Use hidden advanced arguments for user-friendly functions

As a user

Imagine that you are starting to learn how to use a specific R package, lets call it foo. You will look at the vignette (if there is one), use help(package = foo), or look at the reference manual (for example, devtools' ref man). Eventually, you will open the help page for the function(s) you are interested in using.

?function_I_want_to_use

In many packages, there is a main use case that is addressed by the package. A common strategy is to export a main function. That function will likely have a long list of arguments. So as a new user, you are suddenly exposed to a complicated help page and you will want to figure out which arguments you need to use.

As a developer

From the developer's side, you want to give users control over several details. Each detail you want the user to control involves one more argument in your function. Sooner rather than later, you will have a long list of arguments. This increases the learning curve for new users of your package, and can potentially scare them away. That is contradictory of another goal you have as a developer: you want to get people to use your package.

Lets say that you are developing the function use_me(). If the details you want the users to control are actually arguments of other functions used inside use_me(), then you can simplify your function by using the ... argument. This argument is very well explained at The three-dots construct in R (Burns, 2013). It is very useful and can greatly simplify your life as a developer. Plus, it reduces the length of your help pages, thus making your package more user friendly.

center

However, if some of the details in use_me() are not arguments to other functions, then the common strategy is to write two functions. One is a low level function with arguments for all the details which might or might not export. Then, you write a second function that is a wrapper for the low level function and pre-specifies values for all the details. See the next minimal example:

# Don't export this function
.use_me <- function(arg1, arg2, verbose = TRUE) {
    if(verbose) message(paste(Sys.time(), 'working'))
    pmax(arg1, arg2)
}

#' @export
use_me <- function(arg1, ...) {
    .use_me(arg1, 0, ...)
}

## Lets see it in action
use_me(1:3)
## 2014-12-11 17:03:32 working
## [1] 1 2 3
use_me(-1:1, verbose = FALSE)
## [1] 0 0 1

In this example, the help page for use_me() is fairly short and friendly. You don't expect users to be interested in changing arg2 much. Surely you could make it so the non-exported function .use_me() sets a default value for arg2.

Another strategy is to specify inside use_me() the default values for all the arguments you want to use while keeping the list of visible arguments short. That is, maintain the user friendliness of your functions while also giving them control over all the details. That is what you can do using dots() from dots (Collado-Torres, 2014). dots() is a very simple function that checks if ... has a specific argument, and if absent, it returns a default value. It can be seen in action below:

library('dots')
use_me_dots <- function(arg1, ...) {
    ## Default hidden arguments
    arg2 <- dots(name = 'arg2', value = 0, ...)
    verbose <- dots('verbose', TRUE, ...)
    
    ## Regular code
    if(verbose) message(paste(Sys.time(), 'working'))
    pmax(arg1, arg2)
}
use_me_dots(1:3)
## 2014-12-11 17:03:32 working
## [1] 1 2 3
use_me_dots(-1:1, verbose = FALSE)
## [1] 0 0 1
use_me_dots(-1:1, verbose = FALSE, arg2 = 5)
## [1] 5 5 5

dots is my solution to the problem of keeping functions user friendly while giving them control over all the details. The idea is that experienced users will be able to find what the advanced arguments are. While they could find them from the code itself, I do recommend describing the advanced arguments in a vignette targeted for these users.

Complications

Now, while ... is great, you might run into problems when use_me() calls two functions that have different arguments and that don't have the ... argument. Such a scenario is illustrated below.

status <- function(arg3, status = TRUE) {
    if(status) print(arg3)
    return(invisible(NULL))
}
use_me_again <- function(arg1, ...) {
    res <- .use_me(arg1, 0, ...)
    status(res, ...)
    return(res)
}

## Seems to work
x <- use_me_again(1)
## 2014-12-11 17:03:32 working
## [1] 1
## But nope, it doesn't
use_me_again(1, verbose = FALSE, status = FALSE)
## Error in .use_me(arg1, 0, ...): unused argument (status = FALSE)

This scenario can happen when you are using functions from other packages. It's happened to me in cases where the main function does have a ... argument but uses several internal functions that don't use it.

