[Haskell-cafe] Deriving type family data

[andy morris]

2009/4/28 Tuve Nordius <tuve at student.chalmers.se>:
> If I for some data type wants to derive, in this case Data and Typeable for
> use with syb code, but the problem is the same regardless what I want to
> derive.
>
>
>
> data family Something
>
> data Tree = Leaf Something | Fork Something Tree Tree
>        deriving (Data, Typeable)
>
>
> The problem is I want to derive a class for a data type that depends on some
> non instantiated data type.
>
> I can of course rewrite Tree as:
>
> data (Data a) => Tree a = Leaf a | Fork a Tree Tree
>        deriving (Data, Typeable)
>
>
> but then data family Something is redundant, I want to think of Something as
> a not yet instantiated abstract data type
>
> Is there anyway to express that either the instance of Something used in
> Tree should be a member of Data, or even better
> any instance of Something should be a member of Data or maybe even better
> that every instance should  derive data.
>
> data family Something
>        deriving Data
>
>
> //Tuve Nordius_______________________________________________
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>

If you enable flexible contexts, you can have something like

    class Data (Something a) => HasSomething a where
        data Something a

It turns out you can't have contexts on family declarations
themselves, though, hence the dummy class. (BTW, you were missing the
parameters on your family declarations.)

Plus, trying it just now, you can't derive Typeable for family
instances, so you'll need to write it longhand. Data seems to be OK,
though.