How to add a class constraint to a Functor instance declaration in Haskell?

Question

I have defined the following data type:

data Probability a = PD { mass :: [(a, Ratio Int)] } 

Now I want to write that it is an instance of

Answer 1

What you're writing is not a Functor. A Functor's fmap must support arbitrary value types being mapped, not just those that satisfy a certain constraint. You can make it an instance of a more general concept though. For example, the package constrained-categories defines a class of constrained functors.