Assign Argument Value from Option if NULL or Missing

This utility function is designed to be called from within another function. If a function argument is missing or NULL, this function attempts to assign a value from a global option. If neither the argument nor the option is set, an informative error is thrown, unless allow_null = TRUE, in which case NULL is allowed. Optionally, it checks that the resulting value inherits from a specified class.

Usage

assign_from_options(
  arg,
  option_name,
  expected_class = NULL,
  allow_null = FALSE
)

Arguments

arg

Bare name of the argument to check and (potentially) assign. Should be unquoted.

option_name

Character. The name of the global option (as in getOption()) to use as a fallback value.

expected_class

Character vector or NULL; if not NULL, the result must inherit from one of these classes, otherwise an error is thrown.

allow_null

Logical; if TRUE, both the argument and the global option are allowed to be NULL without error. If FALSE (default), an error is thrown if both are NULL.

Value

Invisibly returns the final value of the argument (after assignment, if performed).

Details

This function is intended for use inside another function's body to help set default argument values using global options.

• If the argument is missing, it assigns the value from getOption(option_name), if available.

• If the argument is explicitly supplied but is NULL, it will also assign the value from the option if available.

• If neither the argument nor the option is set, an error is thrown, unless allow_null = TRUE, in which case NULL is allowed.

• If expected_class is provided, the final value is checked for class inheritance.

Author

Ahmed El-Gabbas

Examples

my_fun <- function(x = NULL) {
  ecokit::assign_from_options(x, "my_x_option", expected_class = "numeric")
  x  # x is now set from option if not provided
}

options(my_x_option = 42)

# returns 42
my_fun()
#> [1] 42

# returns 1.5
my_fun(1.5)
#> [1] 1.5

ecokit::remove_options("my_x_option")

# error: Argument `x` is missing/NULL and option `my_x_option` is not set.
try(my_fun())
#> Error in ecokit::assign_from_options(x, "my_x_option", expected_class = "numeric") : 
#>   Argument `x` is NULL and option `my_x_option` is not set. Provide `x` or set options(my_x_option = ...).