Comment by tialaramex
7 hours ago
I mean, yeah, type erasure does give parametricity, but, you can instead design your language so that you monomorphize but insist on parametricity anyway. If you write stable Rust your implementations get monomorphized but you aren't allowed to specialize them - the stable language doesn't provide a way to write two distinct versions of the polymorphic function.
And if you only regard parametricity as valuable rather than essential then you can choose to relax that and say OK, you're allowed to specialize but if you do then you're no longer parametric and the resulting lovely consequences go away, leaving it to the programmers to decide whether parametricity is worth it here.
I don't understand your first paragraph. Monomorphization and parametricity are not in conflict; the compiler has access to information that the language may hide from the programmer. As an existance proof, MLTon monomorphizes arrays while Standard ML is very definitely parametric: http://www.mlton.org/Features
I agree that maintaining parametricity or not is a design decision. However, recent languages that break it (e.g. Zig) don't seem to understand what they're doing in this regard. At least I've never seen a design justification for this, but I have seen criticism of their approach. Given that type classes and their ilk (implicit parameters; modular implicits) give the benefits of ad-hoc polymorphism while mantaining parametricity, and are well established enough to the point that Java is considering adding them (https://www.youtube.com/watch?v=Gz7Or9C0TpM), I don't see any compelling reason to drop parametricity.
My point was that you don't need to erase types to get parametricity. It may be that my terminology is faulty, and that in fact what Rust is doing here does constitute "erasing" the types, in that case what describes the distinction between say a Rust function which is polymorphic over a function to be invoked, and a Rust function which merely takes a function pointer as a parameter and then invokes it ? I would say the latter is type erased.