On Mon, Jan 08, 2018 at 04:31:53PM +0200, Baa wrote:
Actually I don;t know - is some better way to solve such problem? I'm sure many of us communicate with some RESTfull APIs. Then APIs change. And your apps should work with old and new APIs (services), so you should clone existing library to support both versions (together, at the same time, in one client application!).
A somewhat primitive but doable solution would be: instead of using instances pass a datatype around. E.g. data J a = J { readjs :: a -> MyTpe } myfun :: J a -> a -> Int -- when before it probably was -- myfun :: FromJson a => a -> Int You could enforce some sanity at type level with phantom types, e.g.: data InterfaceA data InterfaceB data J a b = -- something -- J is to be exported without (..) creaIntfA :: J a InterfaceA
Are backpacks something like ML modules?
Indeed backpack is a way to have an ML-like module system in Haskell. The question is interesting and a "real life" one. I urge you to post it on cafe- too, I am sure there will be some more ideas/solutions in there.