Relatedly, I think what "pretty" means is project specific. For example, my pretty floats print like "%.2f", because I usually don't need to see anything past the 3rd decimal point. When you're scanning a list of numbers looking for oddities, having everything formatted this way makes a huge difference for readability. But someone else will complain that everything is .99, and will find scientific notation more useful.
I think it's at least technically possible to provide addition formatting behavior through newtypes (e.g. polymorphic, on RealFloat).
Of course python's str() / repr() doesn't worry about formatting either. Maybe it's useful to have a Show-alike somewhere in between Show and Pretty.
That is basically where I was heading at. Cheers, Simon