Hands-On Design Patterns and Best Practices with Julia

Method arguments are covariant

It should be quite intuitive that method arguments are covariant because that is how multiple dispatch work today. Consider the following function:

friend(m::Mammal, f::Mammal) = "$m and $f become friends."

In Julia, method arguments are formally represented as a tuple. In the preceding example, the method argument is just Tuple{Mammal,Mammal}.

When we call this function with two arguments that have type S and T respectively, then it will only be dispatched if S <: Mammal and T <: Mammal. In this case, we should be able to pass any combination of mammals—dog/dog, dog/cat, cat/dog, and cat/cat. The following screenshot proves this:

Let's also check whether a crocodile can join the party:

As expected, Tuple ...