[veets]

I don't doubt you know more about Rust than I do, but this seems pedantic to me. Kind of like correcting someone for pronouncing forte as "for-TAY" instead of "fort" or telling someone "well technically you don't actually touch _anything_ because of the electrostatic force."

If you ask all the developers out there to describe parametric and ad-hoc polymorphism I think a vast majority would give the example of a type parameter (e.g., Java generics or C++ templates) for parametric polymorphism and Java interfaces or Haskell's classes for ad-hoc polymorphism. I can even quote directly from Pierce (Types and Programming Languages):

> Parametric polymorphism, the topic of this chapter, allows a single piece of code to be typed "generically," using variables in place of actual types, and then instantiated with particular types as needed. Parametric definitions are uniform: all of their instances behave the same.

I think Rust and the aforementioned languages fit this definition. Outside of a specific compiler issue, claiming otherwise seems to only confuse the issue, especially for those just casually reading and not familiar with programming language theory.

[eru]

> If you ask all the developers out there to describe parametric and ad-hoc polymorphism I think a vast majority would give the example of a type parameter (e.g., Java generics or C++ templates) for parametric polymorphism and Java interfaces or Haskell's classes for ad-hoc polymorphism. I can even quote directly from Pierce (Types and Programming Languages):

In practice it is often useful to drop our demands for rigour by a bit. But any reasonable definition of parametric polymorphism _has_ to exclude C++ templates.

C++ templates are much closer to the definition of ad-hoc polymorphism.

My practical less-than-rigorous rule-of-thumb for parametric polymorphism amounts to something like: whenever Wadler's Theorems for Free paper applies. https://ttic.uchicago.edu/~dreyer/course/papers/wadler.pdf

Theorems for Free means that eg the type of the identity function (forall a . a -> a) guarantees that even if you supply it a bool, it can't magically turn into an implementation for `not`, or multiply all integers by 2.

[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.

[quicknir]

IIRC, Java interfaces are subtype polymorphism. Ad hoc polymorphism would be, for example, overloading.