test.rs

trait Functor<A>{
  fn fmap<B>(f : |A| -> B, v : A) -> B;
}
impl Functor<i32> for i32 {
  fn fmap<B>(f : |i32| -> B, v : i32) -> B {
    f(v)
  }
}

fn main(){
  let i:i32 = Functor::fmap(|x: i32| x * 2 , 10);
  println!("{}", i);
}