There are so many "slightly different but mostly the same" things in this space. In Rust, this is const generics: https://github.com/rust-lang/rfcs/blob/master/text/2000-cons...

  struct RectangularArray<T, const WIDTH: usize, const HEIGHT: usize> {
      array: [[T; WIDTH]; HEIGHT],
  }

  const X: usize = 7;

  fn main() {
      let x: RectangularArray<i32, 2, 4>;
      let y: RectangularArray<i32, X, {2 * 2}>;
  }

I haven't read enough about singleton types to know the exact details about their similarities, so I'm not sure.