Adding ...
to a function is a powerful technique because it allows you
to accept any number of additional arguments. Unfortunately it comes
with a big downside: any misspelled or extraneous arguments will be
silently ignored. This package provides tools for making ...
safer:
check_dots_used()
errors if any components of ...
are not
evaluated. This allows an S3 generic to state that it expects every
input to be evaluated.
check_dots_unnamed()
errors if any components of ...
are named.
This allows you to collect arbitrary unnamed arguments, warning if
the user misspells a named argument.
check_dots_empty()
errors if ...
is used. This allows you to use
...
to force the user to supply full argument names, while still
warning if an argument name is misspelled.
Thanks to Jenny Bryan for the idea, and Lionel Henry for the heart of the implementation.
Install the released version from CRAN:
install.packages("ellipsis")
Or the development version from GitHub:
devtools::install_github("r-lib/ellipsis")
mean()
is a little dangerous because you might expect it to work like
sum()
:
sum(1, 2, 3, 4)
#> [1] 10
mean(1, 2, 3, 4)
#> [1] 1
This silently returns the incorrect result because mean()
has
arguments x
and ...
. The ...
silently swallows up the additional
arguments. We can use ellipsis::check_dots_used()
to check that every
input to ...
is actually used:
safe_mean <- function(x, ..., trim = 0, na.rm = FALSE) {
ellipsis::check_dots_used()
mean(x, ..., trim = trim, na.rm = na.rm)
}
safe_mean(1, 2, 3, 4)
#> Error: 3 components of `...` were not used.
#>
#> We detected these problematic arguments:
#> * `..1`
#> * `..2`
#> * `..3`
#>
#> Did you misspecify an argument?
Any scripts or data that you put into this service are public.
Add the following code to your website.
For more information on customizing the embed code, read Embedding Snippets.