On Sun, Apr 3, 2011 at 1:00 PM, Tad Doxsee <tad.doxsee@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)