I agree that Haskell currently does it best (along with Rust, which IIRC does it identically). That said, there are two ways in which the situation is not perf

I agree that Haskell currently does it best (along with Rust, which IIRC does it identically). That said, there are two ways in which the situation is not perfect. First, the dynamics you describe mean I can always add (which is great! that's usually what I need to do to get my job done). They don't help much in reorganizing. If I come out with a new package that introduces a typeclass, I'll usually provide instances for common types. If, down the line, we want to move those instances next to their definitions, both my library and theirs need breaking changes. Second, related, if I want to provide instances for types that are provided by heavy-weight dependencies, other packages that might want to depend on me would have to depend on those dependencies. Mutatis mutandis for if I am introducing the types and the libraries have the typeclasses. Peeling instances out into separate packages means a multiplication of packages or retains a coupling of dependencies, and raises maintainability concerns around orphan instances. IIRC, Backpack might fix this.