[steveklabnik]

This isn't Rust specific; it's just the definition of parametric polymorphism. Yes, many programmers may give you a slightly incorrect definition, but especially in an article about Haskell, I'd expect a bit more precision.

Which doesn't mean it's terrible to get it wrong, just want to be clear about what Rust does and does not have. It is important because these kinds of definitions either hold or they don't; "sorta kinda mostly parametric" isn't the way folks tend to think about this. Which makes sense, because they're interested in proofs and formal definitions.

Yes, Pierce is great! But the issue is:

> all of their instances behave the same.

This is not true in Rust, as shown in my comment and the other replies. We have accepted APIs that break this property, and we have other language features on the way that break this property.