Programming Rust, 2nd Edition

Chapter 14. Closures

Save the environment! Create a closure today!

Cormac Flanagan

Sorting a vector of integers is easy:

integers.sort();

It is, therefore, a sad fact that when we want some data sorted, it’s hardly ever a vector of integers. We typically have records of some kind, and the built-in sort method typically does not work:

struct City {
    name: String,
    population: i64,
    country: String,
    ...
}

fn sort_cities(cities: &mut Vec<City>) {
    cities.sort();  // error: how do you want them sorted?
}

Rust complains that City does not implement std::cmp::Ord. We need to specify the sort order, like this:

/// Helper function for sorting cities by population.
fn city_population_descending(city: &City) -> i64 {
    -city.population
}

fn sort_cities(cities: &mut Vec<City>) {
    cities.sort_by_key(city_population_descending);  // ok
}

The helper function, city_population_descending, takes a City record and extracts the key, the field by which we want to sort our data. (It returns a negative number because sort arranges numbers in increasing order, and we want decreasing order: the most populous city first.) The sort_by_key method takes this key-function as a parameter.

This works fine, but it’s more concise to write the helper function as a closure, an anonymous function expression:

fn sort_cities(cities: &mut Vec<City>) {
    cities.sort_by_key(|city| -city.population);
}

The closure here is |city| -city.population. It takes an argument city and returns -city.population. Rust infers ...