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README.Rmd
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---
output: github_document
---
<!-- README.md is generated from README.Rmd. Please edit that file -->
```{r setup, include = FALSE}
knitr::opts_chunk$set(
collapse = TRUE,
comment = "#>",
fig.path = "man/figures/README-",
out.width = "100%"
)
```
# ellipsis
<!-- badges: start -->
[](https://lifecycle.r-lib.org/articles/stages.html)
[](https://github.com/r-lib/ellipsis/actions)
[](https://cran.r-project.org/package=ellipsis)
[](https://codecov.io/gh/r-lib/ellipsis?branch=master)
<!-- badges: end -->
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_evaluated()` 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](https://github.com/jennybc) for the idea, and [Lionel Henry](https://github.com/lionel-) for the heart of the implementation.
## Installation
Install the released version from CRAN:
```{r, eval = FALSE}
install.packages("ellipsis")
```
Or the development version from GitHub:
```{r, eval = FALSE}
devtools::install_github("r-lib/ellipsis")
```
## Example
`mean()` is a little dangerous because you might expect it to work like `sum()`:
```{r}
sum(1, 2, 3, 4)
mean(1, 2, 3, 4)
```
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:
```{r, error = TRUE}
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)
```