Using the functor pattern

A functor is approximately the inverse of a function:

• A function defines a transformation, accepts data, and returns the result of the transformation
• A functor defines data, accepts a function, and returns the result of the transformation

A simple example of a functor is the Rust vector and its accompanying map function:

fn main() {
   let m: Vec<u64> = vec![1, 2, 3];
   let n: Vec<u64> = m.iter().map(|x| { x*x }).collect();
   println!("{:?}", m);
   println!("{:?}", n);
}

Functors are often thought of as only the map function, due to the rules of what constitutes a functor or not. The preceding common case is what's called a structure-preserving map. Functors do not need to be structure-preserving. For example, take the very similar case of a map implemented for a set, as shown in the following code:

use std::collections...