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R Tip: Force Named Arguments

R tip: force the use of named arguments when designing function signatures.

R’s named function argument binding is a great aid in writing correct programs. It is a good idea, if practical, to force optional arguments to only be usable by name. To do this declare the additional arguments after “...” and enforce that none got lost in the “... trap” by using a checker such as wrapr::stop_if_dot_args().


#' Increment x by inc.
#' @param x item to add to
#' @param ... not used for values, forces later arguments to bind by name
#' @param inc (optional) value to add
#' @return x+inc
#' @examples
#' f(7) # returns 8
f <- function(x, ..., inc = 1) {
   wrapr::stop_if_dot_args(substitute(list(...)), "f")
   x + inc

#> [1] 8

f(7, inc = 2)
#> [1] 9

f(7, q = mtcars)
#> Error: f unexpected arguments: q = mtcars

f(7, 2)
#> Error: f unexpected arguments: 2 

By R function evaluation rules: any unexpected/undeclared arguments are captured by the “...” argument. Then “wrapr::stop_if_dot_args()” inspects for such values and reports an error if there are such. The "f" string is returned as part of the error, I chose the name of the function as in this case. The “substitute(list(…))” part is R’s way of making the contents of “…” available for inspection.

You can also use the technique on required arguments. wrapr::stop_if_dot_args() is a simple low-dependency helper function intended to make writing code such as the above easier. This is under the rubric that hidden errors are worse than thrown exceptions. It is best to find and signal problems early, and near the cause.

The idea is that you should not expect a user to remember the positions of more than 1 to 3 arguments, the rest should only be referable by name. Do not make your users count along large sequences of arguments, the human brain may have special cases for small sequences.

If you have a procedure with 10 parameters, you probably missed some.

Alan Perlis, “Epigrams on Programming”, ACM SIGPLAN Notices 17 (9), September 1982, pp. 7–13.

Note that the “substitute(list(...))” part is the R idiom for capturing the unevaluated contents of “...“, I felt it best to use standard R as much a possible in favor of introducing any additional magic invocations.

Posted on Categories Opinion, Programming, StatisticsTags , , , , , 2 Comments on Using wrapr::let() with tidyeval

Using wrapr::let() with tidyeval

While going over some of the discussion related to my last post I came up with a really neat way to use wrapr::let() and rlang/tidyeval together.

Please read on to see the situation and example. Continue reading Using wrapr::let() with tidyeval

Posted on Categories Opinion, Programming, StatisticsTags , , , 5 Comments on Please Consider Using wrapr::let() for Replacement Tasks

Please Consider Using wrapr::let() for Replacement Tasks

From dplyr issue 2916.

The following appears to work.


COL <- "homeworld"
starwars %>%
  group_by(.data[[COL]]) %>%
## # A tibble: 1 x 14
## # Groups:   COL [1]
##             name height  mass hair_color skin_color eye_color birth_year
##            <chr>  <int> <dbl>      <chr>      <chr>     <chr>      <dbl>
## 1 Luke Skywalker    172    77      blond       fair      blue         19
## # ... with 7 more variables: gender <chr>, homeworld <chr>, species <chr>,
## #   films <list>, vehicles <list>, starships <list>, COL <chr>

Though notice it reports the grouping is by "COL", not by "homeworld". Also the data set now has 14 columns, not the original 13 from the starwars data set.

Continue reading Please Consider Using wrapr::let() for Replacement Tasks

Posted on Categories Coding, Opinion, Programming, StatisticsTags , , , , 6 Comments on Why to use wrapr::let()

Why to use wrapr::let()

I have written about referential transparency before. In this article I would like to discuss “leaky abstractions” and why wrapr::let() supplies a useful (but leaky) abstraction for R programmers.

Wraprs Continue reading Why to use wrapr::let()

Posted on Categories Coding, Opinion, StatisticsTags , , , , , , , , , 7 Comments on wrapr: for sweet R code

wrapr: for sweet R code

This article is on writing sweet R code using the wrapr package.

Continue reading wrapr: for sweet R code