Haskell：派生自其他类型类的类型类

Question

Suppose I have a typeclass for entities being kept in database. Some simplified example may look like this:

class Persistent a where
    fetch :: Int -> IO (Maybe a)
    store :: a -> IO Bool

Now I found that for store I may need to know the type of entity, so it should be also Typeable .

Is there some way to tell that all Persistent entities are Typeable without adding deriving (Typeable) to every specific data clause? Eg like this:

class Persistent a deriving (Typeable) where
    fetch :: Int -> IO (Maybe a)
    store :: a -> IO Bool

Answer 1

No, this is not possible.

In class Persistent a , a doesn't have to represent a data type declaration. It's just a type. For example, one can add a Persistent instance for Maybe Integer .

instance Persistent (Maybe Integer) where ...

So it doesn't make much sense to talk about "adding a deriving (Typeable) clause to all a s that happen to be Persistent ". One cannot say data Maybe Integer deriving Typeable or anything like that.

---

If you are absolutely sure that every Persistent thing must be Typeable , you may want to add a constraint to your Persistent class:

class Typeable a => Persistent a where ...

This however doesn't help you in any way with automatic derivation of Typeable . It just requires that for every Persistent instance there should be a Typeable instance, which you still have to produce yourself one way or another (eg by adding deriving (Typeable) to all relevant data types).