value you pass to the function. So hello is never evaluated, it's converted to a symbol.

When you use the magrittr pipe, the value is not passed as itself. All values from the previous calculation are stored in a variable named "." so that's why we see that value when using the pipe operator. The job::job function is expecting a code block, not just a single symbol . hence you get the error.

The native |> pipe operator works because it doesn't create the special value variable and it actually re-writes the code you wrote at the abstract syntax tree level. You can see this if you do

quote(hello %>% nse())
# hello %>% nse()
quote(hello |> nse())
# nse(hello)

The |> operator actually re-writes the code so it doesn't exist.