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

That is entirely besides the point though. My point is: GHC doesn't enforce global instance uniqueness. This is demonstrably true, but exhibiting a program in which global instance uniquess is violated. Whether or not it's easy to avoid is another discussion