I wish Rust had ML-style modules. Traits/typeclasses are great for the common case of a structure over one type. If there's one main type & some auxiliary ones, fine, ok. But ML modules can save your ass because they don't make you feel like you're holding it wrong if you need to parameterize over lots of types and none of them is "the one main type". You just do it and move on, instead of chickening out or getting into "which thing is the main type" hell.