First, what is mapMaybe?
Its type is: (a -> Maybe b) -> [a] -> [b]
It maps the input function over the list, and drops any values which evaluate to nothing. It's like a combination of map and filter, where the input function is given the option to either transform the input or filter it out.
But then I needed more information threaded around in my functions, and the types went from a -> Maybe b to a -> ReaderT r Maybe b.
So I needed:
> mapAlt :: Alternative f => (a -> f b) -> [a] -> f [b]
It's just like mapMaybe, except it works for any Alternative functor.
The output is still in the functor f so I can have it work for effectful monads and such, but it will always return a value (even if it's the empty list).
Here's the implementation:
> mapAlt f xs = go xsLinks:
> where go  = pure 
> go (y:ys) = (pure (:) <*> f y <*> go ys)
> <|> go ys