I have an issues that looks like it should have a trivial solution, but for the life of me, I cannot figure it out.

So I have defined the following template enum

pub enum ModelParameters<const D: usize> {
  Width,
  Height,
  Slope,
  Left(u8),
  Right(u8),
}

The idea here is to use this enum as indices to a model I'm trying to fit. I want to be able to change easily the number of parameters. Here D is the total number of parameters. The left and right parameters are balanced, so if there is N of them, D = 2*N + 3.

My first idea is to have N as the type parameter, but I need D as a template parameter for nalgebra so I ended up using D as template parameter and calculating N when I need, N = (D - 3)/2.

What I want now is to make sure that N makes sense, so I need D to satisfy the following

D >= 3 and D % 2 == 1 and D < 515.

I would like this check to happen once in compile time, but I cannot make this happen.

I have defined the following function

impl<const D: usize> ModelParameters<D> {
  pub const COUNT: usize = D;

  pub const fn number_of_sides() -> usize {
    assert!(D >= 3 && D % 2 == 1 && D < 515);
    let n = (D - 3) / 2;
    return n;
  }

Then I thought that if I can force the compiler to run the function, then it will throw an error. I realised that I cannot do this with assert. Then I tried my luck with const_assert from the static-assertions crate. This give me the following error:

    |
 97 | impl<const D: usize> SkutVolModelParameters<D> {
    |            - const parameter from outer item
...
101 |     const_assert!(D >= 3 && D % 2 == 1 && D < 515);
    |                                           ^ use of generic parameter from outer item
    |
    = note: a `const` is a separate item from the item that contains it

What does this even mean? What is the "outer item"? Is there a way to have a compile time check in this case?

4 posts - 3 participants

Read full topic