Operate on a list of Maybe values

Question

I can multiply a list per 2:

(* 2) <$> [1, 2, 3]

But I want multiply the elements which are Just:

(* 2) <$> [Just 1, Nothing, Just 3]

Error:

* Non type-variable argument in the constraint: Num (Maybe a)
  (Use FlexibleContexts to permit this)
* When checking the inferred type
    it :: forall a. (Num (Maybe a), Num a) => [Maybe a] Prelude Data.List 

Another try:

fmap (* 2) [Just 1, Nothing, Just 3]

Error:

* Non type-variable argument in the constraint: Num (Maybe a)
  (Use FlexibleContexts to permit this)
* When checking the inferred type
    it :: forall a. (Num (Maybe a), Num a) => [Maybe a]

I have tried more things: map2, fmap2, map (*2) map, etc.

Answer 1

You need to map twice: once to get inside the list, second time to get inside the maybe. The operator <$> only maps once, and you can't use the same operator twice, so you have to add a call to fmap:

fmap (* 2) <$> [Just 1, Nothing, Just 3]