Edit

The type inst

Sometime, this is a little bit too precise. The type this always refers to the most precise type. The problem with that is that this is often, but not always, what we want. Let's imagine you are building an algorithm that mixes integers and floats, and you want to be able to compare them. You would create types that would look like:

base class Num uses MyComparable
class NumInt(value: Int) extends Num
class NumFloat(value: Float) extends Num

Now what does the signature of isEqual look like?

class NumInt(value: Int) {
  fun isEqual(value2: NumInt): Bool { ... }
}

But is that really the type we want? We want to be able to compare numeric values, so we want isEqual to work with objects of type Num and not NumInt. The type this in the trait is too precise!

That's what the type inst is for. inst should be read as: the type of the object that used the trait — in this case the type Num.

class MyComparable {
  fun isEqual(inst): Bool;
}

And that's the correct definition. Note that it is possible to reuse the same trait twice, so if an ancestor already was using a trait, it's possible to make that instantiation more precise. An extreme version of that would be to systematically reuse a trait in all the subclasses: that would be strictly equivalent to the type this.