[Haskell-cafe] The Typeable class is changing

[Gregory Crosswhite]

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