efficiency

In http://www.cs.columbia.edu/~cdo/icfp99.ps Okasaki does a couple things I'm not sure I quite understand: He has a datatype newtype Empty a = E () I cannot for the life of me figure out why he didn't just do data Empty a = E The other weird thing was: newtype Pair v w a = P (v a, w a) I can't see why he didn't do data Pair v w a = P (v a) (w a) I was wondering if anyone on this list knew of any reasons the types he chose would be more efficient than mine.... David