Showing Functor result in Haskell

I have defined the following Functor with the following type class ...

-- Let's suppose we want to create a functor that
-- performs the opperation :
--
-- fmap inc (Strength 3)

data Stat a = Strength a | Agility a | Constitution a
          deriving Show

instance Functor Stat where
  -- fmap :: (a -> b) -> Stat a -> Stat b
  fmap g (Strength str)     = Strength (g str)
  fmap g (Agility agi)      = Agility (g agi)
  fmap g (Constitution con) = Constitution (g con)

-- Then it can be called as fmap inc (Strength 3)

-- Let's suppose I use a Type class for this function

class Inc a where
  incStat :: (Num a) => a -> a

instance Inc (Stat a) where
  incStat (Strength a) = Strength a

-- instance Show a => Show (Stat a) where
--   show (Strength a) = show (a)
--   show (Agility a) = show (a)
--   show (Constitution a) = show (a)

It builds ok, but when I try to do the following command

fmap incStat (Strength 3)

I'm getting the following error I don´t fully understand

<interactive>:62:1: error:
• Ambiguous type variable ‘b0’ arising from a use of ‘print’
  prevents the constraint ‘(Show b0)’ from being solved.
  Probable fix: use a type annotation to specify what ‘b0’ should be.
  Potentially matching instances:
    instance Show Ordering -- Defined in ‘GHC.Show’
    instance Show a => Show (Stat a)
      -- Defined at characteristicsFunctor.hs:9:24
    ...plus 26 others
    ...plus 12 instances involving out-of-scope types
    (use -fprint-potential-instances to see them all)
• In a stmt of an interactive GHCi command: print it

Can you help me ? What is the error saying ?

NOTE : take IncStat as identity . I`m just checking the structure of the script

Solution

incStat already understands your functor, it does not need fmap to teach it how. So you may either use incStat directly, without fmap:

> incState (Strength 3)
Strength 3

or you may use fmap directly, without incStat:

> fmap (1+) (Strength 3)
Strength 4

To encapsulate this version you may like to define incStat in terms of fmap:

instance Num a => Inc (Stat a) where
    incStat = fmap (1+)

If you insist on using both incStat and fmap, then you must have an outer functor for the fmap to operate on -- which, if you like, may again be Stat.

> fmap incStat [Strength 3, Strength 4]
[Strength 3, Strength 4]
> fmap incStat (Agility (Strength 3))
Agility (Strength 3)

As an aside, the Num constraint in your Inc declaration is almost certainly a mistake. I recommend deleting it.

class Inc a where incStat :: a -> a

The Num constraints are better placed on the instances, where they are used.

instance Num a => Inc (Stat a) where
    incState (Strength a) = Strength (1+a)