Newtype deriving doesn't work with type families

[rl]

This assumes -XTypeFamiles -XGeneralizedNewtypeDeriving. Example:

class C a where
  type T a
  foo :: a -> T a

instance C Int where
  type T Int = Int
  foo = id

newtype N = N Int deriving(C)

This happily produces an instance C N but no type instance T N. It should either (preferably) generate

type instance T N = Int

or fail. The example also compiles if T is a data family (the Int instance needs to be change accordingly). It should probably fail in this case.

BTW, this also compiles fine, with rather dramatic consequences:

type family T a
class C a where
  foo :: a -> T a

type instance T Int = Int
instance C Int where
  foo = id

type instance T N = Double
newtype N = N Int deriving(C)

I guess this last example is the same bug as #1496. I wonder if the deriving clause could generate something like:

instance T Int ~ T N => C Int

[simonpj]

Good point. For now I'm just going to make it fail. It's quite a bit more work to make it generate the extra instance, and the fact that it does not work at all for 'data' convinced me.

It's annoying that you therefore cannot do newtype-deriving for a class with an associated type. An alternative is to allow top-level declarations for associated types, and check that these exist when doing the instance decl. So this would be valid:

class C a where
  type T a
  op :: a -> T a

type instance T Int = Bool
instance C Int where
  op x = True

So I'll leave this open to record the idea, but meanwhile I'll push a patch to make it fail uniformly.

Simon

[simonpj]

This is the patch

Wed Dec 31 16:43:00 GMT 2008  simonpj@microsoft.com
  * Fix Trac #2721: reject newtype deriving if the class has associated types

Pls merge; then leave the ticket open as feature request for 6.12

Simon

[igloo]

Merged.