What's the difference between generic and non-generic use of interfaces?

Question

In the new Type Parameters Design Draft, you can now apply an interface constraint to a generic function:

func prettyPrint[T Stringer](s T) string {
    ...
}

But, this was already possible by using an interface parameter without generics:

func prettyPrint(s Stringer) string {
    ...
}

What's the difference between using the first and using the second?

Answer 1

I assume the question refers to the latest draft of the Type Parameters proposal, which may end up in Go in 1.18.

---

The first is parametric polymorphism. The compiler verifies that the constraint is satisfied, and then generates code that takes a statically-known type. Importantly, it's not boxed.

The second is runtime polymorphism. It takes a type that's unknown at compile time (the only thing known is that it implements the interface) and works on a boxed interface pointer.

Performance considerations aside, in this simple case you can use either approach. Generics really help with the more complicated cases where the current tools don't work well.