On Mon, Jul 28, 2008 at 07:13:08PM +0100, Claus Reinke wrote:
My suggestion is to split this module into two, and stop the implicit import/export of the incomplete instances from Data.Generics.
I don't think that this is a good idea.
Pro: - the instances are still available, and only one explicit import away, so 'deriving instance Data' for types containing uninteresting functions is still convenient
But suppose you want to make such an instance in a library you write. Then you will import the instance, which means that it will be visible to all users of your library too, so they cannot define their own instance. Nor can they use other libraries that define their own instance. I believe instances really do have to be global.
- the situation is similar to Text.Show.Functions, as the convenience instances don't provide the full expected functionality, just barely enough for deriving to get by
I assumed that the only reason this is in its own module is so that you can have Prelude not export the instance, as H98 requires, but I don't know or can't remember the history behind it. Thanks Ian