Wren Romano and I have decided to maintain consistency in the argument
order, even though this is more annoying for people using lens. Thus
alterF :: (Functor f, Ord k) => (Maybe a -> f (Maybe a)) -> k -> Map k
a -> f (Map k a)
and `Control.Lens.At.at` for maps can be implemented as
at = flip Data.Map.Lazy.alterF
Note that Data.Map.Strict will also offer a version of alterF that
forces any values it installs in the map.
On Sat, May 7, 2016 at 4:45 PM, David Feuer
I managed to find an implementation of Control.Lens.At.at for Data.Map that's fast enough to be useful. The function will be named alterF to match the name of Data.Map.alter. The remaining question is what order the arguments should go in. I had thought to follow those of alter for consistency, giving
alterF :: (Functor f, Ord k) => (Maybe a -> f (Maybe a)) -> k -> Map k a -> f (Map k a)
Edward Kmett thinks the ergonomics of that order are terrible, and prefers to follow lens at, giving
alterF :: (Functor f, Ord k) => k -> (Maybe a -> f (Maybe a)) -> Map k a -> f (Map k a)
How do other people feel about this?
David Feuer