Hands-On Design Patterns and Best Practices with Julia

Understanding the diagonal rule

As we learned earlier, it is a nice feature to be able to match type variables and keep them consistent across method arguments. In practice, there are situations where we want to be even more specific when determining the right type for each type variable.

Consider this function:

diagonal(x::T, y::T) where {T <: Number} = T

The diagonal function takes two arguments with the same type, where the type T must be a subtype of Number. The type variable T is simply returned to the caller.

When T is concrete, it is easy to reason that the types are consistent. For example, we can pass a pair of Int64 values or a pair of Float64 values to the function and expect to see the respective concrete type returned:

Intuitively, ...