On Tue, Jan 27, 2015 at 6:47 PM, Edward Kmett
This works great for lenses that don't let you change types.
This is not the only restriction required for this to be an acceptable solution. As soon as you have a distinct Lens type, and use something Category-like for composition, you are limiting yourself to composing two lenses to get back a lens (barring a terrible mptc 'solution'). And that is weak. The only reason I (personally) think lens pulls its weight, and is worth using (unlike every prior lens library, which I never bothered with), is the ability for lenses, prisms, ismorphisms, traversals, folds, etc. to properly degrade to one another and compose automatically. So if we're settling on a nominal Lens type in a proposal, then it is automatically only good for one thing to me: defining values of the better lens type. -- Dan