14 Jan
2013
14 Jan
'13
2:42 p.m.
On Mon, Jan 14, 2013 at 11:14 AM, Andrea Vezzosi
Have you considered the effect on types like Data.Set that use the uniqueness of typeclass instances to maintain invariants? e.g. even when we have "newtype X = X Y" coercing "Set X" to "Set Y" can produce a tree with the wrong shape for the Ord instance of Y.
I was just going to say that. Changing newtypes changes instances, which isn't safe in the general case. -- Johan