On 7/11/11 11:18 AM, Yitzchak Gale wrote:
The standard way to create a Typeable instance is just to derive it. If you do that, you will not be affected by this change.
This is only the "standard way" if one is willing to sacrifice Haskell98 or Haskell2010 compatibility by using the non-standard DeriveDataTypeable extension.
But it seems that many packages create Typeable instances by explicitly using mkTyCon. If your package does this, it will eventually break, after a deprecation period.
I have done this in at least one of my packages (type-level-natural-number) because I wanted to keep my library compatible with Haskell2010 rather than relying on GHC-specific extensions. To be clear, I think that the new Typeable is an improvement and don't mind migrating my package to use it, I just get the sense after reading the linked exchange that those discussing this have just assumed that the standard way to use Typeable *should be* to use "deriving Typeable" and are missing what I suspect is a major reason why people avoid using it, which is to reduce dependence on GHC-specific extensions. Again, this not a complaint or an objection at all, I'm just sharing my own thoughts on the matter. :-) Cheers, Greg