list2() is equivalent to list(...) with a few additional features:

  • You can splice other lists with the unquote-splice !!! operator.

  • You can unquote names by using the unquote operator !! on the left-hand side of :=.

  • Trailing commas are ignored, making it easier to copy and paste arguments.

For lack of a better name, these features are collectively called "tidy dots".

dots_list() is a lower-level version of list2() that offers additional parameters for dots capture.

dots_list(..., .ignore_empty = c("trailing", "none", "all"),
  .preserve_empty = FALSE, .homonyms = c("keep", "first", "last",
  "error"), .check_assign = FALSE)

list2(...)

Arguments

...

Arguments to collect with !!! support.

.ignore_empty

Whether to ignore empty arguments. Can be one of "trailing", "none", "all". If "trailing", only the last argument is ignored if it is empty.

.preserve_empty

Whether to preserve the empty arguments that were not ignored. If TRUE, empty arguments are stored with missing_arg() values. If FALSE (the default) an error is thrown when an empty argument is detected.

.homonyms

How to treat arguments with the same name. The default, "keep", preserves these arguments. Set .homonyms to "first" to only keep the first occurrences, to "last" to keep the last occurrences, and to "error" to raise an informative error and indicate what arguments have duplicated names.

.check_assign

Whether to check for <- calls passed in dots. When TRUE and a <- call is detected, a warning is issued to advise users to use = if they meant to match a function parameter, or wrap the <- call in braces otherwise. This ensures assignments are explicit.

Value

A list of arguments. This list is always named: unnamed arguments are named with the empty string "".

See also

exprs() for extracting dots without evaluation.

Examples

# Let's create a function that takes a variable number of arguments: numeric <- function(...) { dots <- list2(...) num <- as.numeric(dots) set_names(num, names(dots)) } numeric(1, 2, 3)
#> [1] 1 2 3
# The main difference with list(...) is that list2(...) enables # the `!!!` syntax to splice lists: x <- list(2, 3) numeric(1, !!! x, 4)
#> [1] 1 2 3 4
# As well as unquoting of names: nm <- "yup!" numeric(!!nm := 1)
#> yup! #> 1
# One useful application of splicing is to work around exact and # partial matching of arguments. Let's create a function taking # named arguments and dots: fn <- function(data, ...) { list2(...) } # You normally cannot pass an argument named `data` through the dots # as it will match `fn`'s `data` argument. The splicing syntax # provides a workaround: fn("wrong!", data = letters) # exact matching of `data`
#> [[1]] #> [1] "wrong!" #>
fn("wrong!", dat = letters) # partial matching of `data`
#> [[1]] #> [1] "wrong!" #>
fn(some_data, !!!list(data = letters)) # no matching
#> $data #> [1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s" #> [20] "t" "u" "v" "w" "x" "y" "z" #>
# Empty arguments trigger an error by default: try(fn(, ))
#> list()
# You can choose to preserve empty arguments instead: list3 <- function(...) dots_list(..., .preserve_empty = TRUE) # Note how the last empty argument is still ignored because # `.ignore_empty` defaults to "trailing": list3(, )
#> [[1]] #> #>
# The list with preserved empty arguments is equivalent to: list(missing_arg())
#> [[1]] #> #>
# Arguments with duplicated names are kept by default: list2(a = 1, a = 2, b = 3, b = 4, 5, 6)
#> $a #> [1] 1 #> #> $a #> [1] 2 #> #> $b #> [1] 3 #> #> $b #> [1] 4 #> #> [[5]] #> [1] 5 #> #> [[6]] #> [1] 6 #>
# Use the `.homonyms` argument to keep only the first of these: dots_list(a = 1, a = 2, b = 3, b = 4, 5, 6, .homonyms = "first")
#> $a #> [1] 1 #> #> $b #> [1] 3 #> #> [[3]] #> [1] 5 #> #> [[4]] #> [1] 6 #>
# Or the last: dots_list(a = 1, a = 2, b = 3, b = 4, 5, 6, .homonyms = "last")
#> $a #> [1] 2 #> #> $b #> [1] 4 #> #> [[3]] #> [1] 5 #> #> [[4]] #> [1] 6 #>
# Or raise an informative error: try(dots_list(a = 1, a = 2, b = 3, b = 4, 5, 6, .homonyms = "error"))
#> Error : Arguments can't have the same name. We found these problems: #> * Multiple arguments named `a` at positions 1 and 2 #> * Multiple arguments named `b` at positions 3 and 4
# dots_list() can be configured to warn when a `<-` call is # detected: my_list <- function(...) dots_list(..., .check_assign = TRUE) my_list(a <- 1)
#> Warning: Using `<-` as argument is often a mistake. #> Do you need to use `=` to match an argument? #> #> If you really want to use `<-`, please wrap in braces: #> #> # Bad: #> fn(a <- 1) #> #> # Good: #> fn(a = 1) # Match 1 to parameter `a` #> fn({ a <- 1 }) # Assign 1 to variable `a`
#> [[1]] #> [1] 1 #>
# There is no warning if the assignment is wrapped in braces. # This requires users to be explicit about their intent: my_list({ a <- 1 })
#> [[1]] #> [1] 1 #>