[Haskell-cafe] fundeps => type family

[Ryan Ingram]

On Sun, Apr 3, 2011 at 1:00 PM, Tad Doxsee <tad.doxsee at gmail.com> wrote:

>   "Equality constraints ... enable a simple translation of programs
>    using functional dependencies into programs using family
>    synonyms instead.
>
> So I tried:
>
> class (T s ~ a) => ShapeC a s where
>  type T s :: *
>  draw :: s -> String
>  copyTo :: s -> T s -> T s -> s
>
> but got a compile error:
>
>  Alas, GHC 7.0 still cannot handle equality superclasses: T s ~ a
>
> So my question is, how does one convert the above code to use type
> families instead of functional dependencies?  Is one technique
> preferable over another?
>

Sadly the documentation assumes the feature that you show is missing.  That
said, you don't need that feature for the simple FD you have.

Just do

class ShapeC s where
   type T s :: *
   draw :: s -> String
   copyTo :: s -> T s -> T s -> s


This code should work:

data ShapeD a = forall s. (ShapeC s, a ~ T s) => MkShapeD s

instance ShapeC (ShapeD a) where
    type T (ShapeD a) = a
    draw (MkShapeD s) = draw s
    copyTo (MkShapeD s) x y = MkShapeD (copyTo s x y)
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