Tools for Validating

The fp library contains some functions to validate values. These validation functions are useful for tasks such as validating ROS parameters.

Validate in

fp::validate_in can be used to validate a value is within a set of values.

auto const day = "monday";
auto const weekends = std::vector<std::string>{"saturday", "sunday"};
auto const result = fp::validate_in(weekends, day, "day");
fmt::print("Is \'{}\' in {}?\n{}\n", day, weekends, result);

This this example will generate this result:

Is 'monday' in {"saturday", "sunday"}?
[Result<T>: [Error: [OutOfRange] monday is not in {"saturday", "sunday"}]]

Validate range

fp::validate_range is a struct that can be used to validate numbers are within a range defined by these struct member variables.

Variable	Default	Description
from	-inf	The minimum
to	+inf	The maximum
step	nothing	The step
step_threshold	1e-3	The threshold to use when evaluating if value is at step

An example of using this if you have c++20 to test if a value is in the [-2,inf,3].

auto const result = fp::validate_range<double>{.from=-2,.step=3}(value);

If you instead only have C++17 you can use it like this:

auto const result = []{
  fp::validate_range<double> ret;
  ret.from = 2;
  ret.step = 3;
  return ret;
}()(value);

Validating multiple items

You can combine these functions with the fp::maybe_error function to validate a set of values. This is useful when validating a struct of ROS parameters.

Result<Parameters> validate(Parameters const& params)
{
  if (auto const error = fp::maybe_error(
        fp::validate_in(valid_modes(), params.mode, "mode"),
        fp::validate_range<size_t>{.from = 2}(params.population_size, "population_size"),
        fp::validate_range<size_t>{.from = 2}(params.elite_count, "elite_count")
      ); error) {
    return tl::make_unexpected(error.value());
  }
 
  return params;
}

Summary

In this tutorial you learned about the functions in fp for validating values and how to combine them into a function that validates a set of values.