If you want to be pure, you can add a strict WriterT to your monad stack, or something similar. If you don't mind temporary impurity, Debug.Trace provides some

If you want to be pure, you can add a strict WriterT to your monad stack, or something similar. If you don't mind temporary impurity, Debug.Trace provides some helper functions, but they call unsafePerformIO, which can do unexpected things.