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Signal deprecated argument by using self-documenting sentinel deprecated() as default argument. Test whether the caller has supplied the argument with is_present().






A deprecated() function argument.

Magical defaults

We recommend importing lifecycle::deprecated() in your namespace and use it without the namespace qualifier.

In general, we advise against such magical defaults, i.e. defaults that cannot be evaluated by the user. In the case of deprecated(), the trade-off is worth it because the meaning of this default is obvious and there is no reason for the user to call deprecated() themselves.


foobar_adder <- function(foo, bar, baz = deprecated()) {
  # Check if user has supplied `baz` instead of `bar`
  if (lifecycle::is_present(baz)) {

    # Signal the deprecation to the user
    deprecate_warn("1.0.0", "foo::bar_adder(baz = )", "foo::bar_adder(bar = )")

    # Deal with the deprecated argument for compatibility
    bar <- baz

  foo + bar

foobar_adder(1, 2)
#> [1] 3
foobar_adder(1, baz = 2)
#> Warning: The `baz` argument of `bar_adder()` is deprecated as of foo 1.0.0.
#>  Please use the `bar` argument instead.
#> [1] 3