Tony Morris

in practice people often compile with warnings enabled

In practice, it’s actually rather hard to put yourself in that situation unintentionally. My point is more that global uniqueness isn’t guaranteed, not that it’s hard to achieve :)

so "not in practice (but actually kinda in practice)"

No, I disagree with that. It’s not at all enforced by default, in practice. That’s just far less of an issue, in practice, than people make it out to be.

Fair enough - what did I miss in my (nasty) bit of code? Doesn't it expose exactly the fact that global uniqueness is not guaranteed by ghc? Who's got the bigger type class implementation is another debate entirely and not at all the point I was making, sorry if it was unclear

The argument is practical Write Haskell; never use orphan instances. This is achievable with little/no penalty Write Scala; never use orphan instances. With this commitment, it's difficult & there's no sweet spot Scala does not have type-classes in any general practical sense

The separate discussion that you're answering to is interesting though. I'd like to know how you define a type class, and in which way that definition invalidates Scala's encoding of them. This is neither a challenge nor sarcasm. I'm genuinely interested.