You need very little to make a Prolog-like language Turing-complete (lists and recursive predicates). And so Haskell's type system only needs one (or two) extensions to be Turing-complete, `UndecidableInstances` (and maybe `FlexibleInstances` or `MultiParamTypeClasses`). It is no accident. The name "undecidable" shows that the authors of that extension were well aware that it enables Turing-completeness.