Answer by Mandorman for How to deal with a list of 'Maybe [values]' in Haskell?

Thanks a lot chi, it works perfectly. I just made a new function:

describePaths situacio edges = case lookup situacio edges of    Nothing -> []    Just locs -> map describePath locs

.. and works pretty well:

*TextGame> situacio = "living-room"*TextGame> describePaths situacio edges["There is door going west from here.","There is ladder going upstairs from here."]

But I see that I do not fully understand the <$> operator. I follow the comments of : What does <$> mean in Haskell?

So I follow the suggestion : https://hoogle.haskell.org/?hoogle=%3C$%3E

This is call a Functor:

(<$>) :: Functor f => (a->b) -> f a -> f b

That actually is exactly the same as fmap:

fmap :: Functor f => (a -> b) -> f a -> f b

It seems that are the same:

*TextGame> (*2) <$> [1..3][2,4,6]*TextGame> fmap (*2) [1..3][2,4,6]

But, actually they are different:

*TextGame> map describePath <$> lookup situacio edgesJust ["There is door going west from here.","There is ladder going upstairs from here."]*TextGame> fmap (describePath) (lookup situacio edges)<interactive>:30:22: error:• Couldn't match type ‘[Lloc]’ with ‘Lloc’      Expected type: Maybe Lloc        Actual type: Maybe [Lloc]        ....

Could someone put a little more light on it ? I do not fully understand why 'fmap (describePath) (lookup situacio edges)' do not work?(I am playing with Maybe's and Just's but ...)