patch applied (ghc): Deriving for indexed data types

[Manuel M T Chakravarty]

Simon,

> |   - We cannot derive Typeable.  This seems a problem of notation, more
> | than
> |     anything else.  Why?  For a binary vanilla data type "T a b", we
> | would
> |     generate an instance Typeable2 T; ie, the instance is for the
> | constructor
> |     alone.  In the case of a family instance, such as (S [a] (Maybe
> | b)), we
> |     simply have no means to denote the associated constuctor.  It
> | appears to
> |     require type level lambda - something like (/\a b. S [a] (Maybe b).
> 
> Suppose you have an indexed data type with kind
>         T :: * -> * -> * -> *
> Of these parameters, suppose 2 are type indices and 1 is fully polymorphic.

Just to ensure that I understand exactly what you mean, for indexed data
types (as opposed to indexed synonyms) we dropped this whole arity
business and distinction between arguments that can be indexes and
others that must be parametric.  (We can drop that distinction for
indexed data types as they only ever induce injective type functions;
ie, ones for which decomposition is valid.)

However, for every individual instance of an indexed data type, we could
peel of any trailing variable-only arguments.  (Somewhat like the eta
business for newtype deriving.)  So, for

  data instance T [a] b = ... deriving Typeable 

we could generate 

  instance Typeable a => Typeable1 (T [a]) where ...

However, for

  data instance T [a] (Maybe b) = ... deriving Typeable

we would generate

  instance (Typeable a, Typeable b) => Typeable (T [a] (Maybe b)) where ...

So, for different instances of the same family, we would use different
TypeableX classes.  Do you think that might be too confusing.

> Then shouldn't we generate an instance for Typeable1, *not* Typeable3:
>         instance (Typeable a, Typeable b) => Typeable1 (T a b) where...
> 
> Then there is no difficulty about generating its type representation is there?

I didn't look at the type representation yet, as I stumbled over the
mentiond issue first.  But I think it should be ok.

Manuel