How can I generate random values in Haskell for any Value of an aeson Object

I've started coding Haskell few weeks ago. I'm trying to do something like a random JSON generator using aeson.

Suppose that I have a json object like this:

{
    name: "bob",
    age: 13,
    role: "admin"
}

When I use aeson to decode it, I get this:

let Just json = decode str :: Maybe Object

-- fromList [("name",String "bob"),("age",Number 13.0),("role",String "admin")]

What I want is how can I generate a list of JSON objects based on bob:

[
 {
   name: <any random string>,
   age: <any random number>,
   role: <any random string>
 },
 ...
]

I know that I need to fmap over the HashMap values, but I don't have any clue of how can I produce random values based on its types.

I'm not worried about other complex values that can appear on JSON fields yet. I'm focusing on numbers and strings for now.