TL;DR: change -XSafe to -XTrustworthy. I’ve found myself needing to add some trace statements to System/Environment.hs. The module is marked Safe, so when you try to: import Debug.Trace (trace) you get the following error: Debug.Trace: Can't be safely imported! The module itself isn't safe. I don’t know anything about Safe Haskell and I didn’t want to learn at the time. Removing {-# LANGUAGE Safe #-} made the Safe modules depending on System.