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 ...
