Functional Programming extensions to C++ for ROS projects.
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 = []{
ret.from = 2;
ret.step = 3;
return ret;

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;


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.

constexpr std::optional< E > maybe_error(tl::expected< Args, E >... args)
Definition: result.hpp:227
constexpr Result< T > validate_in(Rng const &valid_values, T const &value, std::string const &name)
Definition: validate.hpp:95
Definition: validate.hpp:52
std::optional< T > step
Definition: validate.hpp:55
T from
Definition: validate.hpp:53