Adding Torque

[dunmatt]

This is a continuation of #114 (which will be closed when #115 goes in) that adds Torque.

I'm starting to look at adding torque, but I have a question. Over in #114 you mentioned that the new Kind will likely want to implement all of the marker traits, but I'm not sure I understand why you said that. What would it mean to divide a torque by an energy? (for example)

Edit: I think I've figured out the answer to my own question (I'd still appreciate confirmation though). I think it never derives inter-Kind mathematical operators. If that's the case, may I suggest changing the docs on the markers? "Trait to denote that a quantity is able to be X with a quantity of the same dimensions." makes it sound like the Kind isn't taken into account.

[iliekturtles]

Kind serves two purposes in uom. Both to distinguish different quantities of the same dimension that are not mutually comparable (e.g. torque and energy) as well as governing what operations (+, -, *, ...) can be used on the quantity. In torque's case we don't want it comparable with energy but we still want to be able to add torques together as well as perform other operations.

The definition should probably look like this:

pub trait TorqueKind: // still haven't done any research to see what the appropriate name would be.
    marker::Add + marker::AddAssign
    + marker::Sub + marker::SubAssign
    + marker::Mul + marker::MulAssign
    + marker::Div + marker::DivAssign
    + marker::Rem + marker::RemAssign
    + marker::Neg + marker::Saturating
{
}

This all works because Kind is one of the associated types of Dimension. impl blocks for traits like Add or Eq only define a single Dimension generic parameter ensuring that you can only add or compare quantities with the exact sameDimension and therefore exact same Kind. Other impl blocks such as for Mul have a generic parameter for each quantity's Dimension but constrain the impl to kinds that implement the marker::Mul trait.

[dunmatt]

That makes sense, and it's consistent with my edit above. The next question, then, is how are lines like

let _: Torque<V> = Force::new::<f::newton>(V::one()) * Length::new::<l::meter>(V::one());

supposed to compile? Right now it's giving me a type mismatch on account of Torque's kind (using the example kind you supplied).

[iliekturtles]

Force * Length can be energy or torque. The Mul trait uses Energy as the result as Energy uses the default Kind. The only previous time Kind has been used was for thermodynamic temperature vs. temperature interval and a calculation result like this wasn't relevant. I see a couple possible solutions:

1. Don't actually use a distinct Kind for Torque. This makes Torque and Energy aliases and directly comparable. Try this one first.
2. Add a method or trait impl (From?) to allow for the explicit conversion between Torque and Energy.