Am Dienstag, 5. August 2014, 12:46:23 schrieb Carter Schonwald:

> i assume 7.6 and 7.8, if we're talking GHC rather than GCC :)
>
> in 7.8 you can't define userland typeable instances, you need only write
> deriving (Typeable) and you're all set.
> add some CPP to select the instances suitable

So you need to be able to change the definition of the data type, in order to
add deriving (Typeable). It's not possible to add a Typeable instance
declaration later.