There's a huge gulf here. undefined doesn't introduce unsoundness, and unsafePerformIO isn't part of the language standard, it's strongly discouraged, and rare

There's a huge gulf here. undefined doesn't introduce unsoundness, and unsafePerformIO isn't part of the language standard, it's strongly discouraged, and rarely used. I've been writing Haskell professionally for 10 years, for example, and I've never written unsafePerformIO.