On 7 May 2016 at 20:45, 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)
I would vote for this order. The other order would be very surprising for anyone who is familiar with other functions of this family. Also I think this function is something a significant number of people will use directly, not as a lens. alter is the most general way to update the value for a key, and the need for its monadic variant will come up from time to time. At least I wanted this function a few times in the past. Regards, Takano Akio