fmap在我的数据类型列表上不起作用

Question

I have defined my datatype as:

data MyMaybe a = Lust a | Lothing deriving (Show,Eq,Ord,Functor, Foldable)

When I type fmap (+3) Lust 4 it gives me the correct answer: Lust 7

But when I type fmap (+3) [Lust 4, Lust 5] it does not work and I get the follwoing error:

Non type-variable argument in the constraint: Num (MyMaybe a)

Please tell me what to do resolve this error.

Answer 1

fmap (+3) works for a single MyMaybe in this case. If you want to map a function over a list, you map that function. Let's give fmap (+3) a name:

myMaybePlus3 :: Num a => MyMaybe a -> MyMaybe a
myMaybePlus3 = fmap (+3)

We can now see that it only works for a single MyMaybe . We can use map (or fmap ) to use it for a list:

map myMaybePlus3 [Lust 4, Lust 5]

But due to referential transparency, we can also simply write

map (fmap (+3)) [Lust 4, Lust 5]

So all you were missing was an additional (f)map .

Answer 2

Look at the type of fmap :

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

When you do

fmap (+3) [Lust 4, List 5]

You are using fmap on a list [] , so fa in this case is Num b => [MyMaybe b]

The implementation of fmap on a list is to map all elements of the list. In this case, the elements are MyMaybe's . But you can't apply +3 on your MyMaybe s because it isn't a Num . If you want you can do:

fmap (fmap (+3)) [Lust 4, List 5]

This will apply the function fmap (+3) on each element of the list.

---

You can use ghci to examine types of simple expressions, for example:

Prelude> :t fmap (+3)
fmap (+3) :: (Functor f, Num b) => f b -> f b

Shows that the argument to fmap (+3) must be a functor of some Num

Answer 3

In your code you are traversing through the list (using fmap ) and are trying to apply (+3) to all elements, but your list elements are of type Num a => MyMaybe a .

You need to apply fmap twice. Once for traversing the list as you did, and another to traverse each element inside the list which, again, has the type Num a => MyMaybe a .

So the code needs to be:

fmap (fmap (+3)) [Lust 4, Lust 5]