To the best of my knowledge, (GHC) Haskell and OCaml don't need to monomorphize parametrically polymorphic code, because they use a uniform representation for m

To the best of my knowledge, (GHC) Haskell and OCaml don't need to monomorphize parametrically polymorphic code, because they use a uniform representation for most objects. Some features of these languages, like GADTs and polymorphic recursion, are difficult to reconcile with monomorphization anyway. On the other hand, Rust generics are monomorphized at compile time, which is closer to how C++ templates work. (The most important difference is that Rust generics are type-checked at definition time, rather than instantiation time.) Another language implementation that uses compile-time monomorphization is MLton, a whole-program Standard ML compiler. Ideally, the language should offer the programmer two kinds of polymorphism: (0) second-class, which is less expressive but easier to type-check and more amenable to optimization, (1) first-class, with the opposite tradeoff.