I know barely enough haskell, passively, to dislike quite a lot of this code (like really, 3 lines on 3 places, just to case on a maybe to throw an error? w/a j

I know barely enough haskell, passively, to dislike quite a lot of this code (like really, 3 lines on 3 places, just to case on a maybe to throw an error? w/a just calling fromMaybe or even fromJust? and manual instances??) - but, could someone more experienced suggest a saner one?