By the way, your original example (`l ?? a`) can already be written as `l
<*> pure a` using the Applicative. So a new operator wouldn't gain that
much.
2014-02-17 0:42 GMT+04:00 Nikita Volkov
I too find such a function useful and deserving a place in the "base", however I don't think that it's a good idea to make it an operator, especially such a misleading one. I bet that I'm hardly alone in perception of question marks as of indicator of a questionable data (e.g., Maybe), but in no way of lists or functions. Therefore I suggest to instead consider some verbal name for it. E.g., "mapApp" or "mapf" (as an opposite to "fmap").
Also, since the variable input of this combinator seems to be on the functor side I suggest to inverse the arguments order order, i.e.:
mapf :: a -> f (a -> b) -> f b
This way it'll target composition. And since it's a combinator over functions it's a pretty important thing. E.g.:
mapf 2 . map (*) $ [1..10]
So, +1 for the combinator, -1 for the proposed name and arguments order .