kmc from #haskell at freenode has told me this is a classic case of "don't need no typeclass here". So I reckon I will take the approach discussed in #haskell and abandon this design. On Fri, Aug 19, 2011 at 4:53 PM, Michael Litchard wrote:

I'm playing around with type families, and using as a starting point, this example:

class GMapKey k where  data GMap k :: * -> *  empty       :: GMap k v  lookup      :: k -> GMap k v -> Maybe v  insert      :: k -> v -> GMap k v -> GMap k v

Here's an instance that will lead to my question

instance GMapKey Int where  data GMap Int v        = GMapInt (Data.IntMap.IntMap v)  empty                  = GMapInt Data.IntMap.empty  lookup k   (GMapInt m) = Data.IntMap.lookup k m  insert k v (GMapInt m) = GMapInt (Data.IntMap.insert k v m)

Here is my experiment

...

class PM m where    data ServerModel m :: * -> *    movetoPreProcess :: m -> FilePath -> IO ()

I want to write an instance for this. As GMap can have an Int with some value v, I would like ServerModel to have a type I create (with another data declaration, call it Foo v) with values I define.

Here's a little more detail for the sake of any explanation that might come my way.

data ModelFoo a = ModelBar a                | ModelBaz a

ModelFoo is the type I want to use with ServerModel. What would the instance declaration look like? If ServerModel is defined within PM, should ModelFoo be as well?