Unwrap a parameter value before a step function is called

pip_param_value() is an extension point for packages that store annotated values (e.g. values with units or labels, or parameter objects that carry UI metadata) as step parameters. When a step runs, the generic is applied once to each unbound argument of the step, that is, to the values that come from params or from the defaults of the step function, and its result is what the step function receives. The stored parameters are not changed.

Usage

pip_param_value(x, ...)

Arguments

x: The parameter value as stored in the step.
...: Currently unused, reserved for future extensions. Methods should accept it.

Value

The value the step function receives for the argument.

Details

The default method returns x unchanged. Methods are meant for your own classes, not for base classes such as numeric or character. Note that this is a hook that unwraps a stored value, not an accessor: use pip_get_params() to read parameters from a pipeline.

The generic is not applied to the outputs of other steps (arguments defined as ~step), to partitions or to failure objects. An error in a method is an error of the step and is handled like any other step error, including pip_run(on_error = "continue").

Examples

# A value that carries annotations
my_number <- structure(
    list(value = 1.23, label = "Number", descr = "I am an example"),
    class = "foo"
)

# Step functions then simply see the plain value
pip_param_value.foo <- function(x, ...) x$value
registerS3method("pip_param_value", "foo", pip_param_value.foo)

p <- pip_new() |>
    pip_add("double", \(x = 1) x * 2)
pip_set_params(p, list(x = my_number))
pip_run(p, lgr = NULL)
pip_collect(p)
#> $double
#> [1] 2.46
#>