[IsTom]

I'm a little behind times on Haskell (haven't used it for some years) – there always were extensions that made it unsound, but the core language was pretty solid.

[g15jv2dp]

Look at the last example in there: https://counterexamples.org/polymorphic-references.html

[IsTom]

Well, it does use unsafePerformIO. It's not particularly horrible most of the time, but in this case it obviously is.