Get name of dataframe passed through pipe in R

Question

I would like to be able to print the name of a dataframe passed through the pipe. Is this possible? I can do.

printname <- function(df){
    print(paste(substitute(df)))
}
printname(mtcars)
#[1] "mtcars"

However, it returns "." when this function is piped using the magrittr pipe.

mtcars %>% printname
# [1] "."

This would be helpful when writing custom error messages of functions used in logged production processes -- it's hard to know where something failed if the only thing in the log is "."

It would probably be enough to return the original call, which would include the mtcars %>% piece.

Answer 1

This is a first attempt, it's kind of a hack, but seems like it might work.

find_chain_parts <- function() {
    i <- 1
    while(!("chain_parts" %in% ls(envir=parent.frame(i))) && i < sys.nframe()) {
          i <- i+1
      }
    parent.frame(i)
}

printfirstname <- function(df){
    ee <- find_chain_parts()
    print(deparse(ee$lhs))
}

mtcars %>% printfirstname
# [1] "mtcars"

The pipe function creates an environment that keeps track of the chain parts. I tried walking up the current execution environments looking for this variable and then use the lhs info stored there to find the symbol at the start of the pipe. This isn't well tested.