如何将应用效果添加到半身像中？

Question

How can I change following code so that each element in Maybe monoid element takes also two arguments (so it would become "double" reader applicative)?

g :: (Monoid a, IsString a) => [Maybe a] -> Maybe a
g = foldr (<>) (Just "")

So that result would be of type:

g2 :: (Monoid a, IsString a) => [b -> c -> Maybe a] -> b -> c -> Maybe a

I guess I could lift <> twice:

> :t (liftA2 $ liftA2 (<>))
(liftA2 $ liftA2 (<>))
  :: (Monoid c, Applicative f1, Applicative f) =>
     f (f1 c) -> f (f1 c) -> f (f1 c)

But I'm stuck trying to compose it with foldr to get the type signature I want.

Answer 1

这就是我会做的

g2 x y = g $ ($ y) <$> ($ x) <$> theList