R: use magrittr pipe operator in self written package

I would like to use the pipe-operator %>% introduced in the magrittr package in a package I wrote myself to chain dplyr data transformations. magrittr is listed as Import in the DESCRIPTION file. After loading my own package and testing the function which uses the pipe-operator I get the following error message:

> Error in functionname(parameter, : could not find function "%>%"

Changing %>% to magrittr::%>% in the function source code does not help either because the package cannot be built anymore.

It should have worked correctly if you had magrittr listed in Depends. However, this is not advised. Instead, you leave magrittr in Imports and add the following line to NAMESPACE:

importFrom(magrittr,"%>%")

I suggest reading Writing R extensions. Your question is covered in paragraphs 1.1.3 and 1.5.1.

There's now an easier way to support the pipe in your packages. The wonderful package usethis has the function use_pipe(). You run that function once and it handles everything. This is how the use_pipe() function is described in the usethis documentation:

> Does setup necessary to use magrittr's pipe internally in your package > and to re-export it for users of your package: > > Adds magrittr to "Imports" in DESCRIPTION > > Creates R/utils-pipe.R with the necessary roxygen template

One additional solution - use the roxygen package. It's implemented as part of the devtools package. Once devtools is installed, calling devtools::document() will update your NAMESPACE for you. It also auto-builds .Rd files with documentation, which is handy.

All you do is add a special comment in the format #' @import packagename to a file to import all functions from that package, or #' @importFrom packagename functionname to import a function. You can have as many of these comments as you want in your files, so you can have a set of them at the top of each file, or with each of your functions that needs an external function.

Then you run devtools::document() and it parses your code looking for those comments, and then it creates an appropriate NAMESPACE file for you. Easy.

Assuming that you're using RStudio, Hadley's devtools package, and listed magrittr in the Imports section of the DESCRIPTION file, here are steps I took to make %>% work in my package function(s).

First, write function foo.R:

#' Convert \code{data.frame} to \code{list}.
#' 
#' @importFrom magrittr %>%
#' @name %>%
#' @rdname pipe
#' @export
#' @param x A \code{data.frame} object.
#' @examples
#' my_result <- foo(iris)
#'
foo <- function(x) {
    x %>%
        as.list()
}

Second, run devtools::document().

Third, run devtools::load_all().

A file like this will be created in your R/ directory and your function should work as expected.