[Haskell-beginners] Trying to understand type families, continued...

[Michael Litchard]

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 <michael at schmong.org> 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?
>