[deredede]

> Classes exist at runtime

Not necessarily, depending on the language. Functional languages and system languages such as OCaml, Haskell, Rust, but also C (painfully) and C++ can represent wrapper types within a nominal type system at no runtime cost.

[nequo]

Haskell implements type classes via dictionary passing that don’t always get optimized away by the compiler so it does have a slight runtime cost:

https://okmij.org/ftp/Computation/typeclass.html#dict

In Rust, using trait objects also generates a vtable for dynamic dispatch so in that case traits are not fully erased:

https://web.mit.edu/rust-lang_v1.25/arch/amd64_ubuntu1404/sh...

[deredede]

My quotation was not good - I intended to reply to the argument that "nominal type do [exist at runtime] to some level". The newtype pattern in either Haskell or Rust is fully transparent at runtime.