In such situations, you might want to use formal_call() from dots. It figures out which are the arguments formally used by the function of interest and drops out un-used arguments from ..., thus avoiding this type of problem.

use_me_fixed <- function(arg1, ...) {
    res <- formalCall(.use_me, arg1 = arg1, arg2 = 0, ...)
    formal_call(status, arg3 = res, ...)
    return(res)
}

## Works now!
use_me_fixed(1, verbose = FALSE, status = FALSE)
## [1] 1

For a more complicated example, see the dots complex example in the vignette.

Conclusions

As a developer, it is possible to keep your functions user friendly while giving experienced users the option to control the fine tuning arguments which you don't expect most users will want to tweak. My solution to this problem is implemented in dots (check it's vignette). I'd love to hear what you think about it! I am specially interested on what users think about the idea of hidden advanced arguments (documented in an advanced users vignette).

I might try to get dots into a repository: probably in Bioconductor since most of the dots code was first implemented for derfinder.

PS I just found a similar function to dots(). It's berryFunctions::owa() and you can find its code here.

References

Citations made with knitcitations (Boettiger, 2014).

[1] C. Boettiger. knitcitations: Citations for knitr markdown files. R package version 1.0.4. 2014. URL: https://github.com/cboettig/knitcitations.

[2] P. Burns. The three-dots construct in R - Burns Statistics. http://www.burns-stat.com/the-three-dots-construct-in-r/. 2013. URL: http://www.burns-stat.com/the-three-dots-construct-in-r/.

[3] L. Collado-Torres. dots: Simplifying function calls. R package version 1.0.0. 2014. URL: https://github.com/lcolladotor/dots.

Reproducibility

## Reproducibility info
library('devtools')
options(width = 120)
session_info()
## Session info-----------------------------------------------------------------------------------------------------------
##  setting  value                                             
##  version  R Under development (unstable) (2014-11-01 r66923)
##  system   x86_64, darwin10.8.0                              
##  ui       X11                                               
##  language (EN)                                              
##  collate  en_US.UTF-8                                       
##  tz       America/New_York
## Packages---------------------------------------------------------------------------------------------------------------
##  package       * version  date       source                                 
##  bibtex          0.3.6    2013-07-29 CRAN (R 3.2.0)                         
##  devtools      * 1.6.1    2014-10-07 CRAN (R 3.2.0)                         
##  digest          0.6.4    2013-12-03 CRAN (R 3.2.0)                         
##  dots          * 1.0.0    2014-11-14 Github (lcolladotor/dots@a933540)      
##  evaluate        0.5.5    2014-04-29 CRAN (R 3.2.0)                         
##  formatR         1.0      2014-08-25 CRAN (R 3.2.0)                         
##  httr            0.5      2014-09-02 CRAN (R 3.2.0)                         
##  knitcitations * 1.0.4    2014-11-03 Github (cboettig/knitcitations@508de74)
##  knitr         * 1.7      2014-10-13 CRAN (R 3.2.0)                         
##  lubridate       1.3.3    2013-12-31 CRAN (R 3.2.0)                         
##  memoise         0.2.1    2014-04-22 CRAN (R 3.2.0)                         
##  plyr            1.8.1    2014-02-26 CRAN (R 3.2.0)                         
##  RColorBrewer  * 1.0.5    2011-06-17 CRAN (R 3.2.0)                         
##  Rcpp            0.11.3   2014-09-29 CRAN (R 3.2.0)                         
##  RCurl           1.95.4.3 2014-07-29 CRAN (R 3.2.0)                         
##  RefManageR      0.8.40   2014-10-29 CRAN (R 3.2.0)                         
##  RJSONIO         1.3.0    2014-07-28 CRAN (R 3.2.0)                         
##  rstudioapi      0.1      2014-03-27 CRAN (R 3.2.0)                         
##  stringr         0.6.2    2012-12-06 CRAN (R 3.2.0)                         
##  XML             3.98.1.1 2013-06-20 CRAN (R 3.2.0)

Want more?

Check other @jhubiostat student blogs at Bmore Biostats as well as topics on #rstats.

I wrote dots a month ago and the post itself today during our bi-weekly blog meeting.

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