A Rust implementation of data Ext b = forall a. Ext a (a -> b)

existential.rs

struct ExistsPrivate<A, F, B> 
where F: Fn(&A) -> B {
  a: A,
  f: F
}

trait AnyExistsPrivate<B> {
  fn extract(&self) -> B;
}

impl<A, F, B> AnyExistsPrivate<B> for ExistsPrivate<A, F, B>
where F: Fn(&A) -> B {
  fn extract(&self) -> B {
    (self.f)(&self.a)
  }
}

struct Exists<'a, B>(Box<dyn AnyExistsPrivate<B> + 'a>);

impl<'a, B: 'a> Exists<'a, B> {
  fn new<A: 'a, F: 'a>(a: A, f: F) -> Exists<'a, B>
  where F: Fn(&A) -> B {
    Exists(Box::new(ExistsPrivate {
      a: a,
      f: f
    }))
  }

  fn extract(&self) -> B {
    self.0.extract()
  }
}

fn main() {
  let e = Exists::new(0, |i: &i32| i+1);
  println!("{}", e.extract());
}