[hajile]

The type system will know if you don't handle a potential conversion.

This restriction is better for pretty much every real-world use case anyway.

If you don't know what you're converting, then you are almost guaranteed to be getting garbage out the other end. If you do know what you are converting, being explicit and covering all your options isn't a big deal.

And of course, you can pack this all away in a module if you really want too. Modules are more flexible than typeclasses in a language like Haskell anyway (as you can't have multiple typeclass definitions in Haskell).

In truth, I believe module typeclasses (typeclasses only definable in modules) would be a nice addition. It would allow flexibility while creating a pattern that actively discourages the abuse you see in Haskell where typeclasses quickly devolve into unreadable garbage.