```
data Id a = Id a
data Const a b = Const a

```

the functor instance of above is

```
instance Functor Id where
  fmap f (Id x ) = Id (f x )
instance Functor (Const a) where
  fmap f (Const x ) = Const x

```

`f` somehow did not apply in the `const`, `const (f x)` I am confusing about how it work because at least it involves one variable.