On Tue Apr 29 2014 at 16:42:40, Lorenzo Tabacchini lortabac@gmx.comhttp://mailto:lortabac@gmx.comwrote: If you could have a single polymorphic "age" function for all types
having an age, you could never have this runtime error, because the compiler would infer which "age" we are referring to. The GHC OverloadedRecordFields extension that Michael linked seems to do what I am looking for (http://www.well-typed.com/blog/84/).
This is accurate. In the meantime avoid partiality as much as possible. If you must have an age accessor, better to define the lens explicitly. module Main where import Control.Applicativeimport Control.Lens main :: IO ()main = let p = Child 10 in print $ p ^. age data Person = Child Int | Adult Int age :: Lens' Person Intage k (Child x) = Child <$> k xage k (Adult x) = Adult <$> k x Yeah it’s boilerplate. You could makeLenses and then not export the automatically generated selector functions.