[tel]

Sure, and if you treat them all as `undefined`/bottom then you get CPO semantics. Usually "Exception" still refers to `Control.Exception.SomeException` and `error` is bottom.

Catching "undefined" from pure code is the danger that leads to massively confusing semantics—it breaks down the "value" concept badly. The `spoon` library is a great example of this and it's scary to see it in places. That said, it's not terrible for wrapping up foreign code that isn't quite unsafe enough to need "IO" treatment.

The best usage of "error" is to mark completely impossible situations. These show up easily when you do dependently typed stuff with GADTs, but can also exist due to various algorithm invariants.